THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 

LOS  ANGELES 


The  RALPH  D.  REED  LIBRARY 

O 

DEPARTMENT  OF  GEOLOGY 

UNIVERSITY  OF  CALIFORNIA 

LOS  ANGELES,  CALIF. 


Tke  RALPH  D.  REED  LIBRARY 


DEPARTMENT  OK  GEOLOGY 

UNIVERSITY  of  CALIFORNIA 

LOS  ANGELES,  CALIF. 


^Xtrf  of  Oil  Companies  of  Southern  Cali- 
fornia, Alumni  and  Faculty  of  Geology  Depart- 
ment and  University  Library. 

1940 


*"  '  * 

C>  f-/  I  O  . 


RALPH  D.  REED 


Hmerican  Science  Series 

Physics.  v 

By  GEORGE  F.  BARKER. 

Chemistry. 

By    IRA   REMSKN,  President  of  tne  Johns   Hopkins   University. 

Astronomy. 

By  SIMON  NK  vcoMBand  EDWARD  S.  HOLDEN. 

Geology. 

By  THOMAS  C.  CHAMBKRLIN  and  ROLLIN  D.  SALISBURY,  Professors 
in  the  University  of  Chicago. 

Physiography. 

By  ROLLIN  D.  SALISBURY,  Professor  in  the  University  of  Chicago. 

Geography. 

By  ROLLIN  D.  SALISBURY,  HARLANH.  RAKKOWS  and  WALTER  S. 
TOWER,  of  the  Department  of  Geography,  the  University  of 
.Chicago. 

Central  Biology. 

By  WILLIAM  T.  SEDGWICK,  Professor  in  the  Mass.  Institute,  and 
EDMUND  B.  WILSON,  Professor  in  Columbia  University. 

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By  CHARLES  E.  BESSBY,  Professor  in  the  University  of  Nebraska. 

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By  A.  S.  PACKARD,  Professor  in  Brown  University. 

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By  H.  NEWELL  MARTIN. 

Psychology. 

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By  JOHN  DEWKV.  Professor  in  Columbia  University,  and  JAMBS 
H.  Turrs,  Professor  in  the  University  of  Chicago. 

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By  FRANCIS  A.  WALKER. 

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PHYSIOGRAPHY 


ROLLIN    D.    SALISBURY 

Professor  of  Geographic  Geology  and  head  of  the  Department  of  Geography 
in  the  University  of  Chicago 


THIRD  EDITION,  REVISED 


NEW    YORK 

HENRY    HOLT    AND    COMPANY 
1913 


COPYRIGHT,  1907 

BY 
HENRY  HOLT  AND  COMPANY 


Geology 
Library 


PREFACE 

THIS  volume  is  intended  for  students  of  early  college  or  normal- 
school  grade,  who  have  no  purpose  of  pursuing  the  study  of  physical 
geography  beyond  its  elements,  but  who  are  yet  mature  enough 
for  work  beyond  the  grade  appropriate  for  the  early  years  of  the 
secondary  schools  —  the  stage  when  physical  geography  is  usually 
studied  previous  to  college  or  normal-school  work.  No  book 
heretofore  prepared  has  been  intended  especially  for  this  class  of 
students.  The  work  outlined  here  is  essentially  the  work  covered  in 
the  University  of  Chicago  in  a  twelve  weeks'  course,  taken  most 
largely  by  students  who  have  but  recently  entered  college.  The 
work  outlined  might  appropriately  be  expanded  to  a  half-year 
course,  where  so  much  time  is  available. 

In  the  preparation  of  the  text,  the  effort  has  been  to  shape 
it,  where  practicable,  so  as  to  lead  the  student  into  the  subject 
under  discussion,  rather  than  to  tell  him  the  conclusions  which 
have  been  reached  by  those  who  have  made  the  subject  their 
special  study.  This  point  is  illustrated,  for  example,  by  the  treat- 
ment of  isothermal  maps.  This  method  of  work  has  been  found 
by  the  author,  and  by  numerous  other  teachers  as  well,  to  be 
eminently  successful  in  practice;  but  the  author  is  far  from  assum- 
ing that  every  teacher  will  approve  of  it,  or  that  it  is  the  best  for 
every  teacher.  That  method  is  best  for  any  teacher  which  he  can 
use  most  effectively.  If  some  method  other  than  that  of  this 
text  leads  to  better  results,  the  teacher  who  uses  the  book  should 
be  free  to  follow  it,  for  text-books  should  be  servants,  not  masters. 

The  book  contains  no  specific  suggestions  to  teachers.  It 
assumes  that  the  teacher  does  not  need  them.  It  is  the  author's 
belief  that  if  a  text  were  to  suggest  the  means  which  any  ingenious 
teacher  may  devise  for  arousing  interest  and  for  holding  it  through 
the  development  and  the  solution  of  problems,  it  would  attempt 
to  do  much  which  should  be  left  to  individual  initiative.  The 
author  does  suggest,  however,  that  the  leading  of  the  student  (1) 


8732 '17 


Iv  PREFACE 

to  raise  questions  pertinent  to  the  topic  under  discussion,  (2)  to 
formulate  them  into  definite  problems,  and  (3)  to  discover  the 
means  (a)  of  solving  the  problems,  and  (6)  of  testing  the  correct- 
ness of  his  solution,  must  always  be,  in  large  part,  the  work  of  the 
teacher,  not  of  the  text-book,  and  that  no  thoroughly  successful 
teaching  can  leave  these  things  undone.  The  text-book  can,  at 
best,  hope  merely  to  supply  the  setting  for  such  problems  and  such 
work. 

The  map  exercises  which  are  suggested  at  various  places  in 
the  book  are  essentially  those  which  are  used  by  the  author  and 
his  colleagues.  These  exercises  may  be  readily  extended,  if  time 
permits.  These  particular  exercises  have  grown  up  from  small 
beginnings,  through  the  collaboration  of  Dr.  Wallace  W.  Atwood 
and  Mr.  Harlan  H.  Barrows. 

Another  phase  of  work  which  should  not  be  neglected  is  work 
out  of  doors.  This  must  form  a  part  of  the  work  of  every  strong 
course  in  this  subject.  Directions  for  local  field  work  cannot  be 
outlined  profitably  in  a  text-book,  for  the  work  must  be  shaped 
with  reference  to  the  specific  locality  where  the  subject  is  studied. 
Both  field  work  and  map  work  should  have  for  their  aim  the  appli- 
cation of  the  principles  studied,  in  such  a  way  as  to  make  the 
subject  vital.  The  aim  of  every  laboratory  exercise  carried  out 
in  connection  with  this  subject  should  be  the  same,  and  any  labora- 
tory work  which  does  not  either  illustrate  and  enforce  principles, 
or  lead  to  them,  is  not  worth  development.  The  student  who 
cannot  apply  what  he  has  learned  in  the  class-room  to  his  out-of- 
door  surroundings,  has  not  secured  the  maximum  good  from  his 
study  of  the  subject. 

It  may  seem  to  some  teachers  into  whose  hands  this  volume 
may  fall  that  some  parts  are  unnecessarily  simple,  and  especially 
that  some  things  are  introduced  which  the  student  should  know 
before  entering  college.  In  the  abstract,  the  author  is  in  sympathy 
with  this  view;  but  it  is  to  be  remembered  that  large  numbers  of 
students  enter  colleges  and  normal  schools  without  any  knowledge 
of  this  subject  except  that  acquired  in  connection  with  general 
geography,  as  studied  in  the  elementary  schools. 

The  writer  is  indebted  to  various  colleagues  for  suggestions  of 
one  sort  and  another  in  the  preparation  of  this  volume,  but  es- 
pecially to  Mr.  Harlan  H.  Barrows,  who  has  read  the  manuscript 
with  great  care  and  intelligence,  and  has  made  many  useful  sug- 


PREFACE  V 

gestions.  Dr.  Atwood  has  also  rendered  important  assistance  at 
various  points.  Many  of  the  illustrations  of  the  volume  have  been 
taken  from  the  larger  work  on  Geology  by  Professor  T.  C.  Chamber- 
lin  and  the  author.  Many  others  have  been  taken  from  the  publi- 
cations of  the  United  States  Geological  Survey,  and  a  few  from  other 
sources,  acknowledged  in  the  text. 

UNIVERSITY  OF  CHICAGO,  December,  1906. 


CONTENTS 


PAGE 

INTRODUCTION 3 


PART  I 

THE  LITHOSPHERE 
CHAPTER  I 

RELIEF  FEATURES 

RELIEF  FEATURES  OF  THE  FIRST  ORDER 5 

The  continental  platforms,  10.  Continuity  and  dis- 
continuity of  continental  platforms  and  oceanic  basins,  11. 
Grouping  of  the  continents,  11.  Origin  of  relief  features 
of  the  first  order,  12. 

RELIEF  FEATURES  OF  THE  SECOND  ORDER 15 

Great  relief  features  of  the  land,  15.  Great  relief 
features  of  the  sea  bottom,  15. 

Plains 16 

Coastal  Plains,  17.  Explanation  of  contour  map,  19. 
Relief,  19.  Drainage,  21.  Culture,  21.  Interior  plains, 
22.  Topography  of  plains,  24.  Extent  and  habitability, 
25. 

Plateaus. ' 28 

Position  and  area  of  plateaus,  30.  Relief  of  plateaus, 
30.  Other  features  of  plateaus,  30.  Origin,  31. 

Mountains 33 

Mountains  in  history,  38.     Origin.  39. 

SUBORDINATE  TOPOGRAPHIC  FEATURES 42 

Land  surface  and  ocean  bottom,  42.  The  development 
of  minor  topographic  features,  42.  Changes  now  taking 
place  on  the  land,  43, 

vii 


viii  CONTENTS 

PAGE 

THE  MATERIALS  OF  THE  LAND 45 

Mantle  rock,  46.  Rock,  47.  Classes  of  solid  rock,  48. 
Sedimentary  rocks,  48.  Igneous  rocks,  51.  Metamorphic 
rocks,  53. 

CHAPTER  II 

THE  WORK  OF  THE  ATMOSPHERE 

MECHANICAL  WORK. — THE  WORK  OF  THE  WIND , .     55 

Dust 55 

Universality,  55.  Sources  of  dust,  56.  Volcanic  dust, 
56.  Loess,  58.  How  held  in  the  air,  61.  Distribution, 
62.  Gradational  effect  of  winds,  62. 

Sand 62 

Sources  of  sand,  62.  Lodgment  of  wind-blown  sand, 
62.  Dunes,  63.  Distribution  of  dunes,  63.  Configuration 
of  dunes,  65.  Destructiveness  of  eolian  sand,  65.  Migra- 
tion of  dunes,  67.  Not  all  eolian  sand  in  dunes,  69.  Ripple- 
marks,  69.  Gradational  effects,  69.  Abrasion  by  the 
wind,  70. 

THE  CHEMICAL  WORK 71 

Weathering,  72. 
CHANGES  BROUGHT  ABOUT  UNDER  THE  INFLUENCE  OF  THE  Am  .  .     72 

Freezing  and  thawing,  72.     Expansion  and  contraction 
of  rock;  rock-breaking,  73. 
SUMMARY 78- 

CHAPTER  III 

THE  WORK  OF  GROUND-WATER 

GENERAL  FACTS 80 

Source  of  land-water,  80.  The  fate  of  rain-water,  81. 
The  existence  of  ground-water,  83.  The  source  of  ground- 
water,  83.  Descent  of  ground-water,  84.  The  ground- 
water  surface,  85.  Amount  of  ground-water,  86.  The 
movement  of  ground-water,  86. 

Springs 89 

Temperature,  89.  Mineral  and  medicinal  springs,  90. 
Geysers,  90.  Artesian  and  flowing  wells,  94. 

THE  WORK  OF  GROUND-WATER 96. 

Chemical  Work 95, 

Solution,  96.  Deposition,  99.  Other  changes,  105. 
Summary,  105. 


CONTENTS  ix 

PAGE 

Mechanical  Work 105 

Abrasion,  105.  Slumping,  sliding,  etc.,  105. 

Weathering 110 

Conditions  att'ecting  weathering,  111. 


CHAPTER  IV 

THE  WORK  OF  RUNNING  WATER 

Sources  of  stream  water,  116. 

THE  EROSIVE  WORK  OF  STREAMS 120 

Load  and  loading,  122.  Carrying,  127.  Amount  of 
load,  128.  Erosion  defined,  129.  Deposition  a  necessary 
consequence  of  erosion,  129. 

Changes  Made  by  Rivers  in  their  Valleys 129 

The  deepening  of  valleys,  129.  Depth-limit,  131.  The 
widening  of  valleys,  131.  Width-limit,  134.  Valley  flats, 
135.  The  lengthening  of  valleys,  140.  Summary,  141. 

The  History  of  a  River  System 141 

The  courses  of  valleys,  144.  The  permanent  stream, 
145.  Not  all  valleys  are  grown-up  gullies,  146.  Growth 
of  tributaries,  147.  Stages  in  the  history  of  a  valley,  148. 
Cycle  of  erosion,  153.  Peneplains,  153. 

Rate  of  Land  Degradation 154 

Conditions  affecting  the  rate  of  erosion,  155. 

Exceptional  Features  Developed  by  Erosion 156 

Canyons  and  gorges,  156.  Bad  lands,  160.  Natural 
bridges,  161.  Rapids  and  falls,  163.  Narrows,  169. 
Rock  terraces,  171.  Monadnocks,  rock  ridges,  etc.,  171. 

Acddents.to  Streams 173 

Drowning,  173.  Rejuvenation,  174.  Ponding,  175. 
Piracy,  176. 

Consequent  and  Antecedent  Streams 177 

DEPOSITION  BY  RUNNING  WATER 179 

Causes  of  Deposition 180 

Loss  of  velocity,  180.     Excess  of  load  from  tributaries, 
181. 
Location  of  Alluvial  Deposits  and  their  Topographic  Forms. ...   182 

At  the  bases  of  steep  slopes,  182.     In  valley  bottoms, 
184.     Flood-plain  meanders,    187.     Fertility    of    alluvial 
plains,    191.      River   floods,  195.      At  debouchures,    198. 
Ill-defined  alluvium,  202. 
Alluvial  Terraces  . .  .  203 


x  CONTENTS 

CHAPTER  V 
THE  WORK  OF  SNOW  AND  ICE 

PAGE 

MINOR  FORMS  OF  ICE . . 207 

Ice  beneath  the  surface,  207.  The  ice  of  lakes,  207. 
Ice  on  the  sea,  210.  Ice-foot,  212.  Ice  in  rivers,  213. 
Ground-ice,  214.  Snow,  214.  Snow-fields,  215.  The 
snow-line,  217.  Ice-fields,  218. 

GLACIERS 219 

The  Valley  Glacier. . .  .< 223 

Its  surface,  223.  Waste  and  supply  of  ice,  229.  Rate 
of  movement,  229.  Conditions  affecting  rate  of  movement, 
230.  Nature  of  glacier  movement,  231. 

Ice-caps 234 

Piedmont  Glaciers 240 

THE  WORK  OF  GLACIERS 242 

Erosion,  242.  Materials  gathered,  251.  Deposition  of 
debris  in  transit,  253. 

Deposition  by  Glaciers 255 

The  terminal  moraine,  2G7.  The  ground  moraine,  257. 
Lateral  moraines,  257.  Distribution  and  disposition  of  the 
drift,  258.  Re'sume',  260. 

Fluvio-glacial  Deposits 265 

Icebergs 269 

ANCIENT  GLACIERS  AND  ICE-SHEETS 270 

Cause  of  Glacial  Epochs : 273 

CHANGES  PRODUCED  BY  THE  CONTINENTAL  GLACIERS 274 

Changes  Produced  by  Erosion 274 

On  elevations,  274.     In  valleys,  274.     Rock  basins,  275. 

Chanqes  Produced  by  Deposition 275 

General  distribution  of  the  drift,  275.  Terminal 
moraines,  276.  The  ground  moraine,  278.  Effect  of  drift 
on  topography,  278.  Effect  of  drift  deposits  on  drainage, 
280.  Stratified  drift,  287.  Effects  of  glaciation  on  human 
affairs,  287. 

CHAPTER  VI 

LAKES  AND  SHORES 

GENERAL  FACTS 292 

Distribution  of  Lakes 293 

In  latitude,  293.  In  mountains,  293.  Along  rivers, 
293.  Along  coasts,  294.  On  plateaus,  294.  Other  situa- 
tions, 295. 


CONTENTS  xi 

PAGE 

Area,  Topographic  Position,  Depth,  etc 29,5 

Area  and  topographic  position,  295.  Depth,  296. 
Volume,  297.  Movements  of  lake  water,  300.  Changes  of 
level,  300. 

Conditions  Necessary  for  the  Existence  of  Lakes 301 

The  sources  of  lake  water,  301. 

Changes  now  taking  Place  in  Lakes 302 

The  filling  of  their  basins,  302.  The  lowering  of  their 
outlets,  303.  Fate  of  lakes,  303. 

The  Origin  of  Lake  Basins 303 

Diastrophism.  303.  Vulcanism,  305.  Gradation,  305. 
River  lakes,  308.  Shore  lakes,  311.  Glacial  lakes,  311. 
Glacial  lakes  an  index  of  topographic  age,  313.  Lakes  due 
to  slumping,  313.  Solution,  weathering,  wind,  etc.,  313. 

Salt  Lakes 314 

The  Climatic  Effect  of  Lakes 316 

Economic  Advantages  and  Disadvantages 316 

THE  TOPOGRAPHIC  FEATURES  OF  SHORES 317 

Gradational  Changes  now  taking  Place  along  Shores 317 

Waves,  undertow,  shore  currents,  318.  Rivers,  329. 
Winds,  330.  Glaciers,  330.  Shore  ice,  333.  Extinct  lakes, 
333. 

CHAPTER  VII 
VULCANISM 

Examples  of  Active  Volcanoes  . . . . 341 

Stromboli,  341.  Vesuvius,  341.  Krakatoa,  348.  Mont 
Pelee  and  Soufriere,  350.  Hawaiian  volcanoes,  361.  Com- 
mon phenomena  of  an  eruption,  366. 

The  Products  of  Volcanoes 367 

Lava,  367.  Cinders,  ashes,  etc.,  368.  Gases  and 
vapors,  368. 

Number,  Distribution,  etc 368 

Number,  368.     Distribution,  368.     Historical,  371. 

Igneous  Phenomena  not  Strictly  Volcanic 371 

Fissure  eruptions,  371.     Intrusions  of  lava,  374. 

Causes  of  Vulcanism 375 

Topographic  Effects  of  Volcanic  Action 378 

Volcanic  cones,  378.  Destruction  of  volcanic  cones, 
382:  Examples  of  fresh  cones,  382.  Examples  of  worn 
cones,  382.  Mt.  Shasta,  "382.  Mt.  Rainier,  383.  Mt. 


xii  CONTENTS 

PACK 

Hood,  383.    The  Marysville  Buttes,  384.    San  Francisco 

Mountain,  384. 

Indirect  Topographic  Effects  of  Vulcanism 385 

Volcanic  necks,  385.     Columnar  structure,  388. 
Mud  Volcanoes  . .  .388 


CHAPTER  VIII 
CRUSTAL  MOVEMENTS.     DIASTROPHISM 

SECULAR  CHANGES 392 

Evidences  of  Elevation  (Relative)  of  Land 393 

Human  structures,  393.  Rocks,  393.  Measurements, 
393.  Organic  remains,  393.  Raised  beaches,  394.  Sea 
cliffs,  394.  Sea  caves,  394. 

Evidences  of  Relative  Depression 395 

Human  structures,  395.  Submerged  forests,  396. 
Submerged  valleys,  397.  An  Italian  temple,  397. 

7s  it  the  Land  or  the  Sea  which  Changes  its  Level  ? 398 

Why  the  Sea-level  Changes 400 

Sedimentation,  400.  Submarine  volcanic  extrusions, 
401.  Diastrophism,  401. 

Why  the  Land  Changes  Level 402 

Changes  of  Level  in  the  Interiors  of  Continents 402 

General  facts,  402.  Extent,  403.  Ancient  changes  of 
level,  403.  Future  changes  of  level,  404. 

Crustal  Deformation 405 

Warping  and  folding,  405.     Faulting,  406. 

EARTHQUAKES 408 

Definition,  408.  Strength  and  destructiveness,  408. 
Examples,  412.  Earthquakes  starting  beneath  the  sea, 
420.  The  earthquake  wave,  424.  Frequency,  427.  Dis- 
tribution, 429.  Causes  of  earthquakes,  430.  Surface 
changes  caused  by  earthquakes,  432. 

CHAPTER  IX 
ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES 

PLAINS 435 

PLATEAUS 437 

MOUNTAINS 437 

Distribution      of      mountains,      439.      Heights,      440. 


PAGE 

Oceanic  mountains,  442.     Changes  taking  place  in  moun- 
tains, 443. 

Origin  of  Mountains 445 

Volcanic  mountains,  445.  Mountains  produced  by  ero- 
sion, 445.  Mountains  produced  by  intrusion  and  uplift, 
447.  Mountains  produced  by  folding,  447.  Mountains 
produced  by  faulting,  448.  Summary,  449. 

Effects  of  Mountains  on  Mankind 450 

Climatic  effects,  450.  Barriers  to  transportation,  452. 
Barriers  to  animals  and  plants,  452.  Ores,  452.  Agricul- 
ture in  mountains,  456.  Scenic  effects,  456. 

THE  OUTLINES  OF  THE  CONTINENTS 457 

Size,  457.  Position,  460.  Relief,  460.  Distribution  of 
various  types  of  irregularities,  461.  Agents  of  gradation, 
462.  Diastrophism  463.  Vulcanism,  464.  Historical 
bearing  465. 

ISLANDS 466 

By  diastrophism,  466  By  vulcanism,  466.  By  grada- 
tion, 466.  By  combinations  of  diastrophism,  gradation, 
and  vulcanism,  467.  By  organic  processes,  468. 

CHAPTER  X 

TERRESTRIAL  MAGNETISM 
Declination,  477.     Dip,  479.     Intensity,  479. 

PART   II 

CHAPTER  XI 
EARTH  RELATIONS 

Form  of  earth,  482      Size,  484. 

Motions 484 

Rotation,  484.  Effect  of  rotation,  488.  Revolution, 
488. 

Latitude,  Longitude,  and  Time 490 

Latitude    490.     Longitude,  491.     Longitude  and  time, 
492.     Lengths  of  degrees,  494.     Inclination  of  axis  and  its 
effects,  497.     Apparent  motion  of  the  sun,  499.     Latitude 
and  sun  altitude,  502, 
The  Solar  System 504 


xiv  CONTENTS 

PART    III 
THE  ATMOSPHERE 

CHAPTER  XII 
GENERAL  CONCEPTION  OF  THE  ATMOSPHERE 

PAGB 

Substantiality,  506.  Relation  to  the  rest  of  the  earth, 
507.  Density  and  altitude,  507.  Height,  508.  Volume, 
510.  Mass,  510.  History,  510. 

CHAPTER  XIII 

CONSTITUTION  OF  THE  ATMOSPHERE 

Principal  constituents,  512.     Minor  constituents,  512. 
Impurities,  513.     Relations  of  constituents  to  one  another, 
513. 
The  Functions  of  the  Atmospheric  Elements 513 

Nitrogen,  513.  Oxygen,  514.  Carbonic-acid  gas,  514. 
Water  vapor,  517.  Dust,  518. 

CHAPTER  XIV 

TEMPERATURE  OF  THE  AIR 

The  thermometer,  520. 

THE  HEATING  OF  THE  ATMQSPHERE 521 

Sources  of  heat,  521.  Sun  heating:  insolation,  522. 
Primary  distribution  of  heat,  523.  Radiation,  526.  Con- 
duction, 526.  Convection,  527.  How  the  sun  heats  the 
atmosphere,  529. 

THE  SEASONS •. 531 

Differences  between  summer  and  winter,  532.  Why  we 
have  summer  when  we  do,  532.  Change  of  seasons,  533. 
Seasons  in  other  latitudes,  534.  Effect  of  varying  distance 
of  the  sun,  536.  Effect  of  altitude  on  temperature,  537. 

REPRESENTATION  OF  TEMPERATURE  ON  MAPS 539 

Isotherms,  539.  Isothermal  charts,  539.  What  deter- 
mines the  positions  and  courses  of  isotherms?  540.  Alti- 
tude, 545.  Isothermal  surfaces,  546. 

RANGE  OF  TEMPERATURE 553 

Daily,  553.     Seasonal,  559. 


CONTENTS  xv 

PAGE 

ATMOSPHERIC  TEMPERATURE  AND  ATMOSPHERIC  MOVEMENT 561 

Land-  and  sea-breezes,  561.  Monsoons,  562.  Moun- 
tain and  valley  breezes,  562.  Vertical  movements  and 
temperature,  563. 

CHAPTER  XV 

THE  MOISTURE  OF  THE  AIR 

Function  of  atmospheric  moisture,  564.  Sources  of 
water  vapor:  evaporation,  565.  Rate  of  evaporation,  566. 
Function  of  the  atmosphere  in  evaporation,  568.  Evapora- 
tion takes  up  heat,  568.  Amount  of  water  vapor  in  the 
air,  568.  Distribution  of  water  vapor,  569.  Atmospheric 
moisture  and  atmospheric  movements,  569.  Saturation, 
569.  Humidity  and  dew-point,  570.  Condensation,  572. 
Condensation  and  temperature,  572.  Dew  and  frost,  573. 
Clouds  and  fog,  574.  Forms  of  clouds,  576.  Precipitation, 
579.  Rain-making,  580.  Summary,  580. 

CHAPTER  XVI 

ATMOSPHERIC  PRESSURE 

The  barometer,  582.     Air  pressures  unequal,  583. 

Representation  of  Pressure  on  Maps  and  Charts 584 

Isobars,  584.  Isobaric  surfaces,  587.  The  courses  of 
isobars,  588.  Isobars  and  parallels,  589.  Relation  of  iso- 
bars to  land  and  water,  589.  Isobars  and  temperature, 
590  Isobars  and  humidity,  594.  High-pressure  belts,  594. 
Permanent  areas  of  low  pressure,  597.  Temporary  and 
local  variations  of  pressure,  597. 


CHAPTER  XVII 

GENERAL  CIRCULATION  OF  THE  ATMOSPHERE 

Prevailing  and  periodic  winds,  598.  The  general  effect 
of  unequal  insolation,  598.  Effect  of  the  extra-tropical 
belts  of  high  pressure,  600.  The  high-latitude  areas  of  low 
pressure,  601  Direction  of  winds,  602.  The  circumpolar 
whirl,  605.  Unequal  heating  of  land  and  water  a  disturbing 
factor,  605.  Summary,  612.  Gradient,  velocity,  and  di- 
rections of  wind,  613. 


xvi  CONTENTS 

PAGE 

The  General  Circulation  and  Precipitation 614 

Rainfall  in  the  zone  of  the  trades,  616.    Rainfall  in  the 
zones  of  the  prevailing  westerlies,  617. 


CHAPTER  XVIII 

WEATHER  MAPS 

Aperiodic  Changes  of  Pressure 620 

Isobars,  620.  Wind,  622.  Cloudiness,  precipitation, 
etc.,  623.  Temperature,  624.  Movements  of  cyclones  and 
anticyclones,  632.  Winds  incidental  to  cyclones  and  anti- 
cyclones, 645.  Origin  of  cyclones  and  anticyclones  of 
intermediate  latitudes,  648.  Tropical  cyclones,  648. 
Weather  predictions,  656.  Failure  of  weather  predictions, 
660.  Property  saved  by  predictions  of  storms,  frosts, 
floods,  etc.,  663. 

Special  Types  of  Storms 663 

Thunder-storms,  663.  Whirlwinds,  666.  Tornadoes, 
667.  Waterspouts,  673.  Foehn  winds,  Chinook  winds, 
etc.,  673. 

CHAPTER  XIX 

CLIMATE 

Definition,  676.     Uniformity  and  variability,  677. 

Classification  of  Climates 683 

Climatic  Zones 684 

Zones  defined  by  latitude,  685.  Zones  defined  by  winds, 
686.  Zones  defined  by  isotherms,  687.  Oceanic  climates, 
688.  Continental  imates,  690.  Mountain  and  plateau 
climates,  691.  Climatic  effect  of  forests,  692. 

The  Climates  of  the  Several  Zones 693 

The  tropical  zone,  693.  Climate  of  intermediate  zones, 
695.  Climate  of  the  polar  zones,  700.  Rainfall  and  agri- 
culture, etc.,  701.  Climate  and  life,  702. 

Changes  of  Climate 703 

Within  historic  time,  703.     In  geologic  time,  704. 


CONTENTS  xvil 

PART   IV 
THE  OCEAN 


The  sea-level,  707.  What  the  physical  geography  of  the 
sea  includes,  709.  Distribution  of  the  ocean  waters,  710. 
Depth,  711.  Mass,  712.  Topography  of  the  bottom,  713. 

CHAPTER  XXI 
COMPOSITION  OF  SEA-WATER 

The  mineral  matter  in  solution,  717.  Withdrawal  of 
mineral  matter  from  the  sea,  718.  A  suggestion  as  to  the 
age  of  the  ocean,  718.  Gases  in  sea-water,  718.  Salinity, 
density,  and  movement,  719.  Salinity  and  color,  720. 

CHAPTER  XXII 
THE  TEMPERATURE  OF  THE  SEA 

Temperature  of  the  surface,  721.  Temperature  and 
movement,  722.  Temperature  beneath  the  surface,  722. 
The  ice  of  the  sea,  726. 

CHAPTER  XXIII 
THE  MOVEMENTS  OF  SEA-WATER 

Causes  of  Movement 727 

Inequalities  of  level,  727.  Wind,  728.  Differential  at- 
traction of  sun  and  moon,  728.  Occasional  causes,  729. 

Types  of  Movement 729 

Waves 729 

Currents    730 

Cause  of  ocean  currents,  733.  Climatic  effects  of  ocean 
currents,  733.  Gradational  effects  of  ocean  currents,  734. 
Historical  suggestions,  735. 

Tides 735 

The  periodicity  and  the  cause  of  tides,  736.  Solar  tides, 
741.  Spring  tides  and  neap  tides,  744.  Other  variations 


xviii  CONTENTS 

FAQB 

in  the  height  of  high  tides,  744.    Cotidal  lines,  748.    Rate 
of  movement,  748.     Effects  of  tides  on  shores,  748. 

CHAPTER  XXIV 
THE  LIFE  OF  THE  SEA 749 

CHAPTER  XXV 
MATERIALS  OF  THE  SEA  BOTTOM 753 

CHAPTER  XXVI 
RELATION  OF  THE  SEA  TO  THE  REST  OF  THE  EARTH.  756 


PLATES 

PLATE  FACING  PAGE 

I  A  narrow  coastal  plain  in  Oregon 20 

II  A  well-drained  plain  in  Kansas 24 

III  An  ill-drained  plain  in  Wisconsin 25 

IV  Fig.     1.      The     Canyon     of     the     Yellowstone     River. 

Fig.  2.   The  Grand  Canyon  of  the  Colorado  River 30 

V     Fig.  1.    Dunes  on  coast  of  New  Jersey.     Fig.  2.    Dunes 
along  Arkansas  River  in  Kansas.      Fig.  3.   Dunes  in 

plains  of  Nebraska 66 

VI     Limestone  sinks  due  to  solution  by  ground-water.     Near 

Pikeville,  Tenn 98 

VII     Streams  disappearing  in  the  sand,  gravel,  etc.,  at  the  base 

of  mountains  in  an  arid  region 132 

VIII     A  stream  widening  its  valley  by  lateral  planation 133 

IX  Fig.  1.  A  meandering  stream.  The  Missouri  River. 
Fig.  2.  A  further  stage  in  the  development  of  a 
meander.  The  Schell  River,  Missouri.  Fig.  3.  A 

plain  in  old  age 138 

X     A  well-developed  river  flat.   Valley  of  the  Mississippi,  near 

Prairie  du  Chien,  Wis 138 

XI     Stream  flats.     The  Missouri  and  Big  Sioux  Rivers 139 

XII    Fig.  1.  Youthful  Valleys.     Shore  of  Lake  Michigan  just 
north  of  Chicago.     Fig.  2.   A  region  in  a  mature  state 

of  erosion 150 

XIII    The  Niagara  Gorge 151 

XIV     Entrenched  Meanders 174 

XV    A  piedmont  alluvial  plain  or  compound  alluvial  fan  in 

Southern  California 184 

XVI     The  alluvial  plain  of  the  Platte  Rivers  in  Nebraska 185 

XVII     Glaciers  on  Glacier  Peak,  Washington 248 

XVIII     A  portion  of  the  Bighorn  Mountains,  showing   glaciated 

valleys -. 249 

XIX    Characteristic  drift  topography 278 

XX    Fig.  1.  Coastal  lakes  formed  by  the  blocking  of  the  ends 
of  drowned  valleys.     Fig.  2.  A  group  of  lakes  on  the 

coastal  plain  of  Florida 294 

xix 


XX 


PLATES 


FACING    PAGE 


XXI    The  upper  end  of  Seneca  Lake,  New  York 295 

XXII    Fig.  1.  A  coast  line  developed  chiefly  by  wave  erosion. 

Fig.  2.  An  island  tied  to  the  mainland  by  a  "beach".  . .  322 

XXIII  A  section  of  the  California  coast,  showing  lands,  near  the 

coast,  which  have  recently  emerged 394 

XXIV  Cushetunk  and  Round  Mountains,  New  Jersey 395 

XXV     Dunning  Mountain,  Pennsylvania 438 

XXVI     An  area  southwest  of  Denver  showing  a  mountain  ridge 

dissected  by  erosion 439 


PHYSIOGRAPHY 


PHYSIOGRAPHY 


INTRODUCTION 

Definition.  The  science  of  physiography  has  been  variously 
defined,  and  while  there  is  still  much  difference  of  opinion  as  to 
the  precise  limits  that  should  be  set  to  it,  there  is  a  strong  dis- 
position, in  the  school  world  at  least,  to  regard  physiography  as 
one  with  physical  geography.  In  England,  physiography  is  often 
regarded  as  a  general  introduction  to  science,  and  is  made  to 
include  the  elements  of  all  the  physical  and  biological  sciences.  In 
some  other  quarters  physiography  is  regarded  as  the  physical 
geography  of  the  land. 

If  physiography  be  regarded  as  another  name  for  physical 
geography,  it  has  to  do  with  (1)  the  solid  part  of  the  earth,  the 
lithosphere,  (2)  the  water  of  the  earth,  the  hydrosphere,  and  (3) 
the  air  or  atmosphere.  Physiography,  however,  does  not  deal  with 
these  several  spheres  exhaustively.  The  science  of  the  atmosphere 
is  Meteorology;  the  science  of  the  ocean,  which  contains  the  larger 
part  of  the  water  of  the  hydrosphere,  is  Oceanography;  and  the 
science  of  waters  in  general  is  Hydrography.  The  complete  study 
of  the  lithosphere  includes  several  subordinate  sciences,  all  of  which 
may  be  considered  to  be  parts  of  the  broad  science  of  Geology, 
which  has  to  do,  to  some  extent,  with  the  atmosphere  and  the 
hydrosphere,  as  well  as  with  the  lithosphere. 

Physiography  may  be  said  to  deal  with  the  atmosphere  only  in 
so  far  as  the  atmosphere  affects  the  land,  the  water,  and  life,  and 
it  deals  with  the  water  primarily  in  its  relations  to  the  land  and  to 
life.  So  far  as  concerns  the  lithosphere,  physiography  deals  with 
its  surface  only,  though  it  is  more  than  a  mere  description  of  the 
surface;  it  involves  a  consideration  of  the  conditions  and  processes 
which  have  brought  the  surface  to  its  present  state.  The  processes 
involved  are  largely  the  result  of  the  activity  of  the  water  and 
the  air,  and  of  the  life  conditioned  by  them;  but  other  factors, 

3 


4  PHYSIOGRAPHY 

such  as  volcanic  forces,  and  the  forces  which  cause  the  slow  warp- 
ings  of  the  outer  part  of  the  lithosphere,  are  also  involved.  In 
other  words,  physiography  has  to  do  primarily  with  the  surface  of 
the  lithosphere,  and  with  the  relations  of  air  and  water  to  it.  Its  field 
is  the  zone  of  contact  of  air  and  water  with  land,  and  of  air  with  water. 

Physiography  is  not  sharply  separated  from  geology.  Geology 
has  to  do  with  the  history  of  the  earth;  while  physiography  has 
to  do  with  a  late  chapter  only  of  that  history, — the  history  of  the 
present  surface.  Every  period  of  the  past  has  had  its  physiography, 
and  the  history  of  the  successive  physiographies,  could  they  be 
fully  known,  would  give,  in  large  part,  the  history  of  the  earth. 

Physiography  is  also  closely  related  to  geography,  but  it  departs 
from  that  science  in  that  it  has  to  do  primarily  with  the  relations 
of  the  lithosphere,  atmosphere,  and  hydrosphere,  and  with  the 
physical  results  of  these  relations,  while  geography,  as  contrasted 
with  physical  geography,  concerns  itself  primarily  with  the  dis- 
tribution of  life  (including  man)  and  human  industries,  as  affected 
by  the  condition  of  the  land  surface,  climate,  resources,  etc.  Physi- 
ography may  be  said  to  be,  on  the  one  hand,  a  special  phase  of 
geography,  namely,  physical  geography,  and,  on  the  other,  a  special 
chapter  of  geology,  namely,  the  latest.  Since  physical  geography 
affects  the  distribution  of  life  and  all  its  activities,  it  is  not  out 
of  place,  in  its  study,  to  touch  again  and  again  the  biological  and 
historical  bearings  of  the  subject. 

Although  the  lithosphere,  the  hydrosphere,  and  the  atmosphere 
seem  very  distinct  from  one  another,  they  are  in  reality  somewhat 
less  sharply  separated  than  they  seem,  for  though  the  larger  part 
of  the  hydrosphere  is  contained  in  the  ocean,  lakes,  and  rivers,  a 
not  inconsiderable  part  has  sunk  into  the  soil  and  rocks,  while  a 
smaller  amount  always  exists  in  the  form  of  vapor  in  the  atmosphere. 
The  water  therefore  invades  both  the  lithosphere  below  and  the 
atmosphere  above.  So,  too,  a  part  of  the  atmosphere  penetrates 
the  soil  and  the  rocks  of  the  land,  and  is  mingled  with  the  water 
of  the  ocean,  lakes,  and  rivers.  Again,  solid  matter  from  the 
lithosphere  is  found  in  suspension  in  streams,  lakes,  etc.,  often 
making  them  muddy,  and  dust  is  always  present  in  the  atmosphere. 
In  spite  of  the  interpenetration  of  these  three  spheres,  they  remain  so 
distinct  that  the  boundaries  between  them  are  usually  well  defined. 

In  the  development  of  our  subject,  the  lithosphere,  the  atmos- 
phere, and  the  hydrosphere  will  be  considered  in  order. 


PART  I 
THE  LITHOSPHERE 

CHAPTER   I 
RELIEF   FEATURES 

THE  oceans  cover  nearly  three-fourths  of  the  surface  of  the 
earth,  while  but  little  more  than  one-fourth  of  the  lithosphere 
rises  above  the  level  of  the  seas,  forming  land.  The  volume  of  the 
water  in  the  oceans  is  so  great  that  if  the  surface  of  the  lithosphere 
were  reduced  to  a  common  level,  that  is  if  the  protuberant  parts 
were  planed  down  and  the  material  deposited  in  the  depressed 
areas,  there  would  be  no  land  at  all,  but  a  universal  ocean  nearly 
two  miles  deep.  The  existence  of  land  therefore  results  from  the 
fact  that  the  surface  of  the  solid  part  of  the  earth  is  uneven,  and 
that  the  water  has  settled  in  the  depressions. 

It  would  help  us  to  get  a  true  picture  of  the  surface  of  the 
solid  part  of  the  earth,  if  we  could  see  it  without  the  oceans;  but 
since  the  oceans  cannot  be  withdrawn,  some  conception  of  its 
surface  may  be  gained  from  a  relief  model  of  the  earth  which  does 
not  represent  the  water  (Figs.  1  and  2);  or,  if  such  a  model  is  not 
available,  relief  maps  and  charts  of  the  ocean  are  serviceable. 

RELIEF  FEATURES  OF  THE  FIRST  ORDER 

The  most  significant  feature  in  the  surface  of  the  lithosphere  is 
the  contrast  between  the  great  depressions,  which  we  call  the 
ocean  basins,  and  the  broad  elevations,  which  we  call  the  continental 
platforms.  The  continental  platforms  and  the  ocean  basins  are 
topographic  features  of  the  first  order.  The  contrast  between  them 
is  emphasized  by  the  fact  that  there  is  almost  everywhere  a  rather 
steep  slope  from  the  one  to  the  other, — a  steep  descent  from  the 


6 


PHYSIOGRAPHY 


continental  platforms  to  the  ocean  basins,  or,  looked  at  from  the 
other  point  of  view,  a  steep  ascent  from  the  ocean  basins  to  the 
continental  platforms  (Figs.  1,  2,  and  3). 

The  ocean  basins  and  the  continental  platforms  divide  the 
surface  of  the  earth  between  them.  Both  the  basins  and  the 
platforms  are  irregular  in  shape  and  irregular  in  distribution.  The 
larger  part  of  the  elevated  areas  is  in  the  northern  hemisphere, 
while  the  depressed  areas  are  far  in  excess  in  the  southern. 


FIG.  l. 


FJG   2. 


FIG.  1. — Photograph  of  the  Jones  Relief  Globe,  showing  the  North  Atlantic 
Basin  depressed  notably  below  the  continents  about  it.  The  vertical 
scale  of  the  globe  is  exaggerated. 

FIG.  2. — Photograph  of  the  Jones  Relief  Globe,  showing  the  basin  of  the 
Indian  Ocean,  with  its  distinctly  marked  borders. 


The  continental  platforms  are  somewhat  larger  than  the  con- 
tinents (Fig.  3),  and  the  ocean  basins  are  somewhat  smaller  than 
the  oceans.  The  oceanic  area  (more  than  143,000,000  square 
miles)  is  nearly  three  times  the  land  area  (nearly  54,000,000  square 
miles),  but  the  area  of  the  ocean  basins  proper  (about  133,000,000 
square  miles),  is  only  about  twice  as  great  as  the  area  of  the 
continental  platforms  (about  64,000,000  square  miles).  The 
discrepancy  between  the  area  of  the  oceans  and  that  of  the  ocean 
basins  results  from  the  fact  that  there  is  more  water  on  the  earth 
than  the  true  ocean  basins  will  hold,  and  this  excess  overruns  the 
rims  of  the  basins,  and  spreads  itself  out  on  the  low  borders  (the 
continental  shelves)  of  the  continental  platforms  (Figs.  4  and  6). 


RELIEF  FEATURES  7 

Some  10,000,000  square  miles  about  the  borders  of  the  continental 
platforms  are  thus  covered  by  shallow  water.  The  result  is  that 
the  area  of  the  continents  falls  short  of  the  area  of  the  continental 
platforms  by  this  amount,  while  the  area  of  the  oceans  corre- 
spondingly exceeds  that  of  the  ocean  basins.  The  waters  which 


Africa 


FIG.  3. — A  diagrammatic  section  of  the  earth  about  the  equator,  showing 
the  elevated  segments  (continents)  and  the  depressed  segments  (ocean 
basins).  Vertical  scale  X  40.  (Based  on  section  in  Stanford's  Atlas  of 
Universal  Geography.) 

lie  on  the  low  borders  of  the  continental  platforms  have  been 
called  epicontinental  (upon  the  continent)  seas. 

If  all  lands  were  graded  to  a  common  level  without  increasing 
or  decreasing  either  their  area  or  the  amount  of  material  they 
contain,  their  height  above  sea-level  would  be  about  2300  feet. 
If  the  bottom  of  the  sea  were  graded  to  a  common  level,  its  area 
remaining  as  now,  the  water  would  be  between  12,000  and  13,000 


8 


PHYSIOGRAPHY 


feet  deep  everywhere.    The  average  height  of  the  land  is  therefore 
a  little  less   than  half  a  mile  above  sea-level,  while  the  average 


§/ 

"K 


depth  of  the  ocean  bottom  is  but  little  less  than  two  and  a  half 
miles  below  sea-level.     The  difference  between  the  average  height 


RELIEF  FEATURES 


9 


o_ 


cf  the  continental  platforms  and  the  ocean  basins  is  therefore 
about  three  miles.  In  other  words,  about  two-thirds  of  the  surface 
of  the  solid  part  of  the  earth  is  sunk  about  three 
miles  below  the  other  third.  Three  miles  is  a 
little  less  than  y^^  of  the  radius  of  the  earth. 

The  surfaces  of  both  the  continental  plat- 
forms and  the  ocean  basins  are  uneven,  and  as 
a  result,  the  maximum  unevenness,  or  relief,  of 
the  surface  of  the  lithosphere  is  much  more 
than  three  miles.  Its  lowest  known  point, 
near  the  Fiji  Islands,  is  nearly  six  miles  (about 
31,000  feet)  below  the  level  of  the  sea,  while 
its  highest  point  (Mt.  Everest  in  the  Hima- 
layas) is  nearly  as  much  (about  29,000  feet) 
above  the  same  plane.  The  maximum  relief 
of  the  lithosphere  is  therefore  nearly  twelve 
miles,  or  about  5|¥  of  the  earth's  radius.  The 
areas  of  those  parts  of  the  ocean  basins  which 
approach  a  depth  of  six  miles  are,  however, 
very  limited  in  extent,  and  the  areas  of  land 
which  approach  the  height  of  six  miles  are 
hardly  more  than  points. 

The  following  table  gives  some  idea  of  the 
relief  of  the  lithosphere: 

Approximate  percent, 
of  total  area  of  the  earth. 

Area  of  land  more  than  6000  feet  above  sea-level    2 . 3 
Area  of  land  between  6000  and  600  feet  above 

sea-level 18 . 6 

Area   of  land  between  600  feet   above   sea-level 

and  sea-level 69 

Area  of  ocean  where  water  is  less  than  600  feet 

deep 7 

Area   of  ocean  where  water  is  between  600  and 

6000  feet  deep 7. 

Area  of  ocean  where  water  is  between  6000  and 

12,000  feet  deep 14 .8 

Area  of  ocean  where  water  is  between  12,000  and 

18,000  feet  deep 39  4 

Area  of  ocean  where  the  depth  of  the  water  ex- 
ceeds 18,000  feet 31 

i  •  ^r^ 

This  table  shows  that  more  than  half  the  lithosphere  is  more 
than  a  mile  below  sea-level. 


2111*3 


O   S-G 


*«*! 

2  «  o-^is 


J3  G 


10 


PHYSIOGRAPHY 


The  following  table  shows  the  proportion  of  land  at  various 
elevations  above  the  sea: 

Percent,  of  land. 

Less  than     600  feet about  21 .90 

Between       600  and     1,500  feet about  21 .63 

Between    1,500  and    3,000  feet about  21 .34 

Between    3,000  and    6,000  feet about  19.51 

Between    6,000  and  12,000  feet about  12.34 

Between  12,000  and  18,000  feet about     2.95 

Above      18,000  feet about       .  33 

This  table  shows  that  about  two-thirds  of  the  land  has  an 

elevation  of  less  than  3000  feet.     About  six-tenths  of  the  land  is 

less  than  500  meters  :   (1640  feet)  above  sea-level,  and  upon  it 

the  larger  part  of  the  population  of  the  earth  lives.     The  facts 

shown  in  the  above  table  are  expressed  diagrammatically 

Metes 

8800       by  Fig.  7. 

The  continental  platforms.   The  continental  tracts  which 
are  commonly  recognized  are  (1)  the  Eurasian,  (2)  the  Afri- 


FIG.  7. — Diagram  showing  the  relative  areas  of  the  lithosphere  at  various 
levels  above  and  below  sea-level.  Less  than  10  percent,  of  the  litho- 
sphere is  as  much  as  700  meters  above  the  sea,  and  only  28  percent, 
is  above  the  sea.  About  half  the  total  surface  of  the  lithosphere  is 
more  than  3500  meters  below  sea-level.  The  diagram  also  shows  that 
the  mean  surface  of  the  lithosphere  is  about  2300  meters  below  sea-level, 
the  mean  ocean  depth  about  3500  meters,  and  the  mean  elevation  of 
.  the  land  above  the  sea-level  about  700  meters.  (After  Wagner.) 

can,  (3)  the  North  American,  (4)  the  South  American,  and  (5)  the 
Australian,  which  includes  New  Guinea  on  the  north.  Besides  these 
elevated  segments  whose  summits  are  the  recognized  continents, 
there  are  other  lesser  though  still  great  segments  not  commonly 
recognized  as  continental.  Of  these  the  largest  is  (6)  Antarctica, 
which  should  probably  be  regarded  as  a  continent,  and  (7)  Green- 
land, which  is  universally  regarded  as  an  island.  Islands,  in 


1  It  is  serviceable  to  remember  that  1  meter  =  3. 281  (approximately)  feet. 


RELIEF  FEATURES 


11 


general,  are  not  to  be  looked  upon  as  relief  features  of  the  first 
order,  and  will  be  referred  to  in  other  connections. 

Continuity  and  discontinuity  of  continental  platforms  and 
oceanic  basins.  The  great  land  areas  are  notably  discontinuous, 
while  the  sea  is  continuous,  though  its  parts  bear  separate  names, 
as  Atlantic,  Pacific,  etc.  In  contrast  with  the  lands  and  the  seas, 
the  continental  platforms  are  much  more  nearly  continuous  than 
the  continental  lands,  while  the  ocean  basins  are  less  continuous 
than  the  oceans.  Thus  the  American  continental  protuberance  is 
connected  at  the  northwest  with  the  Asian  protuberance,  and  is 
but  slightly  disconnected  at  the  northeast  from  the  European, 
while  the  elevated  Eurasian  platform  is  connected  with  the 
Australian  and  the  African.  Of  the  continental  protuberances, 
Antarctica  alone  seems  to  be  really  isolated.  Of  the  ocean  basins, 
the  Arctic  is  measurably  isolated.  It  is  of  interest  to  note  that 
the  most  isolated  basin  is  about  one  pole,  and  the  most  isolated 
protuberance  at  the  other,  so  far  as  present  knowledge  of  the  polar 
regions  allows  of  generalization.  Some  of  the  smaller  deep  basins, 
such  as  those  of  the  Mediterranean  and  the  Gulf  of  Mexico,  have 
some  such  measure  of  isolation  as  some  of  the  larger  islands,  as, 
for  example,  Greenland  and  New  Zealand. 

Grouping  of  the    continents.     The  northern  hemisphere  con- 


FIG.  8. — Land  and  water  hemispheres. 

tains  more  than  twice  as  much  land  as  the  southern.  If  the  earth 
be  divided  into  two  hemispheres  having  their  poles  in  England 
and  New  Zealand,  respectively  (Fig.  8),  the  first  would  contain 
about  f  of  all  the  land,  and  might  be  called  the  land  hemisphere, 
while  the  latter  would  contain  only  about  |  of  the  land,  and  might 


12  PHYSIOGRAPHY 

be  called  the  water  hemisphere.  Even  in  the  land  hemisphere, 
however,  the  water  would  cover  rather  more  than  £  the  surface, 
while  in  the  water  hemisphere  it  would  cover  about  \\  of  it  (Fig.  8). 
Since  the  northern  hemisphere  contains  §  of  the  land  and  a  still 
higher  proportion  of  the  economically  efficient  land,  it  has  always 
supported  the  larger  part  of  the  human  race. 

Taken  together,  the  continents  may  be  looked  upon  as  forming 
a  great  horseshoe-shaped  protuberance  of  the  lithosphere,  ex- 
tending around  the  Atlantic  from  Cape  Horn  through  the 
Americas  and  Europe  to  the  Cape  of  Good  Hope  in  Africa,  with  a 
spur  stretching  to  the  southeast  to  the  East  Indies  and  Australia. 

If  Europe  and  Asia  be  regarded  as  separate,  the  continents, 
except  Antarctica,  may  be  grouped  in  pairs.  The  Americas  form 
one  pair,  Europe  and  Africa  another,  and  Asia  and  Australia  a 
third.  Considered  in  this  way,  the  longest  line  of  each  pair  is  in 
a  general  north  and  south  direction.  The  continents  are  often 
said  to  be  triangular  in  shape,  with  their  broadest  ends  to  the 
north,  and  their  apexes  to  the  south.  This  is  conspicuously  true 
of  South  America,  and  less  conspicuously  true  of  North  America 
and  Africa;  but  it  is  not  true  of  Europe  and  Asia,  either  by  them- 
selves or  combined,  or  of  Australia  or  Antarctica. 

Origin  of  relief  features  of  the  first  order.  The  origin  of  the 
ocean  basins  and  the  continental  platforms  is  not  known  with 
certainty.  It  is  not  certain  that  they  have  always  existed,  and  it 
is  not  likely  that  the  former  have  always  been  depressed  as  much 
as  now  below  the  latter,  though  there  has  apparently  been  little 
change  for  ages.  The  best  opinion  seems  to  favor  the  view  that 
the  sinking  of  the  ocean  basins,  rather  than  the  elevation  of 
the  continental  platforms,  has  been  the  important  factor  in  the 
development  of  the  topographic  features  of  the  first  order.  The 
chief  reason  for  this  view  is  the  general  fact  that  the  earth  is 
cooling,  and  therefore  shrinking.  Shrinking  means  that  the  out- 
side is  getting  nearer  (on  the  average)  to  the  center.  This  must 
result  in  the  depression  of  the  surface  on  the  average,  though  not 
necessarily  at  every  point. 

If  the  subsidence  of  the  ocean  basins  be  the  principal  factor 
in  developing  the  great  relief  features  of  the  lithosphere,  we  might 
think  of  the  continental  platforms  as  having  been  (1)  wedged  up 
(Fig.  9),  or  warped  (Fig.  10)  up  between  the  sinking  parts;  (2)  as 
having  remained  where  they  were  before  the  sinking  of  the  depressed 


RELIEF  FEATURES 

continent 


13 


Fig.  9  expresses  diagrammatically  the  conception  that  the  continents  were 
elevated  and  the  ocean  basins  depressed  by  movement  along  definite 
sliding  planes  or  fault  planes.  The  dotted  line  may  be  taken  to  rep- 
resent a  somewhat  uniform  original  surface,  which  may  be  looked 
upon  as  the  hypothetical  surface  before  continents  and  ocean  basins 
were  developed.  The  diagram  indicates  that  the  continents  have 
risen  above  this  surface,  while  the  ocean  basins  have  sunk  below  it. 

Fig.  10.  This  diagram  represents  the  same  conception  as  Fig.  9,  except 
that  the  movement  was  by  warping  instead  of  faulting. 

Fig.  11.  This  diagram  represents  the  same  conception  as  Fig.  9,  except 
that  the  continental  segment  is  represented  as  not  having  risen. 

Fig.  12.  This  diagram  represents  the  same  conception  as  Fig.  10,  except 
that  the  continental  segment  has  not  risen. 

Fig.  13.  This  diagram  represents  the  same  conception  as  Fig.  11,  except 
that  both  ocean  basin  and  continental  segment  are  represented  as 
having  sunk  below  the  original  level,  the  former  much  more  than  the 
latter. 


14  PHYSIOGRAPHY 

parts  (Figs.  11  and  12);  or  (3)  as  having  sunk,  but  as  having  sunk 
less  than  the  basins  (Fig.  13).  All  these  conceptions  imply  change 
in  the  relative  positions  of  continental  platforms  and  ocean  basins. 
All  may  have  elements  of  truth  in  them,  and  all  may  have  been 
combined,  so  far  as  now  known,  in  the  evolution  of  the  continents. 
Present  knowledge,  however,  does  not  permit  of  a  definite  state- 
ment of  their  relative  value,  nor  does  it  exclude  other  conceptions 
of  the  origin  of  the  topographic  features  of  the  first  order.  It  is, 
for  example,  possible,  or  even  probable,  that  the  surface  of  the 
lithosphere  was  never  uniform,  and  that  the  topographic  features 
of  the  first  order  are  not  entirely  the  result  of  deformation. 

One  conception  of  the  origin  of  ocean  basins  and  continental 
platforms  is  based  on  the  view  that  the  earth  grew  to  its  present 
size  from  a  smaller  ancestral  body  by  the  ingathering  of  matter 
which  was  once  outside  itself,  and  that  this  growth  was  not  equal 
in  all  places.  On  this  view,  its  surface  may  never  have  been 
smooth.  Even  if  this  conception  be  the  true  one,  it  is  alto- 
gether probable  that  movements  in  the  outer  parts  of  the  earth 
have  set  off  the  ocean  basins  and  the  continental  platforms  from 
each  other  more  and  more  sharply  in  the  course  of  the  long  history 
of  the  earth. 

Even  if  we  suppose  that  the  ocean  basins  have  sunk,  or  that  the 
continents  have  been  upraised,  the  times  of  movement  are  no 
better  known  than  the  methods;  but  it  is  probable  that  the  move- 
ments have  been  intermittent  rather  than  constant,  and  that  periods 
of  movement,  for  example  periods  of  sinking  of  the  ocean  basins, 
have  been  followed  by  periods  of  quiet. 

Geological  history  reveals  the  fact  that  the  areas  of  the  ocean 
and  land  have  changed  somewhat  from  time  to  time,  but  it  is 
not  known  that  the  relative  positions  of  ocean  basins  and  continental 
platforms  have  changed  notably.  If  the  bottom  of  the  sea  were 
to  .sink,  the  ocean  basins  would  hold  more  water,  and  some  part 
of  the  epicontinental  sea  would  be  drawn  off  the  submerged  parts 
of  the  continental  platforms,  that  is  off  the  continental  shelves. 
If  the  bottoms  of  the  ocean  basins  were  to  sink  about  600  feet, 
the  water  would  be  drawn  off  the  continental  shelves,  and  the 
continental  lands  would  correspond  with  the  continental  platforms. 
If  the  continental  tracts,  on  the  other  hand,  were  to  sink,  the 
waters  of  the  sea  would  encroach  upon  their  borders  farther  than 
now,  and  the  area  of  the  land  would  be  diminished.  Geology 


RELIEF  FEATURES  15 

teaches  that  such  changes  as  these  have  taken  place  at  various 
times  in  the  past,  so  that  the  lower  portions  of  the  continental 
platforms  have  been  alternately  submerged,  and  converted  into 
land. 

RELIEF  FEATURES  OF  THE  SECOND  ORDER 

The  continental  platforms  and  the  ocean  basins  are  the  relief 
features  of  the  first  order.  The  more  strongly  marked  lineaments 
of  these  two  great  divisions  of  the  lithosphere  are  the  relief  features 
of  the  second  order. 

Great  relief  features  of  the  land.  The  continental  platforms 
are  made  up  of  plains,  plateaus,  and  mountains.  The  plains  are 
the  lowlands  of  the  continents,  and  the  plateaus  and  mountains  are 
the  highlands;  but  no  one  of  these  great  types  can  be  defined  in 
terms  of  altitude  alone.  Most  continental  lands  may  be  readily 
classed  in  some  one  of  these  three  divisions,  but  many  small  islands 
do  not  seem  clearly  referable  to  any  one  of  them.  The  difficulties 
which  they  present  need  not,  however,  be  considered  at  this  point. 

Great  relief  features  of  the  sea  bottom.  The  major  topo- 
graphic divisions  of  the  land  may  be  contrasted  and  compared 
with  those  of  the  sea  bottom.  The  continental  shelves  which,  it 
will  be  remembered,  are  really  parts  of  the  continental  platforms, 
are  submerged  plains.  They  are  below  the  sea-level  by  an  amount 
comparable  to  the  elevation  of  the  land-plains  above  it.  Some  of 
the  land-plains  are,  however,  much  higher  above  the  sea  than 


FIG.  14. — Diagram  to  illustrate  the  relations  of  mountain,  plateau,  plain, 
ocean  basin,  ocean  deep,  etc. 

any  continental  shelf  is  below  it.  The  great  areas  of  ocean  bottom 
covered  by  water  one  to  three  and  a  half  miles  deep  may  be 
compared  to  the  higher  plains  and  the  plateaus  of  the  land,  in 
reverse;  while  the  very  deep  tracts  of  limited  extent  on  the  ocean 
bottom  may  be  compared  to  very  high  plateaus,  such  as  Tibet,  in 
reverse.  Deep  holes  in  the  sea-b3d,  corresponding  to  mountain 
peaks  in  reverse,  are  not  known  to  exist.  From  the  tables  on 
page  9,  and  from  Fig.  7,  it  will  be  seen  that  the  very  deep  areas 


16 


PHYSIOGRAPHY 


I, 
II 


of  the  sea,  say  more  than  12.000  feet,  are  much  more  extensive 
than  the  correspondingly  high  areas  of  the  land  (p.  10),  while 
the  low  areas  of  the  land  (plains)  are 
much  more  extensive  than  the  shallow 
(epicontinental)  part  of  the  sea. 

The  relation  between  the  topography 
of  continental  platforms  and  ocean 
basins  may  be  made  clear  in  another 
way.  Extensive  areas  of  plateaus  and 
lesser  areas  of  mountains  rise  above  the 
average  level  of  the  continental  plat- 
forms, while  a  few  relatively  small 
basins,  some  of  them  occupied  by  lakes, 
sink  far  below  it.  Similarly,  ridges  and 
peaks,  roughly  comparable  to  the  moun- 
tains of  the  land,  and  broad  areas  such  iddresissipi 
as  the  continental  shelves,  comparable 
to  the  plateaus,  rise  well  above  the  gen- 
eral level  of  the  ocean  floor,  while  rela- 
tively small  basins  (deeps)  are  depressed 
far  below  it.  These  relations  are  ex- 
pressed diagrammatically  in  Fig.  14. 
Fig.  7  expresses  the  relations  of  the  sur-  o 
face  of  the  lithosphere  to  sea-level  both 
in  extent  and  in  relief. 

Plains 


The  plains  are  the  lowlands  of  the 
earth,  yet  they  can  hardly  be  denned  in 
terms  of  altitude  above  sea-level,  the 
datum  to  which  all  elevations  are  com- 
monly referred.  They  may  be  but  a  few 
feet  above  sea-level,  or  they  may  be 
thousands  of  feet  above  it.  In  the  lat- 
ter case,  however,  they  are  generally  far 
from  the  sea,  and  distinctly  lower  than 
other  lands  on  at  least  one  side.  Fig.  15 
is  intended  to  give  some  idea  of  the  re- 
lations of  large  plains.  It  will  be  seen  that  plains  may  be  as  high 


RELIEF  FEATURES  17 

above  sea-level  as  low  plateaus  are,  or  even  as  low  mountains, 
though  this  is  not  usually  the  case.  They  are  never  as  high  as 
plateaus  or  mountains  in  their  own  vicinity. 

Plains  differ  widely  among  themselves,  not  only  in  height,  but 
in  position,  in  size,  in  topography,  in  fertility,  in  origin,  and  in 
various  other  ways.  Various  names  are  applied  to  various  types 
of  plains,  the  names  being  intended  to  direct  attention  to  one  or 
another  distinctive  feature.  Considered  as  topographic  features 
of  the  second  order,  the  most  important  classes  of  plains  are 
Coastal  Plains,  which  border  the  sea,  and  Interior  Plains,  which  are 
far  from  the  sea,  or  separated  from  it  by  high  lands. 

Coastal  Plains.  These  plains  occur  on  the  borders  of  many 
continents,  as  along  the  eastern  coast  of  the  United  States  south  of 
New  York.  They  may  be  narrow  or  wide.  A  narrow  plain  is  shown 
in  Fig.  16,  which  represents  a  diagrammatic  plain,  not  an  actual 


FIG.  16. — A  narrow  coastal  plain. 

one.  It  is  low,  and  has  a  nearly  plane  surface  which  slopes  gently 
toward  the  sea.  Its  surface  is  made  uneven  by  the  shallow 
valleys  of  the  streams  which  flow  across  it.  The  inner  edges  of 
coastal  plains  are  not  always  so  clearly  defined  as  in  this 
illustration. 

A  narrow  coastal  plain  may  have  originated  in  either  of  two 
ways:  (1)  It  may  be  a  part  of  the  former  continental  shelf  from 
which  the  sea  has  withdrawn,  or  (2)  the  sediment  washed  down 
from  the  land  may  have  been  deposited  in  the  shallow  water  of 
an  epicontinental  sea,  building  up  (aggrading)  its  bottom  above 
the  surface  of  the  water,  and  thus  converting  it  into  land.  Coastal 
plains  have  been  made  in  both  these  ways,  and  both  processes 
have  often  been  concerned  in  the  making  of  a  given  plain.  Coastal 


IS 


PHYSIOGRAPHY 


RELIEF  FEATURES  19 

plains  may  also  be  made  by  the  degradation  of  coastal   lands 
which  were  once  high. 

Plate  I  represents,  in  another  way,  a  part  of  the  narrow  coastat 
plain  of  Oregon,  and  Fig.  17  shows  the  Cdastal  Plain  of  the  Atlantic 
and  Gulf  coasts  of  the  United  States.  Since  illustrations  of  the 
sort  shown  in  Plate  I  will  be  used  frequently  in  the  following 
cages,  the  principles  on  which  it  is  based  must  be  clearly  understood. 

EXPLANATION   OF   CONTOUR   MAP 

"The  features  represented  on  the  topographic  map  are  of  three  distinct 
kinds:  (1)  inequalities  of  surface,  called  relief,  as  plains,  plateaus,  valleys, 
hills,  and  mountains;  (2)  distribution  of  water,  called  drainage,  as  streams, 
lakes,  and  swamps;  (3)  the  works  of  man,  called  culture,  as  roads,  railroads, 
boundaries,  villages,  and  cities. 

"Relief.  All  elevations  are  measured  from  mean  sea-level.  The  heights 
of  many  points  are  accurately  determined,  and  those  which  are  most  impor- 
tant are  given  on  the  map  in  figures.  It  is  desirable,  however,  to  give  the 
elevation  of  all  parts  of  the  area  mapped,  to  delineate  the  horizontal  outline, 
or  contour,  of  all  slopes,  and  to  indicate  their  grade  or  degree  of  steepness. 
This  is  done  by  lines  connecting  points  of  equal  elevation  above  mean  sea- 
level,  the  lines  being  drawn  at  regular  vertical  intervals.  These  lines  are 
called  contours,  and  the  uniform  vertical  space  between  each  two  contours 
is  called  the  contour  interval.  On  the  maps  of  the  United  States  Geological 
Survey  the  contours  and  elevations  are  printed  in  brown  (see  Plate  I). 

"The  manner  in  which  contours  express  elevation,  form,  and  grade 
is  shown  in  the  following  sketch  and  corresponding  contour  map,  Fig.  18. 

"The  sketch  represents  a  river  valley  between  two  hills.  In  the  fore- 
ground is  the  sea,  with  a  bay  which  is  partly  closed  by  a  hooked  sand-bar. 
On  each  side  of  the  valley  is  a  terrace.  From  the  terrace  on  the  right  a 
hill  rises  gradually,  while  from  that  on  the  left  the  ground  ascends  steeply 
in  a  precipice.  Contrasted  with  this  precipice  is  the  gentle  descent  of  the 
slope  at  the  left.  In  the  map  each  of  these  features  is  indicated,  directly 
beneath  its  position  in  the  sketch,  by  contours.  The  following  explanation 
may  make  clearer  the  manner  in  which  contours  delineate  elevation,  form, 
and  grade: 

"1.  A  contour  indicates  approximately  a  certain  height  above  sea-level. 
In  this  illustration  the  contour  interval  is  50  feet;  therefore  the  contours  are 
drawn  at  50, 100, 150,  200  feet,  and  so  on,  above  sea-level.  Along  the  contour 
at  250  feet  lie  all  points  of  the  surface  250  feet  above  saa;  and  similarly 
with  any  other  contour.  In  the  space  between  any  two  contours  are  found 
all  elevations  above  the  lower  and  below  the  higher  contour.  Thus  the 
contour  at  150  feet  falls  just  below  the  edge  of  the  terrace,  while  that  at 
200  feet  lies  above  the  terrace;  therefore  all  points  on  the  terrace  are  shown 
to  be  more  than  150  but  less  than  200  feet  above  sea.  The  summit  of  the 
higher  hill  is  stated  to  be  670  feet  above  sea;  accordingly  the  contour  at 
650  feet  surrounds  it.  In  this  illustration  nearly  all  the  contours  are  num- 


20 


PHYSIOGRAPHY 


bered.  Where  this  is  not  possible,  certain  contours — say  every  fifth  one — • 
are  accentuated  and  numbered;  the  heights  of  others  may  then  be  ascer- 
tained by  counting  up  or  down  from  a  numbered  contour. 

"2.  Contours  define  the  forms  of  slopes.  Since  contours  are  continuous 
horizontal  lines  conforming  to  the  surface  of  the  ground,  they  wind  smoothly 
about  smooth  surfaces,  recede  into  all  reentrant  angles  of  ravines,  and  project 
in  passing  about  prominences.  The  relations  of  contour  curves  and  angles 
to  forms  of  the  landscape  can  be  traced  in  the  map  and  sketch. 


FIG.  18. — Sketch  and  map  of  the  same  area  to  illustrate  the  representation 
of  topography  by  means  of  contour  lines.     (U.  S.  Geol.  Surv.) 

"3.  Contours  show  the  approximate  grade  of  any  slope.  The  vertical 
space  between  two  contours  is  the  same,  whether  they  lie  along  a  cliff  or  on 
a  gentle  slope;  but  to  rise  a  given  height  on  a  gentle  slope  one  must  go  farther 
along  the  surface  than  on  a  steep  slope,  and  therefore  contours  are  far  aparf 
on  gentle  slopes  and  near  together  on  steep  ones. 

"For  a  flat  or  gently  undulating  country  a  small  contour  interval  is  used; 
for  a  steep  or  mountainous  country  a  large  interval  is  necessary.  The  smallest 
interval  used  on  the  atlas  sheets  of  the  Geological  Survey  is  5  feet.  This 
is  used  for  regions  like  the  Mississippi  delta  and  the  Dismal  Swamp.  In 
mapping  great  mountain  masses,  like  those  in  Colorado,  the  interval  may 


PLATE   I 


ToweHRock"  "  "^ '" 


A  narrow  coastal  plain  in  Oregon.     Scale  2  —  miles  per  inch.     (Port  Orford 
Sheet,  U.  S.  Geol.  Surv.) 


RELIEF  FEATURES  21 

be  250  feet.  For  intermediate  relief  contour  intervals  of  10,  20,  25,  50,  and 
100  feet  are  used. 

"Drainage.  Watercourses  are  indicated  by  blue  lines.  If  the  streams 
flow  the  year  round  the  line  is  drawn  unbroken,  but  it  the  channel  is  dry 
a  part  of  the  year  the  line  is  broken  or  dotted.  Where  a  stream  sinks  and 
reappears  at  the  surface,  the  supposed  underground  course  is  shown  by  a 
broken  blue  line.  Lakes,  marshes,  and  other  bodies  of  water  are  also  shown 
in  blue,  by  appropriate  conventional  signs. 

"Culture.  The  works  of  man,  such  as  roads,  railroads,  and  towns,  together 
with  boundaries  of  townships,  counties,  and  states,  and  artificial  details, 
are  printed  in  black."  ' 

CONTOUR-MAP  EXERCISE2 

1.  Draw  a  contour-line  map  of  a  conical  mountain  the  top  of  which 
is  2000  feet  high,  making  the  contour  interval  200  feet. 

2.  Draw  a  contour-line  map  of  a  plain   five  miles  square,  one  edge 
of  which  is  at  sea-level  and  the  opposite  one  at  an  elevation  of  100  feet. 
The  otherwise  uniform  seaward  slope  of  the  land  is  scarred  by  a  single 
valley,  without  tributaries,  which  extends  across  the  entire  width  of 
the  plain.     Use  a  10-foot  contour  interval,  and  a  horizontal  scale  of  one 
inch  to  the  mile. 

The  Coastal  Plain  of  the  eastern  part  of  the  United  States  has  a 
width  ranging  from  a  few  to  60  miles  in  New  Jersey,  to  100  miles 
or  more  in  the  Carolinas  and  Georgia  (Fig.  17),  and  would  be 
counted  a  wide  coastal  plain.  The  Coastal  Plain  bordering  the 
Gulf  of  Mexico  is  still  wider,  reaching  a  maximum  width  of  several 
hundred  miles  in  the  vicinity  of  the  Mississippi.  The  Coastal  Plain 
of  northern  Eurasia  is  still  wider,  though  locally  interrupted  by 
mountains,  such  as  the  Urals. 

At  their  seaward  edges  coastal  plains  are  commonly  but  little 
above  the  sea.  Their  inland  borders,  on  the  other  hand,  especially 
if  they  be  wide,  may  be  hundreds  of  feet  above  the  sea.  At  its 
landward  edge  a  coastal  plain  may  abut  against  a  plateau  or 
against  mountains  by  a  slope  somewhat  steeper  than  that  of  the 
plain  itself  (Fig.  6).  It  is  this  steep  slope,  rather  than  any  par- 
ticular altitude  above  the  sea,  which  limits  a  coastal  plain  to 
landward.  The  landward  border  of  the  Atlantic  Coastal  Plain 
has  an  altitude  ranging  from  100  feet  or  so  to  several  hundred 
feet.  The  relatively  steep  slope  marking  its  landward  edge  is 

1  From  folio  preface,  U.  S.  Geol.  Surv. 

2  The  author's  experience  has  been  that  students  come  to  an  appreciation 
of  topographic  maps  most  readily  by  making  them. 


22  PHYSIOGRAPHY 

known  as  the  Fall  Line,  and  along  it  are  located  many  impor- 
tant cities,  among  them  Trenton,  Philadelphia,  Baltimore,  Wash- 
ington, Richmond,  Raleigh,  Camden,  Columbia,  and  Augusta. 
The  location  of  these  cities  was  determined  largely  by  the  fact  that 
the  streams  were  readily  navigable  in  the  Coastal  Plain,  but  not 
above.  The  position  of  the  Fall  Line  was  determined  by  the 
inequalities  of  hardness  of  the  underlying  rocks.  Those  to  the 
west  of  it  are  much  harder  than  those  to  the  east.  The  landward 
margin  of  the  Coastal  Plain  of  the  Gulf  of  Mexico  west  of  Alabama 
is  less  well  marked  than  that  of  the  Atlantic  border. 

Such  coastal  plains  as  exist  along  the  eastern  side  of  North 
America  north  of  New  Jersey,  and  along  the  western  coast  of  the 
continent,  are  narrow  and  discontinuous,  and  for  considerable 
stretches  are  wanting  altogether.  Coastal  plains  are  therefore  to 
be  looked  upon  as  common,  but  not  as  universal,  features  of  con- 
tinental borders. 

If  the  epicontinental  seas  were  withdrawn  from  the  submerged 
parts  of  the  continental  platforms,  the  coastal  plains  of  the  present 
land  would  be  seen  to  be  continuous,  topographically,  with  the 
continental  shelves.  These  submerged  parts  of  the  great  continental 
protuberances  of  the  lithosphere  are  therefore  to  be  looked  upon 
as  submerged  coastal  plains.  Many  of  the  existing  coastal  plains 
of  the  land  have  emerged  from  the  sea  in  very  late  stages  of  the 
earth's  history.  The  submerged  coastal  plains  are  more  nearly 
continuous  about  the  continents  than  the  coastal  plains  which 
are  above  sea-level. 

Interior  plains.  These  plains  are  often  higher,  and  sometimes 
much  higher,  than  coastal  plains.  A  large  part  of  the  great  area 
between  the  Appalachian  Mountains  on  the  east,  and  the  Rocky 
Mountains  on  the  west,  is  an  interior  plain.  At  the  south  it  is 
relatively  low,  and  merges  into  the  Coastal  Plain  bordering  the 
Gulf.  At  the  north  this  Interior  Plain  is  much  higher,  attaining 
an  elevation  of  more  than  1000  feet;  but  its  rise  is  so  gradual 
that  it  nowhere  ceases  to  have  the  general  effect  of  a  great  lowland. 
At  the  east,  also,  it  rises  until  the  Appalachian  Mountains  are 
approached.  Along  the  western  border  of  these  mountains  there 
is  a  higher  area,  about  1000  feet  above  sea-level,  often  known  as 
the  Cumberland,  or  Allegheny,  Plateau  (Fig.  17).  This  tract  is 
called  a  plateau,  rather  than  a  part  of  the  plain,  not  more  because 
of  its  altitude  than  because  it  is  often  somewhat  distinctly  set 


RELIEF  FEATURES  23 

off  from  the  lower  area  to  the  west.  To  the  west  the  interior  plain 
rises  gradually,  and  without  any  conspicuous  increase  of  slope, 
until  it  attains  an  altitude  of  several  thousand  feet.  In  spite  of 
this  very  considerable  elevation,  far  greater  than  that  of  the 
Cumberland  Plateau,  the  area  east  of  the  Rocky  Mountains  is 
usually  called  the  Great  Plains.  The  western  part  of  this  region 
is  perhaps  more  properly  a  plateau  than  a  plain;  but  it  is  notably 
lower  than  the  mountains  against  which  it  abuts  on  the  west, 
and  between  its  higher  parts,  next  to  the  Rockies,  and  its  lower 
parts,  adjacent  to  the  Mississippi,  there  is  no  abrupt  change  of  slope. 
It  is  clearly  a  topographic  unit.  If  the  western  part  of  this  area 
be  classed  as  plateau,  the  area  affords  a  good  illustration  of  the 
gradation  of  a  plain  into  a  plateau,  for  the  line  separating  the 
plain-part  from  the  plateau-part  would  be  an  arbitrary  one. 
Even  if  the  higher  western  part  of  the  Great  Plains  be  regarded 
as  plateau,  it  is  still  true  that  portions  of  the  interior  plain  are 
higher  than  many  areas  which  are  called  plateaus.  Areas  like 
the  Great  Plains  are  classed  as  plains,  not  primarily  because  of 
their  altitude  above  sea-level,  but  because  they  do  not  stand 
conspicuously  above  their  surroundings  on  any  side. 

The  general  topographic  relations  of  the  Great  Plains  are 
illustrated  diagrammatically  by  Fig.  15.  If  the  general  slope  of 
the  area  between  the  Rocky  Mountains  and  the  Mississippi  River 
had  been  that  of  the  dotted  line  shown  in  the  middle  of  this 
figure, -Its  western  part  would  doubtless  have  been  classed  as  a 
plateau,  and  the  line  of  separation  between  plateau  and  plain 
would  have  been  a  natural  one. 

Here  and  there  mountains,  snch  as  the  Black  Hills  of  South 
Dakota,  the  Ozark  Mountains  (plateau)  of  Missouri,  and  the 
Ouachita  (pronounced  Wash'-i-ta)  Mountains  of  Arkansas,  Indian 
Territory,  and  Oklahoma,  rise  distinctly  above  the  general  level  of 
this  great  Interior  Plain.  The  Ozark  and  Ouachita  Mountains 
do  not  attain  an  elevation  equal  to  that  of  the  western  margin  of 
the  Great  Plains,  but  they  are  so  distinctively  and  conspicuously 
above  their  immediate  surroundings  that  they  are  not  regarded 
as  parts  of  the  plains,  and  the  summit  area  of  the  Ouachita  Moun- 
tains ,  at  least,  is  so  limited  that  they  cannot  be  regarded  as  a 
plateau.  The  Ozark  Mountain  tract,  on  the  other  hand,  might 
equally  well  have  been  called  the  Ozark  Plateau,  for  the  character 
of  the  region  which  bears  this  name  is  intermediate  between  that 


24 


PHYSIOGRAPHY 


of  a  well-defined  mountain  group  and  a  plateau.  The  Black  Hills 
are  higher,  and  more  distinctly  set  off  from  the  plains,  than  are 
the  Ozark  and  the  Ouachita  Mountains.  They  are  mountains,  in 
spite  of  their  name. 

Interior  plains  have  come  into  existence  in  various  ways. 
Some  of  them  are  former  coastal  plains,  now  partially  shut  off 
from  the  sea  by  the  development  of  highlands  between.  Some 
of  them  represent  areas  which  were  once  high,  but  which  have 
been  worn  down  by  rivers,  and  by  the  other  agents  which  degrade 
lands;  others  may  have  originated  in  other  ways. 


Fio.  19. — A  plain  with  little  relief.     Valley  plain  of  the  Cimarron  River, 
southwestern  Kansas.     (U*.  S.  Geol.  Surv.) 

Topography  of  plains.  The  surfaces  of  plains  are,  on  the 
whole,  much  less  uneven  than  the  surfaces  of  plateaus  and  moun- 
tains. The  surfaces  of  plains  may  indeed  be  nearly  flat,  though 
more  commonly  they  are  somewhat  uneven.  The  relief  is  some- 
times slight  and  sometimes  considerable,  and  in  general  high 
plains  are  rougher  than  low  ones.  To  this  general  statement  there 
are,  however,  local  exceptions,  for  considerable  areas  of  high  plains 
are  sometimes  nearly  flat. 


PLATE  II 


A  well-drained  plain  in  Kansas.     Scale  2—  miles  per  inch. 
(Anthony,  Kan.,  Sheet,  U.  S.  Geol.  Surv.) 


PLATE  III 


An  ill-drained  plain  in  Wisconsin.     Scale   1  —    mile  per  inch       (Silver 
Lake  Sheet.  U.  S.  Gecl.  Surv.) 


RELIEF  FEATURES 


25 


The  unevennesses  of  surface  differ  in  kind,  as  well  as  in  amount. 
Thus  in  some  plains,  or  in  some  parts  of  plains,  all  depressions 
have  outlets  through  which  the  surface  water  flows  away,  while  in 
others  numerous  depressions  have  the  form  of  basins  which  contain 
ponds  and  lakes.  Plains  of  the  former  type  are  well  drained,  if 
the  depressions  are  numerous,  while  those  of  the  latter  are  ill 
drained.  Well-drained  areas  (PI.  II)  of  plain  prevail  in  the  south- 
ern part  of  the  United  States,  as  south  of  the  Ohio  and  the 
Missouri,  while  ill-drained  areas  (PI.  Ill)  abound  farther  north. 

The  topographic  features  of  plains  are  relief  features  of  the 
third  order,  and  will  be  considered  later;  but  the  points  here 
mentioned  have  a  bearing  on  the  topic  of  the  next  paragraph. 


FIG.  20. — A  plain  with  notable  relief.     Iowa.     (Calvin.) 

Extent  and  habitability.  Plains  constitute  the  larger  part  of 
the  area  of  the  land,  and  the  larger  part  of  the  population  of  the 
earth  lives  upon  them  (Figs.  21  and  22).  They  are  the  principal 
theatres  of  human  activity,  partly  because  the  climate  is  on  the 
whole  more  favorable  than  in  higher  regions,  and  partly  because 
there  is  a  greater  proportion  of  land  which  is  nearly  flat,  or  which 
has  but  gentle  slopes.  As  compared  with  higher  lands,  a  larger 
proportion  of  the  surface  of  plains  is  arable,  for  (1)  their  flats  and 
gentle  slopes  are  more  generally  covered  with  soil  than  the  steeper 
slopes  of  higher  lands  are,  and  (2)  a  larger  proportion  of  their 
surfaces  is  not  too  steep  for  cultivation.  The  larger  part  of  the 
agriculture  of  the  world  is  therefore  on  plains. 

When  the  population  of  the  United  States  was  about  50,000,- 
000  l  (1880),  it  was  distributed  as  follows,  with  reference  to  altitude: 

1  Later  data  on  this  point  are  not  available. 


26 


!S 

"Sai 


RELIEF  FEATURES 


27 


28  PHYSIOGRAPHY 

Less  than  100  feet  above  sea-level 15.9% 

Between    100  and    500  feet  above  sea-level 21 .8% 

Between    500  and  1000  feet 38. 7% 

Between  1000  and  1500  feet 14. 7% 

Between  1500  and  3000  feet 6.2% 

Above  3000  feet 2.7% 

Plains  also  favor  transportation  and  intercommunication,  for 
(1)  the  construction  of  roads,  railways,  canals,  etc.,  is  vastly  easier 
in  plains  than  in  higher  and  rougher  regions,  and  (2)  che  streams 
of  plains  are  much  more  commonly  navigable  than  those  of  moun- 
tains and  plateaus.  For  these  reasons,  and  also  because  the  larger 
part  of  the  raw  materials  used  in  manufacturing  is  grown  upon 
the  plains,  the  larger  part  of  the  manufacturing  of  the  world  is  on 
plains.  It  is  noteworthy  that  the  extensive  plains  most  favored 
by  climate  and  soil  border  the  Atlantic  Ocean,  and,  largely  for  this 
reason,  the  borders  of  this  ocean  have  been  the  theatres  of  the 
world's  culture  and  commerce. 

Not  all  plains  support  an  abundant  population.  Thus  the  north- 
ern parts  of  the  great  Eurasian  and  North  American  plains  are  too 
cold  to  be  hospitable  to  varied  industries  or  productions,  and  their 
populations  are  likely  to  remain  small. 

Plateaus 

Plateaus  are  tracts  of  land  so  situated  as  to  appear  high  from 
at  least  one  side,  and  which  have,  at  the  same  time,  considerable 
areas  at  or  near  their  summit  levels.  Thus  if  a  coastal  plain  rises 
gradually  from  the  sea  to  a  height  of  200  feet,  and  then  joins  by  a 
steep  slope  another  tract  of  more  or  less  level  land  which  rises  100 
or  200  feet  higher  (Fig.  6),  the  upper  tract  would  commonly  be 
called  a  plateau,  not  primarily  because  of  its  altitude  above  sea- 
level,  but  because  of  its  distinct  rise  above  the  plain  along  one  side 
of  it.  Traced  landward,  the  low  slope  of  the  Atlantic  Coastal  Plain 
of  the  United  States  gives  place  to  a  steeper  one  at  the  Fall  Line, 
and  the  tract  above,  beyond  the  Coastal  Plain,  is  the  Piedmont 
Plateau.  The  elevation  of  much  of  this  plateau  is,  however,  less 
than  that  of  much  of  the  great  interior  plain  of  the  continent. 

Though  plateaus  are  on  the  whole  higher  than  plains,  it  may 
be  pointed  out  again  that  the  distinction  between  them  is  not 


RELIEF  FEATURES 


29 


more  one  of  elevation  than  of  relations.  A  tract  of  land  is  rarely 
called  a  plateau  unless  it  rises  distinctly  above  adjacent  land  or 
adjacent  water  on  one  or  more  sides. 

In  spite  of  the  broad  distinction  between  plateaus  and  plains, 


these  two  great  topographic  types  grade  into  each  other  so  com- 
pletely that  it  is  often  hard  to  say  whether  a  given  region  should 
be  classed  as  the  one  or  the  other,  and  a  tract  which,  in  its  sur- 
roundings, is  a  plateau,  might  in  other  surroundings  be  a  plain. 


30  PHYSIOGRAPHY 

Distinctions  in  nature  are  often  less  sharp  than  we  seem  to  make 
them  by  arbitrary  (though  often  necessary)  definitions. 

Position  and  area  of  plateaus.  Plateaus  often  lie  between 
mountains  on  the  one  hand  and  plains  on  the  other,  as  in  the  case 
of  the  Piedmont  and  Cumberland  plateaus  already  cited.  They 
also  fre  between  mountains,  as  the  plateaus  of  Central  Asia  (Fig.  24), 
Mexico,  and  the  western  part  of  the  United  States,  and  they  some- 
times rise  directly  from  the  sea,  as  in  the  case  of  Greenland  and 
parts  of  Africa  (Fig.  25). 

JMStA 

E.. 


FIG.  24. — Section  across  Asia  along  the  35th  parallel.  Vertical  scale  greatly 
exaggerated.  The  plateau  between  mountains  is  marked  Pit.  (Alter 
Heidrich.) 

The  aggregate  area  of  plateaus  is  less  than  that  of  plains, 
though  they  constitute  a  very  considerable  fraction  of  the  land. 

Relief  of  plateaus.  The  surfaces  of  plateaus  usually  have 
greater  relief  than  the  surfaces  of  plains,  because  the  valleys  are 
deeper.  The  plateau  of  the  Colorado  in  northern  Arizona  has  an 
elevation  of  about  7000  feet,  and  a  relief  of  a  mile  or  more,  for  the 
Colorado  River  has  a  valley  (canyon)  of  that  depth  (Figs.  27  and  27a). 
From  the  bottom  of  this  valley,  its  slopes  look  like  mountains. 
They  are  indeed  much  higher  and  bolder  than  many  mountains; 


FIG.  25. — Section  across  Africa  along  the  parallel  of  10°  S.     Vertical  scale 
exaggerated  about  fifty  times. 

but  since  there  are  great  stretches  of  land  about  the  canyon  at 
about  the  elevation  of  the  tops  of  these  slopes,  the  area  is  a  plateau 
region,  rather  than  a  mountain  region.  No  plain  has  such  great 
relief  as  this  plateau. 

Other  features  of  plateaus.  Except  for  the  greater  average 
relief  of  plateaus,  their  surfaces  have  much  in  common  with  the 
surfaces  of  plains.  There  are  flat  plateaus,  broken  plateaus,  roll- 
ing plateaus,  etc.,  and  these  topographic  terms  are  often  applicable 
to  different  parts  of  the  same  plateau.  There  are  plateaus  which 


PLATE  IV 


FIG    1. — The  canyon  of  the  Yellowstone  River.     Scale  2—  miles  per  inch. 
(Canyon,  Wyo.,  Sheet,  U.  S.  Geol.  Surv.) 


FIG.  2. — The  Grand  Canyon  of  the  Colorado  River.     Scale  4—  miles  per  inch. 
(TJ.  S.  Geol.  Surv.) 


RELIEF   FEATURES 


31 


are  well  drained,  and  plateaus  which  are  ill  drained;  there  are 
plateaus  which  are  relatively  fertile,  and  plateaus  which  are  essen- 
tially desert.  The  relief  features  of  plateaus,  like  those  of  plains, 
are  relief  features  of  the  third  order. 

The  climate  of  plateaus,  especially  that  of  high  ones,  is  distinctly 
colder  than  that  of  plains  in  similar  latitudes,  and  their  precipita- 
tion is  generally  less.  Except  in  low  latitudes,  they  are  too  cold  to 
be  well  adapted  to  human  habitation,  and  their  rainfall  is  often 
insufficient  for  agriculture.  Their  deep  valleys  are  barriers  to  trans- 


FIG.  26. — A  valley  (canyon)  in  a  plateau.     Snake  River  below  the  mouth  of 
Rattlesnake  Creek.     (U.  S.  Geol.  Surv.) 

portation.  For  these  and  other  reasons,  high  plateaus  are,  on 
the  whole,  less  well  adapted  to  human  habitation  than  plains,  and 
the  population  of  high  plateaus  is  generally  scanty.  On  the  other 
hand,  the  altitude  of  low  plateaus,  such  as  the  Piedmont  and  the 
Cumberland  plateaus,  is  too  slight  to  affect  the  climate  adversely, 
and  such  plateaus  may  be  as  fertile  as  plains,  so  far  as  climate  is 
concerned.  Plateaus  in  low  latitudes  may  have  a  favorable  tem- 
perature, and  may  be  so  situated  as  to  have  an  adequate  supply  of 
water,  as  illustrated  by  some  parts  of  the  plateau  of  Mexico. 

Origin.  Plateaus  attain  their  height  in  various  ways.  (1)  In 
some  cases  their  surroundings  probably  sank  away  from  them. 
If,  for  example,  the  eastern  half  of  the  Great  Plains  was  to  sink 
a  few  hundred  feet,  while  the  western  half  did  not,  the  latter  would 
doubtless  be  called  a  plateau  (Fig.  15).  (2)  Some  plateaus  may 
have  attained  their  height  by  elevation  above  their  surroundings, 


32 


PHYSIOGRAPHY 


RELIEF  FEATURES 


33 


while  still  others  (3)  have  been  built  up  either  from  plains  or  from 
lower  plateaus,  by  the  outpouring  of  lavas.  Such  is  the  lava  plateau 
of  the  northwestern  part  of  the  United  States  (Fig.  401). 

The  term  plateau  is  often  applied,  and  properly,  to  small  areas 
which  may  owe  their  plateau  character  to  other  causes,  such  as 


Fro.  27a. — The  Grand  Canyon  of  the  Colorado.  The  inner  gorge  in  the 
foreground,  and  the  more  distant  cliffs  in  the  background.  The  canyon 
is  about  a  mile  deep.  (Hull.) 

isolation  by  the  degradation  of  the  surrounding  surface.  Such 
plateaus  are  topographic  features  of  a  lower  order,  and  are  not 
here  considered. 

Mountains 

Mountains  are  conspicuously  high  lands  which  have  but  slight 
summit  areas'.  Conspicuously  high  lands  must  be  interpreted  to 
mean  lands  which  are  conspicuously  high  in  their  surroundings, — 
not  necessarily  those  which  have  great  elevation,  measured  in 
feet  or  meters. 


34 


PHYSIOGRAPHY 


Though  the  tops  of  the  highest  mountains  are  between  five  and 
six  miles  above  the  level  of  the  sea,  most  mountains  have  not  half 


FIG.  28. — Sierra  el  Late  Mountains,  Colo.,  with  dissected  mesa  in  the  fore- 
ground.    (Holmes,  U.  S.  Geol.  Surv.) 

this  height.  The  highest  mountains  are  higher  than  any  plateaus, 
but  many  mountains  are  not  so  high  as  the  highest  plateaus.  Rela- 
tively few,  for  example,  reach  the  height  of  the  Plateau  of  Tibet, 
15,000  to  16,000  feet.  Many  elevations  called  mountains  are  not 
even  so  high  above  sea-level  as  the  higher  parts  of  the  higher  plains. 
Mountains  differ  from  plateaus  of  similar  elevation  in  that 


FIG.  29. — The  Needle  Mountains  of  Colorado.  An  illustration  of  mountain 
topography.  Taken  from  an  elevation  of  about  10,700  feet.  (U.  S. 
Geol.  Surv.) 

they  have  little  extent  of  surface  at  the  summit  level.     In  the 
case  of  mountain  peaks  this  is  indicated  by  the  name.     A  moun- 


RELIEF  FEATURES 


35 


tain  ridge  or  range  may  be  long,  but,  as  its  name  implies,  its  crest 
is  usually  narrow.     The  several  ridges  shown  in  Fig.  23  are  ex- 


FIG.  30. — Lake  Agnes,  Canadian  Pacific  Railway.     (Photograph  by  Church.) 

amples.     Numerous  peaks  or  ranges  are  often  associated,  making 
a  mountain  group  (Fig.  28)  or  "a  mountain  chain  (Fig.  23);  but 


FIG.  31. — Cascade  Pass  in  the  Cascade  Mountains.  Washington.     An  illustra- 
tion of  mountain  topography.     (Willis,  U.  S.  Geol.  Surv.) 

even  in  great  mountain  groups  there  is  no  great  continuous  area 
of  high  land.     Land  10,000  feet  high  would  generally  be  called  a 


36 


PHYSIOGRAPHY 


FIG.  32. — A  portion  of  the  Elk  Mountains  of  Colorado. 
(Holmes,  U.  S.  Geol.  Surv.) 


FIG.  33. — Photograph  of  relief-model  of  Texas  and  surroundings.  The 
area  near  the  coast  is  a  part  of  the  Coastal  Plain.  Inland  this  plain 
gives  place  to  a  plateau  tract,  while  at  the  north  and  west  mountains 
rise  above  the  plateau  level.  The  valleys  are  deep,  and  the  relief  is 
greater  in  tl:e  mountains  than  in  the  plateau,  and  in  the  plateau  greater 
than  in  the  plain.  (Hill.) 


RELIEF  FEATURES 


37 


plateau  if  its  summit  area  were  extensive,  a  mountain  if  its  summit 
were  a  peak,  a  mountain  ridge  or  range  if  its  crest  were  a  narrow 
ridge,  or  a  mountain  area,  a  mountain  group,  or  a  mountain  system, 
if  composed  of  a  succession  of  peaks  and  ridges. 


CALIFORNIA 


/         -*i.v- 


FIG.  34. — Topographic  map  of  California.     The  State  is  largely  mountainous, 
but  the  central  plain  is  conspicuous.     (Model  by  Drake.) 

Considered  in  a  large  way,  mountains  are  in  contrast  with  plains 
and  plateaus,  and  are  the  third  of  the  three  topographic  types  of 
the  second  order,  as  they  appear  on  the  lands  of  the  earth. 

High  mountains  are  on  the  whole  the  most  impressive  and 
awe-inspiring  features  of  the  earth's  surface.  This  is  especially 


38  PHYSIOGRAPHY 

the  case  where  they  rise  abruptly  to  great  heights  above  their 
surroundings.  In  not  a  few  cases  they  rise  from  low  warm  plains 
to  such  heights  that  their  summits  are  continually  covered  with 
snow.  Nowhere  else  are  such  contrasts  of  climate  found  in  such 
close  proximity. 

In  this  grouping  of  mountains,  as  the  third  great  topographic 
features  of  the  lands,  it  must  be  noted  that  only  great  groups  or 
systems  of  mountains,  such  as  the  Appalachians,  the  Rockies,  the 
Sierras,  the  Alps,  the  Caucasus,  the  Himalayas,  the  Andes,  and  others 
of  comparable  extent  and  magnitude  are  included.  Since  the  term 
mountain  is  applied  to  any  point  or  ridge  of  such  steep  slopes  and 
so  much  above  its  surroundings  as  to  be  very  conspicuous,  if,  at 
the  same  time,  its  summit  area  is  so  small  that  it  is  not  a  plateau,  it 
follows  that  many  elevations  called  mountains  do  not  belong  to 
the  great  physiographic  type  which  is  to  be  brought  into  contrast 
with  plains  and  plateaus.  From  this  category  we  must  exclude 
many  minor  and  isolated  elevations  called  mountains,  especially 
those  of  such  small  size  that,  in  surroundings  other  than  their* 
own,  they  would  not  be  regarded  as  mountains. 

Mountains  in  history.  Mountains  are  always  more  or  less 
formidable  barriers,  and  as  such  have  played  important  roles  in 
history.  They  have  sheltered  nascent  civilizations  from  invasion, 
and  they  often  determine  the  boundaries  of  political  states.  The 
mountains  of  western  and  southwestern  Europe  were  an  important 
factor  in  producing  the  many  small  political  divisions  of  those 
sections,  so  in  contrast  with  Russia.  Mountainous  highlands  have 
frequently  become  a  refuge  for  weak  peoples,  driven  by  their 
stronger  enemies  from  the  more  desirable  lowlands.  The  rela- 
tively inaccessible  highlands  of  Scotland,  Wales,  and  parts  of 
India  enabled  such  peoples  to  maintain  their  independence  for 
long  periods.  The  Appalachian  Mountains  confined  the  English 
settlements  to  the  rim  of  the  continent  for  nearly  a  century  and 
a  half,  and  influenced  their  life  in  many  ways.  Later,  grave 
political  dangers  arose  from  the  effectual  isolation  of  the  Ohio 
Valley  settlements  from  the  Atlantic  seaboard. 

The  scant  soils  and  low  temperatures  of  most  mountains  in- 
hibit agriculture,  while  the  difficulties  of  communication  help  to 
restrict  commerce  and  social  intercourse.  Poverty  is,  accordingly, 
the  common  lot  of  the  mountaineer,  save  in  certain  mining  and 
lumbering  areas.  Shut  out  from  the  progressive  life  of  the  plains, 


RELIEF  FEATURES 


39 


mountain  peoples  are  proverbially  conservative,  maintaining  old 
customs  and  habits,  and  supporting  the  established  order  of  things. 
In  the  Civil  War  the  Southern  Appalachians  became  a  zone  of 
disaffection  through  the  heart  of  the  Confederacy,  sending  100,000 
men  to  the  northern  armies. 

The  most  distinctive  industry  of  the  mountains  is  mining; 
yet  many  mountains  have  no  ores  or  mineral  matter  of  commer- 
cial value,  while  many  ores  and  many  mineral  substances  which 
are  not  ores  are  mined  in  plains  and  plateaus.  This  is  true,  fcr 
example,  of  most  of  the  iron  and  coal  now  mined  in  the  United 
States. 


FIG.  35. — Cross-section  illustrating  the  structure  of  the  Appalachian    Moun- 
tains.    (After  Rogers.) 


T»I*V  TRIASA^WCRJURA  UP^tR  JURA  CRCTACCO'US' ' "  TERT1A«V. 

FIG.  36. — Section  of  the  Alps  from  Saint  Gothard  South.     (After  Heim.) 


FIG.  37. — Cross-section  of  the  Elk  Mountain  Range,  Colo. 
(Holmes,  U.  S.  Geol.  Surv.) 


Pine  Forest  Mts. 


Volcanic  Mesa          puebb 


Stein  Mts. 


Barren  Val]^ 


FIG.  38. — Faulted  Mountain  (Block  Mountain)  structure,  Nevada. 
(Russell,  U.  S.  Geol.  Surv.) 

Origin.     Mountains  have  originated  in  various  ways.     In  their 
formation,  the  layers  of  rock  of  which  they  are  composed  were 


40  PHYSIOGRAPHY 

often  folded  and  crumpled,  sometimes  on  a  grand  scale.     Figs. 
35-37  illustrate  types  of  mountain  structure  common  to  the  great 


FIG.  39.— Sketch  of  the  Abert  Lake,  Ore.     (Russell,   U.  S.   Geol.   Surv.) 

ranges  which  belong  to  the  topographic  features  of  the  second 
order. 

TOPOGRAPHIC  MAPS1   SHOWING  GREAT   PHYSIOGRAPHIC 

TYPES 

Note.  The  conventions  used  on  the  topographic  maps  are  explained 
on  their  backs.  The  meaning  of  each  should  be  noted. 

A  Plain  Region.  Maumee  Bay,  Ohio  Sheet.  This  map  shows  a 
nearly  level  plain  whose  surface  slopes  very  gently  to  the  northeast. 
The  general  flatness  may  be  read  at  a  glance  from  the  fewness  of  the 
contour  lines  (only  four  appear  upon  the  entire  map),  and  also  from 
the  fact  that  the  railroads  run  long  distances  in  straight  lines.  Calculate 
the  average  slope  of  the  surface  per  mile.  *.j  r>  ^  • 

A  Plateau  Region.  Echo  Cliffs,  Ariz.  Sheet.  The  numbers  upon  the 
contour  lines  show  this  to  be  an  elevated  region,  and  the  disposition  of 
the  contours  shows  that  there  are  considerable  areas  of  the  high  land, 
and  that  the  region  is  therefore  a  plateau.  The  very  deep  valley  of  the 
Colorado  River  also  indicates  great  height  of  land,  for  such  valleys 
are  found  only  in  regions  far  above  sea-level.  Note  that  the  Paria 
Plateau,  at  the  northwest,  is  bordered  by  an  abrupt  descent.  This  is 
often  true  of  plateaus  upon  at  least  one  side. 

1  These  maps  are  topographic  maps  of  the  U.  S.  Geological  Survey. 
They  may  be  had  of  the  Director  of  that  Survey,  Washington,  D.  C.,  at 
$3.00  per  100.  Many  of  these  maps  will  be  referred  to  in  the  following  pages. 
See  list  at  end  of  Part  I. 


RELIEF  FEATURES  4l 

A  Mountain  Region.  Hummelstown,  Pa.  Sheet.  The  massing  of  the 
contours  along  northeast-southwest  lines  in  the  northern  part  of  the 
area  shows  a  series  of  relatively  steep  slopes  extending  in  that  direction. 
The  crests  between  the  steep  slopes  are  narrow.  The  numbers  on  the 
contour  lines  show  that  the  elevations  are  of  mountainous  heights. 

SUBORDINATE  TOPOGRAPHIC  FEATURES 

It  has  already  been  noted  that  the  surfaces  of  plains  and  pla- 
teaus are  often  somewhat  uneven,  while  the  very  name  of  moun- 
tain suggests  roughness  of  surface.  In  many  cases  the  degree  of 
unevenness  of  surface  is  more  or  less  closely  related  to  altitude 
above  sea-level,  increasing  roughness  going  with  increasing  alti- 
tude, though  altitude  is  by  no  means  the  only  factor  which  deter- 
mines roughness  and  smoothness  of  surface.  The  minor  uneven- 
nesses  of  surface  which  affect  plains,  plateaus,  and  mountains  are 
topographic  features  of  the  third  order.  Some  of  these  irregularities  of 
surface  consist  of  elevations  above  the  general  level  of  their  sur- 
roundings, and  some  of  depressions  below  it.  Thus  on  the  plains 
there  are  ridges  and  hills  above  the  general  level,  and  valleys  and 
sometimes  basins  (depressions  without  outlets)  below  it,  while 
-fiats  may  be  interspersed  among  the  uneven  tracts.  The  eleva- 
tions and  the  depressions  are  bordered  by  slopes,  which,  when 
steep,  are  cliffs.  These  subordinate  features,  ridges,  hills,  valleys, 
basins,  flats,  etc.,  affect  plateaus  as  well  as  plains;  but  the  corre- 
sponding features  of  plateaus  are  often  more  pronounced,  some- 
times so  much  more  pronounced  that  they  receive  different  names. 
Many  of  the  same  features,  often  on  a  still  larger  scale,  affect  moun- 
tains; but  here  the  more  or  less  isolated  elevations,  instead  of 
being  merely  ridges  or  hills,  are  often  of  mountainous  size,  and 
receive  individual  names.  And  so,  as  terms  are  now  used,  it  is 
difficult  to  distinguish,  in  words,  between  mountains  which  are 
topographic  features  of  the  second  order  and  mountains  which 
are  topographic  features  of  the  lower  order,  though  in  reality  the 
distinction  is  clear  enough.  Thus  the  Appalachian  Mountains 
are  a  topographic  feature  of  the  second  order,  but  any  minor  ridge 
or  peak  in  the  system,  though  still  a  mountain,  is  a  feature  of  the 
third  order,  and  is  to  be  compared  with  the  hills  and  buttes  of 
plains  and  plateaus. 

The  depressions  in  the  surface  of  plains  or  plateaus  are  of 
different  sizes,  shapes,  and  origins,  and  will  be  the  object  of  future 


42  PHYSIOGRAPHY 

study.  Similarly  the  hills  and  ridges  of  plains  and  plateaus,  and 
the  larger,  mountain-big  hills  of  mountainous  regions,  are  of  dif- 
ferent sizes,  shapes,  and  origins,  and  their  history  is  often  intimately 
connected  with  the  history  of  the  depressions  with  which  they 
are  associated.  Slopes  w?ere  developed  when  elevations  and  de- 
pressions were  developed,  and  largely  by  the  same  means.  The 
origin  of  the  topographic  features  of  the  third  order  is  in  general 
well  understood,  for  the  processes  which  have  developed  them 
are  still  in  operation,  and  their  results  in  past  times  may  be  in- 
ferred with  much  confidence.  These  processes  we  shall  study  in 
some  detail. 

Land  surface  and  ocean  bottom.  Were  the  water  removed 
from  the  ocean  basins,  the  surface  of  the  ocean  bed  would  appear 
much  less  uneven  than  the  surface  of  the  land.  While  its  aggre- 
gate relief  is  a  little  greater  than  that  of  the  land  (p.  9),  much 
larger  tracts  of  it  are  nearly  plane,  and  minor  irregularities,  such  as 
hills  and  valleys  with  their  accompanying  slopes,  the  most  wide- 
spread of  the  minor  irregularities  of  the  land,  are  of  much  less 
common  occurrence;  are,  indeed,  entirely  absent  from  the  larger 
part  of  the  ocean's  floor. 

Why  this  difference  between  land  and  sea  bottom?  Without 
discussing  this  subject  at  this  point,  it  may  be  noted  that  the 
atmosphere  and  running  water,  both  of  which  are  in  almost  con- 
stant motion,  are  always  in  contact  with  the  surface  of  the  land, 
while  the  atmosphere  is  excluded  from  the  ocean  bottom,  and  the 
water  which  covers  it  is  practically  motionless,  except  where  the 
water  is  very  shallow.  It  will  be  seen  in  the  sequel  that  the 
differences  in  topography  between  the  land  and  the  sea  bottom 
are  largely  due  to  the  contact  with  air  and  running  water  in  the 
one  case,  and  with  standing  water  in  the  other. 

The  Development  of  Minor  Topographic  Features 

Since  the  minor  topographic  features  of  plains,  plateaus,  and 
mountains  have  been  developed  in  similar  ways,  their  origin  and 
history  may  be  considered  independently  of  their  association  with 
one  or  another  of  these  great  physiographic  divisions.  The  key 
to  the  history  of  topographic  features  of  the  third  order  is  found 
in  the  changes  which  the  surface  of  the  land  is  now  undergoing, 
or  which  it  has  undergone  in  such  recent  times  that  their  records 
are  still  clear. 


RELIEF  FEATURES  43 

Changes  now  taking  place  on  the  land.  Certain  familiar 
changes  are  always  taking  place  on  the  land.  Some  of  them  are 
brought  about  by  the  atmosphere,  some  by  water,  some  by  ice, 
and  some  by  the  life  of  the  earth.  The  same  agencies  produce, 
directly  or  indirectly,  certain  changes  on  the  sea  bottom,  but  the 
changes  there  are  not  only  less  important  than  those  on  land, 
but  they  are  essentially  different  in  their  effects  on  the  topography. 

1.  The  air  is  nearly  always  in  motion,  and  whenever  it  blows 
over  a  surface  on  which  there  is  dust,  some  of  the  dust  is  picked  up 
and  blown  to  some  other  place.     Even  sand,  the  particles  of  which 
are  much  larger  than  those  of  dust,  is  blown  about  in  the  same 
way.    The  wind  is,  therefore,  one  of  the  forces  which  is  changing 
the  surface  of  the  land.    The  winds  also  help  to  distribute  the 
moisture  of  the  atmosphere,  and  so  influence  the  amount  and  the 
distribution  of  rain  and  snow.  Though  winds  do  not  blow  at  the  bot- 
tom of  the  sea,  dust  and  sand  blown  out  from  the  land  are  dropped 
into  the  ocean  and  sink  to  its  bottom,  and  the  winds  generate 
water  waves  which  work  upon  the  shores  of  the  seas,  and  affect 
their  bottoms  where  the  water  is  very  shallow.    The  winds  there- 
fore are  not  without  their  effects  on  the  sea  bottom,  though  these 
effects  are  slight  compared  with  those  on  the  land. 

2.  On  both  the  land  and  sea,  rains  and  snows  fall.    The  rain 
which  falls  on  the  land  disappears  in  various  ways,  but  a  part 
of  it  runs  off  over  the  surface.   When  the  snow  of  the  land  melts, 
the  water  follows  the  same  course.    The  water  which  runs  off  over 
the  land  in  streams  is  the  most  important  single  agent  modifying 
the  land  surface.     The  streams  carry  much  sediment  from  the  land 
to   the   sea,  and  its  deposition  has  its  effect  on  the  sea  bottom, 
especially  near  the  land. 

The  rain-  and  snow-water  which  sinks  beneath  the  surface 
of  the  land  dissolves  more  or  less  mineral  matter,  which  appears 
in  spring  water  and  in  well  water.  This  solution  of  mineral  matter 
beneath  the  land,  and  its  transfer  by  the  water  to  the  surface,  and 
thence  through  streams  to  the  sea,  also  help  to  lower  the  land. 

While  the  waters  which  fall  on  the  land  have  an  indirect  effect 
on  the  bottom  of  the  sea,  as  indicated,  those  which  fall  on  the 
sea  itself  have  little  influence  on  its  bed.  Precipitation,  therefore, 
whether  in  the  form  of  rain  or  snow,  modifies  the  surface  of  the 
land  notably,  but  has  little  influence  on  the  ocean  bottom,  except 
near  its  borders,  where  most  of  the  sediment  from  the  land  is 
left. 


44  PHYSIOGRAPHY 

3.  Great  bodies  of  ice,  called  glaciers,  move  slowly  over  the 
surface  of  the  land  in  some  places,  especially  on  high  mountains 
and  in  high  latitudes.   Glaciers,  which  originate  in  perennial  fields 
of  snow,  work  notable  changes  on  their  beds.    They  sometimes 
push  out  into  the  sea  for  short  distances,  but  they  never  advance 
into  deep  water.    At  most  they  only  affect  the  submerged  edge 
of  the  continental  platform. 

The  winds,  the  streams,  and  the  glaciers  all  tend  to  develop  un- 
evennesses  of  surface  on  the  land.  Since  this  is  the  case,  and  since 
these  agents  are  not  in  operation  beneath  the  sea,  we  infer  that 
they  have  had  much  to  do  with  developing  the  differences  between 
the  topography  of  the  land  and  that  of  the  sea  bottom. 

4.  The  waves  of  the  sea  and  of  the  many  lakes  which  lie  on 
the  land  are  continually  modifying  the  position  and  the   outlines 
of  their  shores.    The  changes  thus  effected  are  slight  in  short  periods 
of  time,  but  they  have  been  very  great  in  the  course  of  the  long 
ages  of  the  earth's  history.    They  change  the  outlines  of  the 
land  rather  than  its  relief,  but  they  alter  the  relief  of  the  sea  or 
lake  bottom  near  shore  in  an  important  way. 

The  winds,  rivers,  glaciers,  and  waves  are  agents  of  gradation. 
They  degrade  the  surface  at  some  points,  and  aggrade  it  (build 
it  up)  at  others.  In  general  they  degrade  the  land  more  than 
they  aggrade  it,  for  much  of  the  material  moved  by  them  finds 
its  resting-place  in  the  sea.  Conversely,  they  aggrade  the  sea 
bottom  more  than  they  degrade  it.  Waves  may  degrade  it  effec- 
tively, but  only  where  the  water  is  shallow. 

5.  Still    another   series   of   changes    in   the    surface   is  being 
brought  about  through  the  agency  of  life.     Man,  for  example, 
grades  down  elevations,  and  he  grades  up  depressions,  as  along 
railroads.     He  makes  dams  across  rivers,  converting  portions  of 
them  into  ponds,  or  at  the  outlets  of  lakes,  raising  their  levels; 
he  raises  and  changes  the  banks  of  streams,  modifying  their  natu- 
ral courses  and  their  natural  work;    he  drains  marshes  and  lakes, 
and,  more  important  than  all  else,  he  clears  (removes  the  forests) 
and  tills  the  land,  and  in  so  doing  destroys  the  native  vegetation 
and  stirs  up  the  soil,  thus  preparing  the  way  for  the  more  effective 
action  of  wind  and  running  water.     Man's  direct  influence  on  the 
sea  bottom  is  slight. 

Plants  and  animals  affect  both  land  and  sea  bottom.  Deposits 
due  to  organisms  of  one  sort  and  another,  especially  those  due 


RELIEF  FEATURES  45 

to  plants,  are  somewhat  wide-spread  in  the  marshes  and  shallow 
lakes  of  the  land;  but  they  are,  on  the  whole,  of  little  consequence 
compared  with  the  deposits  of  shells,  skeletons,  and  other  solid 
matter  made  by  marine  animals  on  the  sea  bottom,  more  especially 
in  shallow  water.  Organic  agents  are  in  some  sense  gradational, 
chiefly  aggradational;  but  they  belong  to  a  different  category 
from  the  inorganic  gradational  agents. 

Various  forms  of  life  have  a  protective  effect  on  the  surface. 
This  is  especially  true  of  vegetation  on  the  land.  The  forests, 
and  even  the  prairie  vegetation,  greatly  restrict  the  erosive  work 
of  wind  and  running  water,  and  so  decrease  the  rate  of  degrada- 
tion which  would  otherwise  obtain. 

6.  Volcanoes  affect  both  land  and  sea  bottom,  and  with  approx- 
imate equality.     Volcanoes  often  give  rise  to  mountain  heights, 
but  the  mountains  to  which  they  give  rise  are  topographic  features 
of  the  third  rather  than  the  second  order.     The  great  processes  of 
vulcanism,  that  is  the  movement  of  liquid  rock  from  great  depths 
up  to  or  toward  the  :  surf  ace,  affect  the  surface  of  land,  and  doubt- 
less of  sea  bottom,  in  other  ways,  which  will  be  mentioned  in  other 
connections. 

7.  Tt  is  well  known  that  the  surface  of  the  lithosphere  seems 
to  be  rising  in  some  places  and  sinking  in  others.     This  has  been 
true  in  the  past,  for  beds  of  sediment  (sand,  clay,  etc.),  containing 
sea-shells,  etc.,  and  therefore  once  beneath  the  sea,  occur  at  levels 
high  above  it,  and  areas  once  land  are  now  beneath  the  sea.     Crustal 
movements  are  probably  responsible  in  large  measure  for  the  ocean 
basins  and  continental  platforms,  and  for  plains,  plateaus,  and 
mountains,  that  is  for  the  topographic  features  of  both  the  first  and 
second  orders.     All  sorts  of  crustal  movements,  of  whatever  nature, 
are  grouped  together  under  the  name  diastrophism. 

The  processes  of  gradation,  vulcanism,  and  diastrophism  will 
be  taken  up  in  order,  but  before  entering  upon  the  study  of  grada- 
tion, the  materials  on  which  the  agents  of  gradation  act  must  be 
briefly  reviewed. 

THE  MATERIALS  OF  THE  LAND 

The  land  is  nearly  everywhere  covered  with  vegetation.  In 
some  places  it  is  dense  enough  to  form  a  thick  mat  over  the  sur- 
face, while  in  others  it  is  meagre,  or  even  wanting.  The  surface 
well  clothed  with  vegetation  is  the  surface  with  which  we  are  most 


46  PHYSIOGRAPHY 

familiar;  but  there  are  tracts  of  sand  on  which  little  or  nothing 
grows,  and  cliffs  where  the  rock  is  bare,  save  for  scattered  patches 
of  moss  or  lichen.  In  the  polar  regions  and  on  lofty  mountains 
also,  the  land  is  often  covered  by  thick  beds  of  snow  on  which  there 
is  no  vegetation  of  the  types  with  which  we  are  familiar. 

Mantle  rock.  Beneath  the  vegetation  there  is,  in  most 
regions,  a  layer  of  loose  material,  composed  of  clay,  loam,  sand, 
gravel,  etc.,  of  variable  thickness.  This  layer  of  earthy  matter 
may  be  a  few  inches  in  thickness,  or  it  may  be  scores  or  even 
hundreds  of  feet  deep.  This  loose  material  is  mantle  rock,  because 
it  covers  and  conceals  the  solid  rock  which  lies  below.  It  is  also 
known  by  other  names,  among  which  are  rock  waste  and  regolith. 

The  uppermost  portion  of  the  mantle  rock  is  commonly  called 
soil.  In  color  the  soil  may  be  black,  gray,  brown,  or  even  dull 
red  or  yellow.  It  may  be  either  clayey  and  compact,  or  sandy 
and  porous.  In  most  cases  it  is  made  up  largely  of  small  particles 
of  mineral  or  rock.  If  a  piece  of  any  common  sort  of  rock  be  put 
into  a  mortar  and  ground  to  powder,  this  powder  will  somewhat 
resemble  soil.  In  general  we  cannot  recognize  the  kinds  of  rock 
from  which  the  mineral  particles  of  the  soil  came,  for  they  are 
usually  very  small.  In  addition  to  the  mineral  matter,  the  soil 
contains  more  or  less  partly  decayed  vegetable  matter.  Bits  of 
roots  may  often  be  seen  in  it,  and  sometimes  fragments  of  de- 
cayed leaves.  Both  the  mineral  and  the  organic  matter  are  neces- 
sary parts  of  a  good  soil,  but  their  proportions  vary  within  wide 
limits.  The  mineral  matter  is  usually  far  in  excess  of  the  organic, 
but  locally,  as  in  bogs  and  marshes  which  have  been  drained,  the 
organic  matter  is  the  more  abundant.  That  part  of  the  mantle 
rock  which  is  properly  called  soil  ranges  from  a  few  inches  to  a  few 
feet  in  thickness. 

The  distribution  and  prosperity  of  population  often  bear  a 
very  direct  relation  to  the  fertility  of  the  soil.  The  fertile  "Blue 
Grass"  region  of  Kentucky  was  the  first  extensive  area  to  be  settled 
in  the  Ohio  basin;  its  inhabitants  have  always  been  progressive 
and  well-to-do.  Some  of  the  hilly  land  to  the  east  was  slowly 
occupied  by  a  sparse  population,  condemned  by  a  poor  soil  to 
financial  and  intellectual  poverty.  The  cotton  and  tobacco  lands 
of  the  Coastal  Plain  were  partly  responsible  for  the  institution  of 
slavery. 

Where  the  mantle  rock  is  thicker  than  the  soil,  the  soil  grades 


RELIEF  FEATURES 


47 


down  into  earthy  matter  of  somewhat  different  composition,  known 
as  subsoil.  Between  the  two  there  is  commonly  no  distinct  separa- 
tion, but  the  subsoil  is  often,  though  not  always,  more  compact 
than  the  soil,  and  its  color  is  often  different.  Like  the  soil,  it  con- 
tains both  mineral  and  organic  matter,  though  the  latter  is  less 
abundant  than  in  the  soil.  Only  the  larger  roots,  and  the  roots 
of  the  larger  plants,  penetrate  the  subsoil  in  great  numbers.  The 
thickness  of  the  subsoil  is  often  much  greater  than  that  of  the  soil, 
but,  on  the  other  hand,  it  is  sometimes  absent  altogether. 

Rock.  Beneath  the  subsoil  is  rock.  When  a  geologist  speaks 
of  rock,  he  does  not  necessarily  mean  solid  rock,  for  sand,  gravel, 
clay,  etc.,  in  large  quantities  and  in  the  proper  relations  are  in- 
cluded under  this  term.  The  subsoil  itself  is  a  sort  of  rock.  As 
commonly  used,  however,  the  term  rock  implies  solid  rock,  and  be- 
neath the  mantle  rock  the  larger  part  of  the  earth  down  to  the 


FIG.  40. — Soil  grading  down  into  rock.     Sandstone,  south  central  Wisconsin. 

(MacNeille.) 

lowest  accessible  depths,  and  far  beyond,  is  solid  rock.     It  is 
probable  indeed  that  the  body  of  the  earth  is  solid  to  the  core. 

In  many  places  the  mantle  rock  grades  down  into  the  solid 
rock  in  such  a  way  as  to  show  that  the  former  was  made  by  the  de- 
cay of  the  latter  (Fig.  40).  It  is  this  fact  which  makes  the  name 
rock  waste  appropriate  for  the  mantle  rock.  Mantle  rock  of  this 
sort  is  local.  It  is  made  up  of  materials  derived  from  the  rock 
below.  In  other  places  the  plane  of  separation  between  the  sub- 
soil and  the  solid  rock  below  is  distinct,  with  no  suggestion  of 
gradation  (Fig.  41).  In  such  cases  the  mantle  rock  often  con- 


48 


PHYSIOGRAPHY 


tains  materials  which  could  not  have  been  derived  from  the  rock 
below.  They  have  been  transported  to  their  present  position  from 
some  other  source. 

Classes  of  solid  rock.    The  solid  rocks  of  the  earth  are  of  many 
kinds.    They  differ  from  one  another  in  color,  in  strength,  in 


FIG.  41. — Section  showing  loose  material  (glacial  drift)  on  solid  rock. 
Moines  County,  la.     (la.  Geol.  Surv.) 


Des 


texture,  in  composition,  in  origin,  etc.;  but  the  common  rocks  may 
be  grouped  into  three  great  classes,  namely,  sedimentary  rocks, 
igneous  rocks,  and  metamorphic  rocks. 

1.  Sedimentary  rocks.  These  rocks  were  once  sediments  not 
unlike  the  muds,  sands,  and  gravels  now  being  deposited  in  rivers, 
lakes,  and  seas.  They  are  generally  arranged  in  layers  or  beds, 
varying  from  a  few  inches  to  several  feet  in  thickness.  Because 
of  this  structure  they  are  often  called  stratified  rocks.  The  layers 
or  strata  are  sometimes  horizontal  (Fig.  42),  but  in  other  cases 
they  are  tilted  or  inclined  at  various  angles. 

Among  the  common  forms  of  stratified  rock  are  conglomerate, 
sandstone,  and  shale.  Conglomerate  is  gravel,  the  pebbles  and  stones 


RELIEF  FEATURES 


49 


of  which  are  cemented  together.  Similarly,  sandstone  is  sand, 
the  grains  of  which  are  cemented  together,  while  shale  is  mud,  the 
particles  of  which  are  so  compacted' or  cemented  that  they  cohere 
into  a  solid  mass.  Various  sorts  of  mineral  matter  serve  as  cement 
for  sedimentary  rocks.  In  general  the  cementing  matter  was 


FIG.  42. — Stratified  rock.     Trenton  Limestone,  Fort  Snelling,  Minn. 

(Calvin.) 

deposited  between  the  grains  or  particles  of  sediment,  from  water 
which  held  it  in  solution,  and  which  at  some  time  overlay,  filled, 
or  passed  through  the  sediments.  The  stones  of  the  gravel,  the 
grains  of  the  sand,  and  the  tiny  particles  of  the  mud,  were  all 
derived  from  some  older  rock  which  was,  in  some  way,  broken  to 
pieces.  The  destruction  of  one  kind  or  generation  of  rock  there- 
fore furnishes  the  material  for  another  and  younger  generation  of 
rock. 

Limestone  is  another  common  sort  of  stratified  rock,  but  in  this 
case  the  mineral  matter  which  makes  the  rock  was  chiefly  de- 
rived from  the  shells  or  other  hard  parts  of  animals  which  lived  in 
the  sea.  It  is  not  of  pebbles,  sand  grains,  or  mud  particles  derived 
directly  from  the  breaking  up  of  older  rock.  Even  the  material 
of  the  limestone,  however,  comes  from  older  rock,  from  which  it 
was  dissolved,  and  taken  in  solution  to  the  sea. 

Great  layers  of  gravel,  sand,  mud,  shells,  etc.,  are  being  formed 
in  the  ocean,  in  lakes,  etc.  We  conclude,  therefore,  that  conglom- 
erate, sandstone,  shale,  and  limestone  were  formerly  beds  of  gravel, 


50 


PHYSIOGRAPHY 


sand,  mud,  shells,  etc.,  accumulated  in  similar  situations.  Since 
these  materials,  as  now  deposited,  are  arranged  in  nearly  horizontal 
layers,  it  is  inferred  that  a  nearly  horizontal  position  is  the  orig- 
inal position  of  the  beds  of  sedimentary  rock. 


FIG.  43. — Massive  rock.     The  Upper  Yosemite  Falls     Compare  the  struc- 
ture of  the  rock  with  that  shown  in  Fig.  42. 


Stratified  rocks  are  more  wide-spread  beneath  the  mantle  rock 
than  the  rocks  of  the  other  classes.  They  are  found  even  in  very 
elevated  mountain  regions,  where  the  strata  are  sometimes  tilted 
and  folded  in  a  very  complicated  way.  Even  in  these  high  places 
they  often  contain  the  shells  or  other  relics  of  animals  which  once 
lived  in  the  sea. 

From  these  facts   the   following  conclusions  may  be  drawn: 


RELIEF  FEATURES 


51 


(1)  The  materials  of  which  many  of  the  rock  formations  of  the 
land  are  composed  were  laid  down  beneath  the  sea;  and  (2)  these 
deposits  have  been  consolidated,  many  of  them  tilted  out  of  their 
original  positions,  and  some  of  them  raised  to  great  heights,  since 


FIG.  44. — Granitic  rock,  about  half  natural  size.  The  white  patches  repre- 
sent crystals  of  one  or  two  kinds  of  mineral,  and  the  dark  parts  represent 
crystals  of  others. 

their  formation.  Such  rocks  contain  parts  of  the  record  of  the 
earth's  history,  and  point  to  very  notable  changes  in  its  sur- 
face. 

2.  Igneous  rocks.  From  volcanoes,  hot  liquid  rock  frequently 
comes  to  the  surface  from  unknown  depths.  This  liquid  rock  is 
lava.  Some  of  the  lava  which  rises  from  within  the  earth  stops 
before  it  reaches  the  surface,  and  cools  where  it  stops,  and  becomes 
solid  rock.  All  sorts  of  rock  formed  by  the  solidification  of  lava 
are  known  as  igneous  rocks.  They  do  not  commonly  occur  in 


52 


PHYSIOGRAPHY 


distinct  beds  or  strata,  and  so  are  said  to  be  non-stratified  or  massive 
(Fig.  43). 

Lavas  vary  much  in  composition.  They  also  harden  under 
different  conditions,  all  of  which  have  their  effect  on  the  character 
of  the  rock.  The  result  is  that  there  are  many  sorts  of  igneous 


FIG.  45. — Metamorphic  rock.     (Ells.  Can.  Geol.  Survey.) 

rock.  One  of  the  best  known  is  granite.  It  is  composed  chiefly 
of  three  or  four  minerals  which  have  the  form  of  imperfect  crystals. 
The  minerals  are  sufficiently  different  in  color  and  outline  to  be 
readily  distinguished,  if  the  crystals  are  large  enough  to  be  distinctly 
seen  (Fig.  44) ;  but  in  some  igneous  rocks  they  are  so  small  as  not 
to  be  distinct.  When  lava  cools  very  quickly,  the  mineral  matter 
of  the  liquid  lava  sometimes  fails  to  crystallize.  It  then  forms  a 
glassy  sort  of  rock. 

When  igneous  rocks  decay,  as  all  igneous  rocks  do,  the  decayed 
particles  at  the  surface  may  be  blown  or  washed  away,  and  may 


RELIEF  FEATURES 


53 


then  accumulate  as  sediment  in  proper  situations.     Igneous  rocks 
may  therefore  give  rise  to  sedimentary  rock. 

3.  Metamorphic  rocks.  This  is  the  name  given  to  the  third  class 
of  rocks,  and  means  rock  which  has  been  notably  altered  from  some 
previous  condition.  Either  sedimentary  rocks  or  igneous  rocks 
may  be  changed  into  metamorphic  rocks,  especially  through  (1) 
the  influence  of  great  pressure,  which  alters  the  structure  of  the 


FIG.  46. — Columnar  structure  in  igneous  rock.     Sierra  Nevada  Mountains 

rock  (Fig.  45);  (2)  the  action  of  water,  which,  by  dissolving  out 
some  parts  and  depositing  new  matter,  changes  the  composition- 
of  the  rock;  and  (3)  heat,  which  sometimes  causes  the  mineral 
matter  to  crystallize  anew  in  new  forms.  In  these  ways  either 
sedimentary  rock  or  igneous  rock  may  be  greatly  changed. 

All  large  bodies  of  rock,  whether  sedimentary,  igneous,  or  meta- 
morphic, are  traversed  by  cracks  or  joints  which  break  them  up 
into  larger  or  smaller  masses.  The  joints  may  be  vertical  or  in- 
clined at  any  angle.  Sometimes  they  are  close  together  (Fig.  46) 
and  sometimes  they  are  far  apart  (Fig.  42). 


54  PHYSIOGRAPHY 


EXERCISE  IN  THE  READING  OF  TOPOGRAPHIC  MAPS 

I.  Study  the  following  maps  in  preparation  for  conference  on  the  maps:1 

List  of  Maps 2 

1.  Mt.  Mitchell,  N.  C.  7.  Glassboro,  N.  J. 

2.  Harrisburg,  Pa.  8.  Watrous,  N.  M. 

3.  Shasta  Special,  Cal.  9.  Marsh  Pass,  Ariz. 

4.  Tooele  Valley,  Utah.  10.  Mesa  de  Maya,  Colo. 

5.  Donaldson ville,  La.  11.  Muskego,  Wis. 

6.  Fargo,  N.  D  —  Minn. 

Note.     In  studying  a  topographic  map,  notice  at  the  outset: 

(a)  In  what  part  of  the  country  the  region  is  situated. 
(6)  The  contour  interval  used, 
(c)  The  horizontal  scale  used. 

II.  Apply  the  following  questions  to  each  of  the  above  maps: 

1.  Is  the  region  represented  by  the  map  a  plain,  a  plateau,  or  a 

mountain   tract?      If   more   than   one  of  these  great  types  is 
shown,  indicate  the  fact,  and  locate  each  definitely. 

2.  What  range  in  elevation  is  shown  on  the  map? 

3.  Is  the  climate  of  the  region  wet  or  dry?    Basis  for  answer. 

4.  Is  the  region  thickly  or  sparsely  settled? 

5.  What  occupations  seem  to  be  favored  in  this  region? 

REFERENCES. — The  following  globes,  models,  and  charts  are  useful  in  the 
study  of  the  topics  discussed  in  this  chapter: 

1.  The  Jones'  Reliej  globe,  known  as  "The  Model  of  the  Earth":  Chicago. 

2.  Howell's  models  0}  the  United  States,  North  America,  South  America, 
Eurasia,  Africa,  and  Australia:   Washington,  D.  C. 

3.  Coast  Survey  Charts.     The  illustrated  catalog  of  these  charts  may  be 
had  of  the  U.  S.  Coast  and  Geodetic  Survey,  Washington,  D.  C.,  and  from 
it  charts  may  be  selected  intelligently. 

1  The    author  has   carried  on  the  conference  work  here  referred  to  as 
follows:  The  class  is  divided  into  groups  of  four,  and  each  group  meets 
the  instructor  for  a  half-hour  or  for  an  hour,  as  the  case  may  be,  for  the 
discussion  and  interpretation  of  the  maps  assigned.     The  maps  (also  relief- 
models,  photographs,  etc.)  are  studied  in  advance  by  the  students.     This 
sort  of  work  is  regarded  as  of  the  utmost  importance. 

2  As  in  the  preceding  and  following  lists,  these  are  sheets  of  the  topo- 
graphic maps  issued  by  the  U.  S.  Geological  Survey.     See  foot-note,  page  40. 


CHAPTER  II 
THE  WORK   OF  THE  ATMOSPHERE 

THE  atmosphere  is  nearly  everywhere  in  direct  contact  with 
the  surface  of  the  land,  and  it  penetrates  the  soil  and  the  rock 
beneath  to  considerable  depths.  Its  effects  on  the  soil  and  rock 
are  many  and  varied,  and  only  a  few  of  the  more  important  ones 
will  be  noticed  here.  Some  of  them  are  brought  about  by  the 
movements  of  the  air,  some  by  the  chemical  activity  of  the  elements 
of  the  air,  while  some  are  conditioned  by  the  air,  rather  than 
accomplished  by  it. 

MECHANICAL  WORK. — THE  WORK  OF  THE  WIND 
Dust 

Universality.  The  atmosphere  is  never  free  from  dust.  On 
windy  days  in  dry  regions  the  amount  of  dust  in  the  air  is  so  great 
that  it  may  be  readily  seen.  Even  when  the  air  seems  perfectly  still, 
dust  is  present.  This  might  be  inferred  from  the  fact  that  even 
on  still  days  dust  settles  in  houses  and  in  enclosures  of  all  sorts, 
and  it  may  be  seen  directly  by  allowing  light  to  enter  a  darkened 
room  through  a  narrow  crack  or  a  small  hole.  In  the  light  thus 
entering,  myriads  of  dust-motes  may  be  seen.  Dust  extends 
high  up  in  the  atmosphere,  for  it  is  found  in  abundance  in  the  air 
over  even  the  highest  mountains.  It  is  carried  far  from  its  sources, 
for  it  often  falls  at  sea  many  miles  from  land,  and  it  occasionally 
settles  on  the  decks  of  vessels,  even  in  mid -ocean,  in  such  quan- 
ties  as  to  be  readily  seen. 

The  universality  of  dust  in  the  atmosphere  may  be  shown  in 
another  way.  If  rain-water  which  has  just  fallen  be  evaporated, 
a  slight  amount  of  sediment  remains.  This  sediment  represents 
the  dust  which  was  brought  down  by  the  falling  drops.  Similarly, 
if  fresh-fallen  snow  be  melted  and  evaporated,  there  is  a  residue 

55 


56 


PHYSIOGRAPHY 


of  dust.  This  is  the  case  even  if  the  snow  be  taken  from  mountain 
tops,  or  from  such  a  place  as  Greenland,  far  from  the  cultivated 
lands  and  streets  which  furnish  much  of  the  dust  in  regions  which 
are  thickly  settled.  Since  all  rams  and  snows  bring  down  dust, 
we  infer  that  dust  is  everywhere  present  in  the  lower  part  of  the 
atmosphere. 

Sources  of  dust.  All  the  small  particles  of  solid  matter  held 
in  suspension  in  the  atmosphere  are  called  dust.  Fine  particles 
of  earthy  matter  caught  up  from  the  surface  of  the  land  are  most 
abundant,  but  the  solid  particles  of  smoke,  the  pollen  of  flo\vering 
plants,  the  spores  of  plants  which,  like  the  puffball,  do  not  blossom, 
and  minute  organisms  of  other  sorts  are  also  abundant  in  the  dust 
of  the  air.  Fine  particles  of  rock  blown  out  of  volcanoes  are 
abundant  in  the  vicinity  of  many  active  volcanoes,  and  a  trifling 
amount  of  dust  reaches  the  earth  from  extra-terrestrial  sources. 
On  windy  days  quantities  of  dust  are  gathered  from  streets 
and  plowed  fields,  and  from  any  dry  land  surface  which  is  not 
well  covered  with  vegetation.  Where  the  surface  is  very  dry, 
as  in  desert  regions,  and  the  wind  strong,  such  "clouds"  or 
"whirls"  of  dust  are  sometimes  swept  up  by  the  rising  currents 
of  air,  so  as  to  be  seen  for  miles.  From  surfaces  densely  covered 
with  vegetation  the  air  gets  little  dust,  except  pollen.  Little  or 
none  is  gathered  from  surfaces  which  are  wet,  or  from  surfaces 
covered  with  snow  or  ice. 

Volcanic  dust.    Volcanoes  whose  eruptions  are  explosive  often 

send  quantities  of  mineral  matter, 
broken  up  into  fine  particles,  high 
into  the  air.  This  is  volcanic  dust,  or 
volcanic  "ash."  The  latter  name  is 
not  a  good  one,  because  the  dust  is 
not  the  product  of  burning.  It  is 
lava,  blown  into  tiny  bits  by  explosion 
(Fig.  47).  The  force  of  explosion  is 
sometimes  so  great  that  the  dust  is 
sent  up  high  into  the  air,  and  once  in 
that  position  it  is  blown  hither  and 
thither  by  the  winds,  sometimes  being 
carried  great  distances. 

In  August  of  1883  a  violent  volcanic  eruption  took  place  on 
the  island  of  Krakatoa,  between   Java  and   Sumatra.     Half  of 


FIG.  47. — Particles  of  volcanic 
dust. 


THE  WORK  OF  THE  ATMOSPHERE 


57 


the  island  was  blown  away,  and  enormous  quantities  of  dust  were 
projected  high  into  the  air.  The  course  of  this  dust  in  the  air 
was  traced,  roughly,  by  means  of  its  effects  upon  the  coloring  of 
the  sunsets.  In  this  way  it  was  estimated  to  have  been  blown 
completely  around  the  earth  in  about  fifteen  days.  The  course 
of  most  of  the  dust  was  around  the  earth  in  latitudes  near  the 
equator,  but  from  this  low  latitude  it  spread  notably  toward  the 
poles.  It  has  been  estimated  that  some  of  the  dust  was  still  in 


FIG.  48. — Thick  layer  of  volcanic  dust  (5  or  6  feet)  on  the  Richmond  estate, 
Island  of  St.  Vincent,  five  miles  from  the  crater  of  the  Soufriere.  After 
the  eruption  of  1902.  (Hovey,  Am.  Mus.  Nat.  Hist.) 

the  air  three  years  after  the  eruption,  and  that  some  of  it  went 
several  times  around  the  earth  before  settling.  It  is  probable  that 
the  dust  from  this  single  volcanic  eruption  found  its  way  to  nearly 
all  parts  of -the  earth.  This  example  may  serve  to  illustrate  the 
extent  to  which  dust  is  carried  in  the  upper  part  of  the  air,  and 
the  length  of  time  it  may  be  held  in  suspension.  Dust  in  the 
lower  part  of  the  air  is  not  usually  held  so  long  or  carried  so  far, 
partly  because  the  winds  are  less  strong,  and  partly  because  the 
dust  encounters  all  sorts  of  obstacles,  such  as  hills,  trees,  etc., 
against  which  it  lodges.  Large  quantities  of  dust  were  ejected 


58 


PHYSIOGRAPHY 


from  the  Soufriere  and  from  Pel£e,  in  the  West  Indies,  in  the 
eruptions  of  1902  (Fig.  48). 


FIG.  49. — Bluff  of  loess  at  Kansas  City.     (Mo.  Geol.  Surv.) 


FIG.  50. — Vertical  face  of  loess  near  Huang-tu-Chai  in  northern  Shan-si. 
(Willis,  Carnegie  Institution.) 

Loess.  In  some  parts  of  China,  in  parts  of  Europe,  and  over 
considerable  areas  in  the  Mississippi  basin  there  are  considerable 
thicknesses  of  a  distinctive  earthy  material,  the  particles  of  which 


THE  WORK  OF  THE  ATMOSPHERE 


59 


PlG.  51. — A  bluff  of  loess  in  China  on  which  stands  a  temple. 
(Willis,  Carnegie  Institution.) 


Kio.  52. — Facade  of  a  group  of  buildings  in  a  bluff  of  loess,  Province  o* 
Shan-si,  China.     (Richthofen.) 


PHYSIOGRAPHY 


FIG.  53. — Dwellings  in  loess,  Province  of  Shan-si,  China. 
(Blackwelder,  Carnegie  Institution.) 


FIQ.  54. — A  roadway  in  China  which  has  been  deepened  by  the  removal 
of  loess  by  wind  and  water.     (Willis,  Carnegie  Institution.) 


THE  WORK  OF  THE  ATMOSPHERE 


61 


are  smaller  than  sand  grains,  but  larger  than  the  particles  of  c4ay. 
It  is  known  as  loess,  and  much  of  it,  at  least,  was  deposited  by  the 
wind.  From  the  flood  plains  of  such  rivers  as  the  Missouri  clouds 
of  dust  are  swept  up  and  out  over  the  adjacent  high  lands  at  the 
present  time,  whenever  the  surface  of  the  flood  plain  is  dry  and 
the  wind  strong.  This  dust  is  very  like  loess,  if,  indeed,  it  be  not 
loess.  The  loess  has  the  remarkable  property  of  standing  with 


FIG.  55. — Slopes  of  loess  in  China,  terraced  for  agricultural  purposes. 
(Willis,  Carnegie  Institution.) 

steep  or  vertical  faces  (Figs.  49-51)  for  long  periods  of  time.  In 
China  the  loess  is  said  to  be  several  hundred  feet  thick  locally, 
but  in  the  Mississippi  basin  it  rarely  reaches  a  thickness  of  more 
than  30  to  50  feet.  In  parts  of  China  the  people  have  excavated 
houses  in  successive  tiers  along  the  faces  of  the  soft  though  steep 
slopes  of  the  loess  (Figs.  52  and  53). 

How  held  in  the  air.  Though  made  up  largely  of  mineral 
matter  which  is  much  heavier  than  the  air,  dust  is  kept  in  sus- 
pension, first,  because  the  particles  are  so  small  that  their  surfaces 
are  large  in  proportion  to  their  masses,  so  that  the  friction  involved 
in  their  descent  through  the  air  is  great;  and  second,  because 
there  are  numerous  upward  currents  in  the  atmosphere,  and  these 


62  PHYSIOGRAPHY 

rising  currents  carry  particles  of  dust  upward  in  spite  of  gravity, 
which  is  always  tending  to  bring  them  down.  As  a  matter  of  fact, 
the  dust  of  the  atmosphere  is  always  settling  somewhere,  and  the 
supply  is  being  constantly  renewed. 

Distribution.  In  view  of  what  is  known  concerning  the  move- 
ments of  dust  in  the  air,  it  would  probably  involve  little  exagger- 
ation to  say  that  every  square  mile  of  the  earth's  surface  may 
have  received  dust  from  every  other  square  mile  which  is  capable 
of  furnishing  dust  to  the  atmosphere.  Much  of  the  dust  of  the 
atmosphere  falls  into  the  ocean,  or  into  other  bodies  of  water, 
where  it  is  safe  from  further  disturbance  by  winds;  but  that  which 
lodges  on  land  may  be  picked  up  and  blown  about  again  and  again. 

Gradational  effect  of  winds.  Since  dust  is  being  blown  con- 
stantly from  the  land  to  the  sea,  and  since  the  sea  is  making  no 
commensurate  return  to  the  land,  the  shifting  of  dust  by  the 
wind  tends,  on  the  whole,  to  lower  the  land  and  to  build  up  the 
sea  bottom.  Locally,  however,  the  wind-deposited  dust  aggrades 

the  land. 

Sand 

Sources  of  sand.  Even  gentle  winds  pick  up  and  carry  dust; 
strong  ones  pick  up  and  carry  grains  of  sand,  and  even  tiny  pebbles. 
Like  the  finer  material,  sand  is  blown  about  only  when  it  is  dry. 
Abundant  sand  is  found  along  many  shores  of  seas  and  lakes,  along 
the  bottoms  of  some  valleys,  in  desert  regions,  and  in  some  other 
situations.  In  most  of  these  places  it  is  dry  at  times,  and  in  some 
of  them  it  is  dry  most  of  the  time. 

Lodgment  of  wind-blown  sand.  Sand  grains  are  not  often 
carried  up  to  such  great  heights  as  particles  of  dust,  nor  do  they 
remain  so  long  in  the  air.  Because  of  their  greater  mass,  they 
drop  through  the  atmosphere  more  promptly  when  the  velocity 
of  the  wind  is  checked.  Because  they  are  carried  chiefly  in  the 
lower  part  of  the  atmosphere,  they  are  much  more  likely  than 
dust  to  be  stopped  by  obstacles  on  the  surface  of  the  land.  Thus 
every  tree,  log,  stump,  building  and  fence,  and  every  mound  and 
hill  against  which  sand  is  blown,  is  likely  to  cause  the  lodgment  of 
some  of  it,  just  as  they  are  likely  to  cause  the  lodgment  of  wind- 
blown snow.  It  follows  that  sand,  instead  of  being  somewhat 
evenly  distributed,  as  dust  is,  is  often  accumulated  in  mounds  and 
ridges,  which  begin  their  growth  about  almost  any  sort  of  obstacle 
on  the  surface. 


THE  WORK  OF  THE  ATMOSPHERE  63 

Dunes.  Mounds  and  ridges  of  wind-blown  or  eolian  sand  are 
dunes  (Fig.  56).  Once  started,  a  dune  becomes  an  obstacle  to 
blowing  sand,  and  the  lodgment  of  more  sand  causes  the  dune  to 
grow.  In  this  way,  mounds  and  ridges  of  sand,  scores  and  some- 
times even  hundreds  of  feet  high,  are  built  up  by  the  wind.  Small 
dunes  are  much  more  numerous  than  large  ones. 


FIG.  56. — A  ripple-marked  sand  dune  in  a  western  valley.     (U.  S.  Geol.  Surv.) 

Distribution  of  dunes.  Dunes  are  found  principally  near  the 
sources  of  abundant  sand.  Thus  they  are  common  along  the 
Atlantic  Coast  of  the  United  States  south  of  New  York.  The  sand 
is  here  washed  up  on  the  beach  by  the  waves,  and  whenever  it  dries 
it  may  become  the  prey  of  the  wind.  Winds  from  the  west  blow 
the  sand  into  the  sea;  those  from  other  directions,  but  especially 
from  the  east,  drift  it  up  onto  the  land,  making  dunes.  Dunes 
abound  along  the  eastern  side  of  Lake  Michigan,  and  some  of  them 
are  very  large ;  but  they  are  essentially  absent  from  the  west  shore 
of  the  lake.  This  is  because  both  the  prevailing  and  the  strongest 
winds  are  from  the  west.  Dunes  are  also  more  common  on  the 
leeward  than  on  the  windward  sides  of  valleys.  Thus  where  west- 


64 


PHYSIOGRAPHY 


erly  winds  prevail,  dunes  are  more  common  on  the  east  sides  of 
valleys  than  on  the  west  sides.     They  are  on  the  whole  more  com- 


Fio.  57. — A  group  of  dunes  at  the  head  of  Lake  Michigan.    Dune  Park,  Ind. 

(Meyers. ) 

mon  on  the  south  than  on  the  north  sides  of  valleys,  because  the 
storm  winds  of  winter  are  from  the  north  of  west,  rather  than 
from  the  south  of  west.  Dunes  abound  over  tracts  of  thousands 


FIG.  58. — Dunes  at  Longport,  coast  of  New  Jersey,  showing  the  irregular 
forms  developed  by  winds  which  erode. 

of  square  miles  in  extent  in  the  semi-arid  tracts  of  the  Great  Plains, 
as  in  western  Nebraska  and  western  Kansas.     The  dune  area  be- 


THE  WORK  OF  THE  ATMOSPHERE 


65 


tween  the  Arkansas  River  and  the  Cimarron  was  the  most  difficult 
portion  of  the  famous  Santa  Fe  trail.  Dunes  of  great  size  occur 
also  in  the  west-central  part  of  Wyoming.  They  reach  their 
greatest  development  in  still  more  arid  regions,  such  as  Sahara. 

Locally  dunes  are  the  most  conspicuous  feature  of  the  surface. 
They  are,  on  the  whole,  more  common  on  plains  and  low  plateaus 
than  in  mountains. 

Configuration  of  dunes.  The  shapes  of  dunes  vary  widely. 
When  they  take  the  shape  of  mounds,  they  may  be  round  or  oval, 
or  they  may  be  very  irregular  in  outline.  When  they  take  the  form 
of  ridges  they  may  be  short  or  long,  straight  or  curved.  In  gen- 
eral one  slope,  the  leeward,  is  steeper  than  the  other,  the  wind- 
ward. The  shape  of  the  same  dune,  however,  shifts  from  time  to 
time.  When  dunes  are  in  process  of  destruction  by  the  wind,  their 
forms  are  often  very  irregular  (Fig.  58).  This  is  sometimes  because 
the  vegetation  growing  on  them  holds  the  sand  in  which  it  is  rooted. 

Associated  with  the  dunes  there  are  often  depressions  (PL  V). 
Some  of  them  are  without  outlets,  while  others  have  outlets. 
Some  of  these  depressions  were  scooped  out  by  the  wind,  and  some 
of  them  were  enclosed  by  the  building  up  of  dunes  about  thed. 


FIG.  59. — Lee  side  of  a  sand  dune,  Cape  Henry,  Va.     The  dune  is  advancing 
on  a  forest  and  burying  the  trees.     (Hitchcock.) 

Destructiveness   of   eolian  sand.     The  piling  up  of  sand  into 
dunes  sometimes  does  great  damage.     Narrow  tracts  of  arable 


66 


PHYSIOGRAPHY 


FIG.  60. — Sand  dune  showing  the  effect  of  a  building  on  the  disposition  c 
the  sand.  The  wind  reflected  from  the  building  keeps  sand  from  ac 
cumulating  against  it.  Manistee,  Mich.  (Hitchcock.) 


" 


FIG.  61.— Sand  drifted  over  railway.     Rowena,  Wash.     (Dept.  of  Agr.) 


PLATE  V 


FIG.  1. — Dunes  on  coast  of 
New  Jersey.  Scale  1  — 
miles  per  inch .  (Cape  May 
Sheet,  U.  S.  Geol.  Surv.) 


»  L.S.&TAT/ON 

L  y  ff£ A  C  H 


FIG.  2.  —  Dunes  along  Arkan- 


sas River  in  Kansas.   Scale 


2—  miles  per  inch.    (Lar- 
nedSheet,  U.S.  Geol.  Surv.) 


Fio.  3. — Dunes  in  plains  of  Nebraska.     Scale  2—  miles  per  inch.     (Camp  Clarke  Sheet, 

U.  S.  Geol.  Surv.) 


THE  WORK  OF  THE  ATMOSPHERE  67 

land  along  seacoasts  have  been  made  desolate  by  the  formation 
of  dunes.  Even  forests  of  large  trees  are  sometimes  buried  be- 
neath them  (Fig.  59).  Some  sorts  of  trees  make  heroic  efforts 
to  maintain  their  life  against  the  burying  sands  by  throwing  out 
roots  far  above  their  original  bases.  In  this  way  some  of  them 
survive  until  they  are  nearly  covered  up;  but  if  the  sand  finally 
covers  their  tops,  they  are  killed.  Occasionally,  too,  the  sand 
buries  abandoned  buildings.  It  rarely  accumulates  so  rapidly 
about  a  building  that  it  may  not  be  kept  away  by  human  effort. 
Drifting  sand  is  sometimes  the  occasion  of  much  trouble  along 
railways,  as  shown  in  Fig.  61.  Many  caravans  have  been  destroyed 
in  the  African  desert  by  sand  storms,  and  an  army  of  Cambyses 
numbering  50,000  is  said  to  have  been  overwhelmed  and  buried. 

Migration  of  dunes.  Dunes  are  often  migratory.  Sand  is 
blown  from  their  windward  sides  and  dropped  on  the  leeward  sides. 
The  continued  shifting  of  sand  from  the  windward  to  the  leeward 
side  causes  the  slow  movement  of  the  dune  in  the  direction  of  the 
prevalent  winds.  The  migration  of  dunes,  like  their  first  de- 


FIG.  62. — A  resurrected  forest.     After  burying  and  killing  the  forest,  the 
sand  was  blown  away,  exposing  the  dead  trees.     (Meyers.) 

velopment,  often  works  destruction  to  cultivated  lands,  to  forests, 
to  buildings,  etc. 

Some  idea  of  the  extent  to  which  dunes  migrate  may  be  gained 
both  from  natural  phenomena  and  from  historic  records.*  Thus 
when  dunes  which  have  buried  forests  move  on,  the  forests  which 


68 


PHYSIOGRAPHY 


were  buried  and  killed  are  again  discovered.  This  is  illustrated 
by  Fig.  62.  The  movement  of  the  dune  sand  may  discover  other 
things  also.  At  one  locality  on  the  coast  of  North  Carolina  a  sand 
area  was  utilized  for  a  cemetery.  The  wind  has  now  blowTn  the 


FIG.  63. — Migration  of  dune  sand,  exposing  bones  in  a  cemetery. 
Hatteras  Island,  N.  C.     (Cobb.) 

sand  away  to  such  an  extent  as  to  expose  the  bones  of  the  bodies 
buried  (Fig.  63).  Buildings  buried  by  dunes  are  sometimes  again 
revealed  after  the  dune  has  moved  on.  This  is  shown  by  Fig.  64. 

Recent  discoveries  indicate  that  "there  are  hundreds,  per- 
haps thousands,  of  square  miles  of  buried  towns  and  cities  "*  in 
Central  Asia.  At  least  a  part  of  these  cities  have  been  buried  by 
migrating  dunes. 


FIG.  64. — Diagram  illustrating  the  migration  of  dunes.    Kurische  Nehrurg. 

(Credner.) 

So  disastrous  is  the  migration  of  dunes  along  some  coasts  that 
steps  are  taken  to  prevent  their  movement.  If  a  dune  becomes 
clothed  with  vegetation,  its  position  is  not  likely  to  be  changed 
so  long,  as  the  vegetation  remains,  for  the  plants  have  the  effect  of 

1  Nat.  Geog.  Mag.,  Vol.  XVI,  1905,  p.  499. 


THE  WORK  OF  THE  ATMOSPHERE 


69 


pinning  the  sand  down.  Trees,  shrubs,  etc.,  adapted  to  such  situa- 
tions are  sometimes  planted  in  the  sand  to  prevent  its  further 
drifting  (Fig.  65).  This  is  done  at  various  points  along  the 
western  coast  of  Europe,  where  land  is  valuable.  It  has  been 
done  to  some  extent  in  our  own  country,  as  at  San  Francisco, 
where  shrubs  have  been  planted  on  the  coastal  slope  to  prevent 


FIG.  65. — Dune  sand  held  by  brush  fences  on  Kurische  Nehrung. 

the  sand  of  the  shore  from  blowing  over  the  Golden  Gate  Park. 
Between  1826  and  1838  the  Government  spent  $28,000  to  fasten 
the  dune  sands  on  the  harbor  shores  of  Provincetown,  Mass. 
Even  in  such  cases,  however,  additional  sand  may  be  deposited 
on  the  plant-covered  dune. 

Not  all  eolian  sand  in  dunes.  Eolian  sand  is  not  always  built 
up  into  dunes.  It  is  sometimes  spread  somewhat  evenly  over 
the  surface  where  it  lodges.  Eolian  sand  is  probably  much  more 
wide-spread  than  dunes  are. 

Ripple -marks.  The  surface  of  wind-blown  sand  is  often 
marked  by  pronounced  ripple-marks  (Fig.  56),  very  like  those 
which  affect  the  surface  of  sand  deposited  beneath  water. 

Gradational  effects.  Much  sand  is  blown  from  the  land  into 
the  sea;  but  the  waves  wash  some  of  the  sand  up  on  the  beach 
again.  The  one  process  reduces  the  volume  of  the  land,  while 
the  other  increases  it.  The  relative  importance  of  these  two  pro- 
cesses is  not  known.  On  the  whole,  the  degradation  effected  by 
the  blowing  of  sand  exceeds  the  aggradation  effected  by  its  deposi- 
tion, so  far  as  the  land  is  concerned.  The  aggradational  effects 
are,  however,  very  conspicuous  locally. 


70 


PHYSIOGRAPHY 


The  amount  of  dust  and  sand  shifted  about  on  the  land,  or 
from  the  land  to  the  sea  by  the  wind,  is  very  great.  It  has  been 
estimated  that  in  violent  dust-storms  the  amount  of  dust  and 


FIG.  66. — A  phase  of  wind-carving  on  sandstone.     Wyoming.     (Bastin.) 


FIG.  67. — A  phase  of  wind  carving  near  Klondike,  Wyo.     (Leffingwell.) 

sand  in  the  air  may  amount  to  as  much  as  126,000  tons  per  cubic 
mile  of  air.  The  average  amount  of  dust  in  the  air,  however,  is 
probably  but  a  very  small  fraction  of  one  percent,  of  this  amount. 


THE  WORK  OF  THE  ATMOSPHERE 


71 


If  we  knew  how  many  tons  of  sand  and  dust  were  blown  from  the 
land  to  the  sea  each  day,  the  figures  would  doubtless  be  most  im- 
pressive, but  the  amount  has  never  been  measured  or  even  estimated. 
Abrasion  by  the  wind.  Sand  and  dust  blown  against  a  sur- 
face of  rock  have  the  effect  of  a  sand-blast,  and  wear  away  even  hard 
rock.  If  the  surface  against  which  sand  is  driven  is  of  unequal 
hardness,  the  softer  parts  are  worn  more  rapidly  than  the  harder. 


FIG.  68. — Erosion  Columns  in  Monument  Park,  Colo.;    partly  the  product 
of  wind  erosion.     (Fairbanks.) 

In  regions  where  abundant  sand  is  driven  by  the  wind,  projecting 
rocks  are  often  carved  into  fantastic  forms  (Figs.  66-68).  Abra- 
sion by  wind-driven  sand  is  of  little  consequence  in  a  plain  country 
where  the  climate  is  moist,  and  where  bare  rock  is  rarely  exposed; 
but  it  is  of  much  consequence  in  semi-arid  regions  where  the  topog- 
raphy is  rough,  and  where  hills  and  points  of  bare  rock  are  nu- 
merous. Wind-driven  dust  is  much  less  efficient  than  sand  in  the 
wear  of  rock  surfaces. 

THE  CHEMICAL  WORK  OF  THE  Am 

One  of  the  principal  constituents  of  the  atmosphere  is  oxygen, 
and  oxygen  is  a  substance  which  is  chemically  active,  especially  in 


72  PHYSIOGRAPHY 

the  presence  of  moisture.  This  is  readily  seen  when  a  piece  of  steel, 
such  as  a  knife-blade,  is  exposed  to  the  air.  It  promptly  rusts. 
This  means  that  both  oxygen  from  the  air,  and  water,  have  entered 
into  combination  with  the  iron,  and  the  iron-rust  contains  all 
three  substances,  united  into  one.  It  is  a  matter  of  common 
knowledge  that  the  iron-rust  scales  off,  and  that  a  knife-blade  will 
soon  be  "eaten  away"  by  this  process,  that  is  converted  entirely 
into  rust.  When  oxygen  unites  with  iron  the  iron  is  said  to  be 
oxidized.  If  water  enters  into  combination  at  the  same  time,  as 
it  does  when  iron  rusts,  the  iron  oxide  is  said  to  be  hydrated.  Iron- 
rust  is  therefore  the  hydrated  oxide  of  iron.  The  amount  (weight)  of 
oxygen  and  water  in  iron-rust  is  greater  than  the  weight  of  the  iron. 

Similar  changes  go  on  in  the  rocks.  Iron,  in  some  combination 
or  other,  is  abundant  in  some  rocks,  and  present  in  most  of  them, 
and  the  iron  in  the  rocks  is  subject  to  changes  similar  to  those 
suffered  by  the  knife-blade;  and  in  the  rocks,  as  in  the  knife-blade, 
the  oxidation  of  the  iron  generally  leads  to  the  crumbling  of  the 
rock  of  which  it  is  a  constituent.  Other  substances  in  the  rocks 
also  are  oxidized  and  hydrated,  usually  with  the  result  of  tending 
to  break  up  the  rock. 

Other  constituents  of  the  atmosphere  are  also  active  in  chang- 
ing some  of  the  minerals  of  the  common  rocks.  This  is  the  case, 
for  example,  with  the  carbonic  acid  gas  (CO2)  of  the  atmosphere, 
which  enters  into  combination  with  certain  constituents  of  the 
rock.  The  union  of  carbonic  acid  gas  with  constituents  of  the 
rock  is  known  as  carbonation.  Like  oxidation,  carbonation  usually 
results  in  the  crumbling  of  the  rock  affected. 

Weathering.  All  changes  which,  like  oxidation  and  carbonation, 
lead  to  the  breaking  up  of  the  rock  are  phases  of  the  general 
process  of  weathering,  which  includes  most  of  the  natural  processes 
by  which  rock  at  or  near  the'  surface  is  caused  to  crumble.  The 
processes  of  weathering  are  very  important.  Much  of  the  soil 
and  subsoil  (mantle  rock)  of  the  earth  have  been  produced  by 
them.  Furthermore,  the  weathering  of  the  rock  is  a  necessary 
preparation  for  its  ready  transportation  by  wind  and  water. 

CHANGES  BROUGHT  ABOUT  UNDER  THE  INFLUENCE  OF  THE  AIR 

The  surface  of  the  land  is  subject  to  great  changes  of  tempera- 
ture, and  these  changes  are  of  importance  in  its  physiography. 
The  effects  of  temperature  changes  on  the  rocks  of  the  earth  are 


THE  WORK  OF  THE  ATMOSPHERE  73 

much  more  obvious  in  some  regions  than  in  others;  but,  directly 
or  indirectly,  they  are  of  more  or  less  importance  everywhere. 

Freezing  and  thawing.  In  many  regions  where  the  surface 
is  well  covered  with  soil,  the  soil  freezes  in  winter;  that  is,  the 
water  in  the  soil  freezes,  so  that  the  soil  becomes  solid.  While  the 
soil  is  frozen  it  cannot  be  blown  or  washed  away.  In  low  tem- 
peratures, too,  the  precipitation  falls  as  snow  instead  of  rain,  and 
the  snow  does  not  immediately  have  the  same  effect  as  rain  on 
the  land.  When  it  is  melted  the  water  runs  over  the  surface  much 
as  rain-water  would;  but  if  the  soil  beneath  the  melted  snow  be 
frozen,  the  effect  of  the  running  water  is  relatively  slight. 

Where  the  soil  is  thin,  the  waters  which  sink  beneath  the  surface 
may  freeze  in  the  cracks  of  the  rock  below.  Since  water  ex- 
pands about  one-tenth  of  its  volume  on  freezing,  the  ice  which 
forms  in  the  cracks  (joints)  of  the  rock  when  they  are  nearly  full 
of  water,  acts  like  a  wedge,  widening  the  cracks  and  forcing  the 
parts  of  the  rock  asunder.  The  effect  is  illustrated  by  the  frequent 
breaking  of  bottles  or  other  vessels  in  which  water  is  allowed  to 
freeze.  This  process  of  rock-breaking  is  most  important  where 
there  is  abundant  moisture,  and  where  the  changes  of  temperature 
above  and  below  the  freezing-point  of  water  are  frequent,  that  is 
in  middle  latitudes,  or  in  altitudes  which  have  the  temperatures 
of  middle  latitudes. 

Expansion  and  contraction  of  rock;  rock -breaking.  When 
solid  rock  has  little  or  no  covering  of  loose  material,  as  is  often  the 
case  on  steep  slopes,  it  is  heated  by  day  and  cooled  by  night.  At 
high  altitudes,  and  especially  on  slopes  and  cliffs  exposed  to  the 
noonday  sun,  the  daily  changes  of  temperature  of  the  surface  of  the 
rock  are  great.  In  such  places  the  surface  of  the  rock  may  become 
very  hot  while  the  sun  shines.  Since  rock  is  a  poor  conductor  of 
heat,  only  its  outermost  portion  is  heated  notably.  Heat  expands 
rock,  and  as  the  heated  part  expands  it  is  likely  to  scale  off  from 
the  cooler,  unexpanded  or  less  expanded  part  beneath.  As  the 
sun  descends,  the  surface  cools  and  contracts.  The  outermost 
film  of  rock  cools  first,  and  most,  and  tends  to  break.  The  break- 
ing of  cool  glass  by  touching  it  with  hot  water,  or  of  hot  glass  by 
touching  it  with  cold  water,  involves  the  same  principle. 

The  breaking  of  rock  by  heating  and  cooling,  even  when  ice  is 
not  formed,  is  a  very  common  phenomenon.  Thus  on  hot  days 
in  summer  the  blocks  of  cement  in  cement  walks  sometimes 


74 


PHYSIOGRAPHY 


expand  so  much  as  to  break  (Fig.  69).  The  heat  of  the  sun  some- 
times so  expands  the  rock  in  the  floor  of  a  rock  quarry  that  it  is 
similarly  bowed  up  and  broken.  This  has  been  seen  repeatedly 
in  the  limestone  quarries  about  Chicago,  and  on  the  floor  of  the 
Drainage  Canal  before  the  water  was  turned  in.  Many  bowlders 
which  lie  on  the  surface  are  seen  to  be  "shelling  off"  (Fig.  70), 


FIG.  69. — A  cement  walk  broken  under  expansion  by  sun-heat. 

and  the  same  process  is  sometimes  seen  on  mountain  tops  (Fig.  71). 
In  high  mountain  regions  where  the  changes  of  temperature  of 
the  rock  are  great  and  sudden,  the  exposed  rock  is  often  much 
broken.  So  far  has  this  gone  that  the  surface  of  many  a  sharp 
mountain  peak  is  covered  with  cracked  and  broken  rock,  so  in- 
secure that  a  touch  or  a  step  will  loosen  numerous  pieces  and  start 
them  down  the  mountain  (Fig.  72).  Quantities  of  such  debris 


FIG.  70. — Concentric  weathering,  or  exfoliation  of  bowlder.  Eastern  California. 

(Fairbanks.) 

(called  talus')  bury  the  bases  of  many  of  the  western  mountains 
many  hundreds  of  feet  (Fig.  73).  The  pieces  of  talus  range  in 
size  from  tiny  bits  up  to  masses  tons  in  weight. 

The  breaking  of  rock  through  changes  of  temperature  is  not  the 


THE  WORK  OF  THE  ATMOSPHERE 


75 


FIG.  71. — Exfoliation  on  a  mountain  slope.     Mount  Starr-King,  Cal. 


FIG.  72. — Crumbling  on  a  mountain  top.     Kearsarge  Pass,  Sierra  Nevada 

Mountains. 


76 


PHYSIOGRAPHY 


work  of  the  atmosphere;  but  the  atmosphere  has  much  influence 
on  the  changes  of  temperature,  on  which  the  process  depends. 

This  process  of  rock  breaking  is  a  phase  of  weathering.  The 
debris  loosened  in  this  way  moves  from  higher  to  lower  levels 
under  the  influence  of  gravity,  if  it  moves  at  all.  The  general 
effect  of  the  process  is  to  make  high  places  lower,  and  to  build  up 
the  lower  surfaces  about  the  bases  of  steep  slopes. 

There  are  many  other  phases  of  weathering  not  due  to  the 
atmosphere,  and  not  altogether  conditioned  by  it.  Some  of  these 
are  illustrated  by  Figs.  74  and  75. 


FIG.  73. — Talus  slope. 

The  roots  of  the  plants  penetrate  the  soil,  loosening  it,  and 
thereby  make  it  easier  for  water  to  get  below  the  surface.  Roots 
sometimes  grow  in  cracks  in  the  rock,  and  as  they  grow  they  act 
like  wedges  (Fig.  74) .  Large  masses  of  rock  are  sometimes  loosened 
in  this  way.  When  a  tree  is  uprooted,  the  ground  is  torn  up,  and 
rock  material  to  the  depth  of  several  feet  is  sometimes  exposed 
to  the  action  of  freezing  water,  air,  and  rain  (Fig.  75).  Again, 
when  plants  decay,  acids  are  formed  which  increase  the  dissolving 
power  of  ground-water.  Burrowing  animals  of  all  sorts  loosen 
the  ground  and  develop  channels  for  the  entrance  of  water.  Even 


THE  WORK  OF  THE  ATMOSPHERE 


77 


FIG.  74. — Tree  growing  in  crack  in  a  rock,  and  by  its  growth 
splitting  the  rock. 


FIG.  75. — Upturned  tree,  showing  the  disturbance  of  soil  and  rock. 
(U.  S.  Geol.  Surv.) 


78  PHYSIOGRAPHY 

such  small  animals  as  ants  and  earthworms  perform  an  important 
work  in  this  connection.  In  Massachusetts  the  ant  has  been 
estimated  to  bring  one-fourth  of  an  inch  of  fine  earth  to  the  surface 
each  year,  while  Darwin  estimated  that  the  earthworm  brings 
seven  to  eighteen  tons  of  material  per  acre  to  the  surface  each 
year. 

SUMMARY 

On  the  whole,  the  tendency  of  the  work  of  the  atmosphere 
and  of  the  work  which  is  controlled  by  it,  is  to  lower  the  surface 
of  the  land,  and  to  loosen  materials  of  the  surface  so  that  they 
may  be  readily  moved  to  lower  levels  by  other  agencies.  The 
most  important  phase  of  the  degradational  work  cf  the  atmos- 
phere is  weathering,  or  the  preparation  of  material  for  removal 
by  other  and  more  powerful  agents  of  degradation.  As  we  shall 
see,  however,  the  atmosphere  is  not  the  only  agent  concerned  in 
weathering  (see  p.  110). 

REFERENCES 

1.  CORNISH,  On  the  Formation  of  Sand  Dunes:    Geog.  Jour.,  Vol.  IX, 
1897,  pp.  278-309. 

2.  UDDEN,  Erosion,   Transportation,  and  Sedimentation  performed  by  the 
Atmosphere:   Jour,  of  Geol.,  Vol.  II,  pp.  318-331,  and  Pop.  Sci.  Mo.,  Sept., 
1896. 

3.  CHAMBERLIN  AND  SALISBURY,  Geologic  Processes,  Chapter  II :    Henry 
Holt  &  Co.,  1903. 

4.  GEIKIE,  Earth  Sculpture,  Chapter  XII:  Putnam. 

5.  HITCHCOCK,  Controlling  Sand  Dunes  in  the  United  States:   Nat.  Geog. 
Mag.,  Vol.  XV,  pp.  43-47,  1904. 

6.  COBB,  Nat.  Geog.  Mag.,  1906,  pp.  310-317. 

7.  MERRILL,   Principles  of  Rock   Weathering:    Jour,   of  Gecl.,   Vol.    VI, 
pp.  704-724  and  850-871 ;— also  Rocks,  Rock  Weathering,  and  Soil.-t:  Macmillan. 

8.  COWLES,  The  Plant  Societies  of  Chicago  and  Vicinity:     Bull.  '2,  Geog. 
Soc.  of  Chicago.     (Deals  with  relations  of  dunes  and  vegetation.) 

9.  SALISBURY  AND  ALDEN,  Geography  of  Chicago  and  its  Environs:     Geog. 
Soc.  of  Chicago,  1899,  pp.  60-63. 

10.  All  standard  text-books  on  Geology. 

11.  The  following  Topographic  Sheets  of  the  U.  S.  Geol.  Surv.: 
Long  Beach,  N.  J.              North  Platte.  Neb.  Great  Bend,  Kan. 
Sandy  Hook,  N.  J.             Green  Run,  Md.-Va.  Kinsley,  Kan. 
Barnegat,  N.  J.                   Fire  Island,  N.  Y.  Hutchinson,  Kan. 
Atlantic  City,  N.  J.            Springfield,  Colo.  Lakin,  Kan. 
Asbury  Park,  N.  J.            Dodge,  Kan.  Pratt,  Kan. 
Browns  Creek,  Neb.           Larned,  Kan. 

St.  Paul,  Neb.  Kingman,  Kan. 

12.  The  following  Folios  of  the  U.  S.  Geol.  Surv.: 

Norfolk,  Va.  Camp  Clarke,  Neb.  Scotts  Bluff,  Neb. 


THE  WORK  OF  THE  ATMOSPHERE  79 


TOPOGRAPHIC  MAPS  SHOWING  EFFECTS  OF  WIND-WORK 

I.  Study  the   following  topographic  maps  in  preparation  for  the  con- 

ference : ' 

1.  Sandy  Hook,  N.  J.— N.  Y.  5.  Barnegat,  N.  J. 

2.  Cape  May,  N.  J.  6.  Lamed,  Kan. 

3.  Toleston,  Ind.  7.  Pratt,  Kan. 

4.  St.  Paul,  Neb.  8.  Kinsley,  Kan. 

II.  Geologic  Folios  2  to  be  studied: 

1.  Norfolk,  Va.— N.  C.  2.  Camp  Clarke,  Neb. 

III.  Suggestions  for  the  study  of  the  above  maps: 

On  these  maps  mounds,  hills,  and  ridges  of  wind-blown  sand, 
or  dunes,  are  represented.     In  studying  the  dunes  note: 

1.  The  various  shapes,  sizes,  and  modes  of  aggregation  of  the 
dunes  in  the  several  regions. 

2.  The  average  height  of  the  dunes  above  their  surroundings 
in  the  different  regions. 

3.  The  distribution  of  the  Kansas  dunes  with   reference   to 
the  stream  courses.     Is  any  law  of  distribution  suggested  by  the 
map?     If  so,  how  is  it  to  be  accounted  for? 

4.  What  are  the  immediate  sources  of  the  sand  which  forms 
the  dunes  in  the  different  regions,  so  far  as  can  be  inferred  from 
the  maps? 

5.  Note  that  in  several  of  the  areas  depressions  are  numerous 
among  the  dunes.     May  the  wind  be  responsible  for  them? 

6.  Locate  the  dunes  on  the  "Topographic  Sheet"  of  the  Camp 
Clarke  folio.     Then  turn  to  the  next  map  of  the  folio,  the  "Areal 
Geology  Sheet."     Here  the   dune   areas  are   colored   yellow    (see 
legend).     From  the  topographic    map   form  a  mental  picture  of 
"Jail  Rock,"  "Smokestack  Rock,"  and  "Chimney  Rock."     Test 
your  picture  by  reference  to  Figs.  18,  20,  and  23  in  the  back  of 
the  folio.     The  wind  is  playing  an  important  part  in  the  reduction 
of  these  elevations.     How? 

1  See  foot-note,  page  54. 

2  The  folios,  like  the  topographic  maps,  are  published  by  the  U.  S.  Geo- 
logical Survey.     Most  of  them  can  be  purchased  at  25  cents  each. 


CHAPTER  III 
THE  WORK  OF  GROUND-WATER 

GENERAL  FACTS 

WATER  is  one  of  the  most  active  agents  working  on  the  land. 
Its  activity  is  seen  on  every  slope  during  heavy  falls  of  rain  and 
when  snow  is  melting  rapidly,  and  it  is  seen  in  every  stream  and 
in  the  waves  of  lakes  and  seas.  Even  the  water  in  the  soil  and 
in  the  rocks  beneath  the  soil  is  active,  though  its  effects  are  less 
obvious  than  those  of  streams  and  waves. 

The  great  activity  of  the  water,  like  that  of  the  air,  is  due 
primarily  to  its  mobility;  but  its  greater  weight  makes  moving 
water  much  more  effective  than  moving  air  when  it  comes  in  con- 
tact with  the  land. 

Source  of  land-water.  The  waters  of  the  land  have  fallen 
from  the  atmosphere,  which  always  contains  some  moisture  in 
the  form  of  water  vapor.  This  vapor  is  constantly  passing  up 
into  the  air  from  all  moist  surfaces  by  evaporation,  a  process  which 
will  be  considered  in  another  chapter;  but  the  familiar  fact  that 
any  moist  surface  soon  dries  in  the  sunshine,  or  in  any  warm 
dry  place,  may  be  taken  to  illustrate  what  is  going  on  at  all  times, 
both  from  moist  land  surfaces  and  from  water  surfaces. 

Under  certain  conditions  some  of  the  moisture  in  the  air  is 
condensed  into  drops  and  falls  as  rain;  or  if  the  temperature  at 
which  the  water  vapor  condenses  is  below  the  freezing-point  of 
water,  the  moisture  freezes  as  it  condenses,  forming  snowflakes 
(Fig.  76)  instead  of  rain-drops.  The  average  amount  of  rain 
and  snow  (snow-water)  which  falls  on  the  land  is  something  like 
40  inches  per  year  (10  to  12  inches  of  snow  being  counted  as  one 
of  water).  In  other  words,  the  precipitation  (the  rain  and  snow) 
which  falls  on  the  land  each  year  is  enough  to  make  a  layer  of 

80 


THE  WORK  OF  GROUND-WATER  81 

water  rather  more  than  three  feet,  or  about  one  meter,  deep  over 
the  surface  of  the  land,  if  it  were  equally  distributed.  Forty 
inches  of  water  over  the  land  would  make  about  35,000  cubic 
miles  of  water.  Since  the  rivers  discharge  but  about  6500  cubic 
miles  of  water  into  the  sea  yearly,  it  is  clear  that  the  larger  part 
of  the  rainfall  is  not  carried  to  the  sea  by  rivers. 


FIG.  76. — Photographs  of  snowflakes,  showing  something  of  their  diversity 

of  form.     (Bentley.) 

The  fate  of  rain-water.  The  water  which  falls  on  the  land 
disappears  in  various  ways.  A  part  of  it  sinks  at  once  beneath 
the  surface,  a  part  forms  pools  or  lakes  upon  the  surface,  a  part 
runs  off  over  the  surface  directly,  and  a  part  of  it  is  evaporated. 
The  proportion  of  the  rainfall  in  any  given  place  which  follows 
each  of  these  courses  depends  upon  several  conditions,  among 
which  are  (1)  the  topography  of  the  surface,  (2)  the  rate  of  rain- 
fall (or  the  rate  at  which  snow  melts),  (3)  the  porosity  of  the  soil 
or  rock,  (4)  the  amount  of  water  which  the  soil  already  contains 


82  PHYSIOGRAPHY 

when  the  rain  falls  or  the  snow  melts,  (5)  the  amount  of  vegetation 
on  the  surface,  and  (6)  the  dryness  of  the  atmosphere. 

Considering  these  points  in  order,  we  find  (1)  that  the  steeper 
the  slope  on  which  rain  falls  or  snow  melts,  the  more  rapidly  the 
water  runs  off,  and  the  larger  the  proportion  which  follows  this 
course;  for  when  it  flows  off  rapidly  there  is  little  time  for  it  to 
sink  in  or  evaporate. 

(2)  The  more  rapidly  the  rain  falls,  the  less  the  proportion 
which  sinks  in.     The  sinking  of  the  water  is  a  slow  process,  espe- 
cially if  the  soil  is  compact.     In  sinking,  the  water  first  fills  the 
pores  of  the  surface,  and  no  more  can  enter  until  that  already  in 
has  had  time  to  sink  out  of  the  way.     If,  therefore,  the  rain  falls 
rapidly,  less  sinks  in  and  more  runs  off  over  the  surface  than  if 
it  falls  slowly. 

(3)  Loose  or  open  soil,  such  as  sand  or  gravel,  takes  in  the 
water  more  readily    than    clay   or  other   compact   material.     A 
clayey  soil,  therefore,  causes  a  larger  part  of  the  rainfall  to  run 
off  over  the  surface,  because  it  allows  less  to  sink  in  in  a  given 
time.     Not  only  this,  but  a  porous  soil  will  hold  more  water, 
because  the  pore  space,  that  is  the  space  between  its  constituent 
parts,  is  larger.     A  special  case  of  compactness  arises  in  connection 
with  changes  of  temperature.     When  the  ground  is  frozen,  that 
is  when  the  water  in  it  is  frozen,  the  soil  is  rendered  solid  and 
less  porous,  and  surface  water  can  penetrate  it  but  slightly,  even 
if  thet  .surface  water  remains  unfrozen.     When  the  soil  beneath 
snow  is  frozen,  the  water  produced  by  the  melting  of  the  snow  does 
not  readily  enter  it,  and  so  a  large  part  is  allowed  to  run  off  over 
the  surface. 

(4)  When  the  soil  contains  much  water,  less  can  enter,  and 
more  runs  off  over  the  surface. 

(5)  Vegetation   serves   to   restrain  the  flow  of  surface  water, 
and  holds  it  longer  on  the  surface.      As  a  result  there  is  more 
time  for  the  water  to  sink  in,  and  less  runs  off  without  sinking. 

(6)  If  the  air   is  very  dry,  a  larger  proportion  of  the  rainfall 
evaporates  directly,  leaving  less  to  run  off  or  sink  in.     The  effect 
of  dryness  on  evaporation  is  conspicuous  in  arid  regions,  where 
the  surface  dries  quickly  after  a  shower,  and  where  light  snows 
often  entirely  disappear  in  a  short  time  by  evaporation,  even  when 
the   temperature   is   constantly   far   below   the  freezing-point   of 
water. 


THE  WORK  OF  GROUND-WATER 

The  water  which  sinks  into  the  ground  becomes  ground-water, 
while  that  which  flows  off  over  the  surface  without  sinking  is  the 
immediate  run-off.  Much  of  the  ground-water  ultimately  reaches 
the  surface  again,  and  some  of  it  joins  the  immediate  run-off  in 
the  streams.  All  the  water  which  the  streams  carry,  whether 
it  has  been  beneath  the  surface  or  not,  is  the  run-off. 

The  existence  of  ground-water.  The  soil  of  most  regions  is 
damp  at  a  depth  of  a  few  inches,  or  at  any  rate  a  few  feet,  even 
when  it  seems  dry  at  the  surface.  In  deep  holes,  like  wells,  water 
seeps  or  flows  in  from  the  sides,  and  collects  in  the  bottom.  In 
thickly  populated  farming  communities  there  are  wells  on  almost 
every  farm,  and  all  are  supplied  with  water.  Illinois  has  more 
than  250,000  farms,  and  it  is  probable  that  the  number  of  wells 
in  the  state  is  double  the  number  of  farms.  The  total  number  of 
wells  in  the  United  States  must  be  several  millions,  and  the  amount 
of  water  drawn  out  through  them  each  day  is  very  great;  yet 
the  wells  do  not  ordinarily  go  dry.  Mines,  too,  usually  encounter 
water  at  no  great  depth  from  the  surface. 

The  source  of  ground-water.  Since  rain-water  and  melted 
snow  are  constantly  sinking  beneath  the  surface,  since  rain  and 
snow  seem  like  an  adequate  source  of  supply  for  the  ground-water, 
and  since  no  other  source  l  whence  it  can  come  is  known,  it  is 
inferred  that  surface  water  is  the  source  of  ground-water.  Other 
phenomena  point  to  a  close  connection  between  the  two.  Thus 
in  time  of  drought  many  shallow  wells  and  some  springs  go  dry. 
When  the  drought  is  broken  by  renewed  rainfall,  the  wells  again 
contain  water,  and  the  springs  again  flow.  This  seems  to  establish 
a  direct  connection  between  precipitation  from  the  atmosphere 
and  the  supply  of  water  beneath  the  surface. 

While  the  proportion  of  rainfall  which  sinks  beneath  the  surface 
is  determined  by  the  conditions  already  suggested,  the  amount 
of  water  beneath  the  surface,  other  things  being  equal,  is  great 
where  the  rainfall  is  great.  The  amount  of  ground-water  in  any 
region  is,  however,  not  entirely  dependent  on  the  rainfall  of  that 
region,  for  water  falling  in  one  place  may  flow  underground  to*  an- 
other. Thus  rain-water  which  falls  in  the  Rocky  Mountains  flows 

1  Water  rarely  sinks  into  the  ground  from  lakes,  rivers,  etc.,  though 
it  does  so  under  some  circumstances.  Rivers  and  lakes  are,  however,  fed 
by  rain-water,  either  before  or  after  it  has  been  underground,  so  that  ground- 
water  derived  from  lakes  and  rivers  comes  from  atmospheric  precipitation. 


84  PHYSIOGRAPHY 

underground  through  porous  beds  of  rock  out    under   the   Great 
Plains,  where  it  is  brought  to  the  surface  through  wells  (Fig.  77). 


FIG.  77. — Diagram  showing  how  water  tailing  in  one  place  may  flow  under- 
ground to  another  and  there  be  brought  to  the  surface.  The  layer  a 
is  porous  and  water  entering  it  in  the  mountains  follows  it  to  the  plains. 

Descent  of  ground-water.  The  manner  in  which  water  enters 
the  soil  is  readily  seen.  It  sinks  in  through  all  the  pores  and  cracks. 
In  the  soil  and  subsoil  pores  are  more  common  than  cracks,  but 
in  the  solid  rock  beneath,  cracks  are  common,  and  while  pores  are 
not  absent,  they  are  often  much  smaller  than  the  cracks.  Since 
the  cracks  in  rock  run  in  various  directions,  the  water  descends  not 
only  vertically,  but  in  oblique  directions  as  well.  Water  descends 
as  long  as  there  are  cracks  and  pores  or  openings  of  any  sort  not 
already  filled  with  water;  but  the  smaller  these  passageways 
become,  the  more  difficult  it  is  for  the  water  to  pass  through  them. 
If,  for  example,  small  pores,  such  as  those  which  occur  in  compact 
soil  or  rock,  be  diminished  in  size  one-half,  the  difficulty  of  the 
descent  of  water  is  much  more  than  doubled. 

Generally  speaking,  the  rocks  near  the  surface  have  more 
and  larger  pores  than  those  at  greater  depths.  It  follows  that 
as  the  pores  get  fewer  and  smaller  with  increasing  depth,  the 
difficulty  with  which  water  descends  increases. 

It  cannot  be  stated  definitely  how  far  down  cracks  and  openings 
exist;  but  it  seems  probable  that  all  openings  become  very  small 
at  a  depth  of  a  mile  or  two,  and  that  none  exist  below  a  depth  of 
five  or  six  miles.  At  this  depth  the  rock  is  under  the  pressure  of 
a  column  of  rock  five  miles  high,  and  the  weight  of  such  a  column 
is  so  great  that,  in  any  ordinary  sort  of  rock,  cracks  and  pores 
would  be  closed,  if  once  formed.  Since  different  sorts  of  rock 
have  unequal  strength,  pores  and  cracks  might  exist  in  different 
sorts  of  rock  at  somewhat  different  depths,  but  probably  in  no 
rock  at  a  depth  of  more  than  about  six  miles. 

For  these  reasons  it  is  not  probable  that  the  water  descends 
more  than  five  or  six  miles,  and  the  amount  of  water  below  the 


THE  WORK  OF  GROUND-WATER 


85 


depth  of  even  one  or  two  miles  is  probably  far  less  than  the  amount 
above  that  level. 

The  ground-water  surface.  Though  the  amount  of  water 
beneath  the  surface  is  very  great,  as  common  phenomena  show, 
the  porous  rock  and  soil  are  rarely  altogether  full  of  water.  This 
is  shown  by  the  fact  that  it  is  necessary  in  many  regions  to  dig 
wells  to  a  depth  of  several  scores  or  even  hundreds  of  feet  before 
an  adequate  supply  of  water  is  obtained.  The  surface  soil  is 
rarely  full  of  water  except  immediately  after  a  heavy  rain,  or 
when  snow  is  melting. 


FIG.  78. — Represents  a  series  of  wells  sunk  in  a  flat  tract  of  land. 

If  a  series  of  wells  be  dug  in  a  flat  region,  where  the  soil  and 
rocks  are  essentially  uniform,  they  would  need  to  be  dug  to  about 
the  same  depth  in  order  to  secure  a  constant  supply  of  water. 
This  is  illustrated  by  Fig.  78.  If  the  well  at  a  is  dug  to  a  given 
depth,  a  well  at  b  will  need  to  be  dug  to  about  the  same  depth  in 
order  to  secure  an  equal  supply  of  water  Other  wells  at  c  and  d 
will  also  need  to  be  of  approximately  the  same  depth.  Under 
these  circumstances,  the  water  in  the  several  wells  will  stand  at 
about  the  same  level.  This  means  that  the  rocks  and  subsoil  of 


FIG.  79. — Diagram   illustrating   the   position   of   the   ground-water   surface 
(the  dotted  line)  in  a  region  of  undulating  topography. 

the  region,  below  the  level  of  the  water  in  the  several  wells, 
are  full  of  water.  The  underground  surface  below  which  the  rocks 
etc.,  are  full  of  water  in  any  given  region  is  the  water  surface  (or 
water  table}  for  that  region.  In  one  region  the  water  surface 
may  be  10  feet  below  the  actual  surface,  and  in  another  100  feet. 
In  dry  regions  it  may  be  even  deeper,  but  where  the  rainfall  is 


86  PHYSIOGRAPHY 

sufficient  for  agricultural  purposes,  the  water  surface  is  rarely 
more  than  a  few  score  feet  below  the  surface  of  the  land. 

Where  the  surface  is  uneven,  the  ground-water  surface  usually 
undulates  with  it,  but  to  a  less  extent,  as  shown  in  Fig.  79. 

Amount  of  ground-water.  The  amount  of  ground-water  is 
not  very  definitely  known,  but  the  best  estimates  which  have  been 
made  indicate  that  the  water  in  the  soil,  rocks,  etc.,  of  the  land 
would  probably  make  a  layer  not  more  than  1000  feet  deep,1  if  it 
were  spread  out  over  the  surface  of  the  land.  The  amount  of  water 
in  the  rocks  beneath  the  ocean  bed  is  probably  less  per  square 
mile  than  beneath  the  land  surface,  because  the  rocks  there  are 
probably  less  porous.  Even  if  the  amount  of  ground-water  beneath 
the  sea  were  as  great  as  that  in  the  land,  square  mile  for  square 
mile,  the  total  amount  of  ground-water  would  be  but  a  fraction  of 
that  in  the  sea. 

The  movement  of  ground-water.  Ground-water  is  in  con- 
stant movement.  This  is  shown  in  many  ways.  If  all  the  water 
is  pumped  out  of  a  well,  it  soon  fills  again  to  its  former 
level.  This  shows  that  water  flows  in.  The  constant  flow  of 
the  thousands  of  springs  shows  that  ground-water  is  in  movement, 
for  only  thus  can  the  springs  be  supplied  with  water.  The  seepage 
of  water  into  mines,  quarries,  etc.,  tells  the  same  story. 


Ground  Water  Surface 


B 


——-—Ground  Water  Surface 


FIG.  80. — In  the  upper  part  of  the  figure  (A)  the  water  surface  is  level. 
If  a  heavy  rain  takes  place  in  the  area  at  the  left  of  that  represented 
by  the  figu-e  the  water  surface  at  the  left  will  be  raised  as  indicated  by 
the  lower  part  (B)  of  the  figure.  Movement  of  the  ground-water  will 
follow. 

The  reasons  why  ground-water  is  in  movement  are  readily 
understood.  The  rainfall  is  not  equally  distributed.  If  there 
be  a  heavy  local  shower  in  a  flat  region,  where  the  water  surface 
is  level  or  nearly  so,  the  soil  and  rock  in  the  area  where  the  rain 
falls  are  more  or  less  completely  filled  with  water.  The  result  is 

1  Estimates  have  ranged  from  3000  feet  to  100  feet. 


THE  WORK  OF  GROUND-WATER  87 

that  the  water  surface  for  the  region  is  temporarily  raised,  as 
shown  at  c  in  Fig.  80.  Since  water  is  mobile,  this  is  a  condition  of 
instability,  and  the  water  from  c  will  tend  to  flow  off  in  all  direc- 
tions to  places  where  the  water  surface  is  lower.  The  principle 
involved  is  precisely  the  same  as  that  which  would  be  in  operation 
if  a  mound  of  water  were  placed  on  a  level  surface.  It  would 
promptly  spread.  In  the  subsoil,  or  rock  beneath,  the  water 
tends  to  spread  just  as  it  would  at  the  surface,  but  its  movement 
is  miLch  slower  because  of  the  friction  of  the  water  with  the  rock, 
etc.,  through  which  it  passes. 

In  a  region  of  uniform  rainfall,  but  of  uneven  surface,  the  water 
surface  is  not  level.  Other  things  being  equal,  it  is  somewhat 
higher  beneath  high  land,  and  somewhat  lower  beneath  low  land 
(Fig.  79).  Where  this  is  the  case  the  water  surface  would  ulti- 
mately become  level  if  there  were  no  rain;  but  in  moist  regions 
rain  is  so  frequent  that  the  water  surface  under  the  hills  rarely 
or  never  sinks  down  to  the  level  of  the  water  surface  under  the 
surrounding  low  land,  before  it  is  raised  again  by  additional  rains. 
As  a  result  of  inequalities  of  rainfall  and  of  topography,  ground- 
water  is  constantly  moving  out  from  areas  where  the  water  surface 
is  higher  to  areas  where  it  is  lower. 

While  the  flow  of  ground-water  is  determined  primarily  by  the 
ground-water  surface,  and  while  it  always  tends  to  flow  from 
higher  to  lower  levels,  it  is,  in  some  situations,  forced  upward. 
Thus,  if  water  moving  down  through  a  porous  layer  of  rock  (a, 
Fig.  77)  between  beds  (&  and  c)  which  do  not  allow  it  to  penetrate 
them,  finds  an  opening  through  the  impervious  layer  (6),  it  may 
escape  upward.  It  may  even  issue  with  great  force,  if  the  source 
of  supply  be  much  higher  than  the  point  of  issue.  Water  also 
rises  by  capillary  action,  but  not  in  such  quantity  as  to  give  rise 
to  springs  or  visible  seepage. 

In  addition  to  the  water  which  comes  out  from  beneath  the  sur- 
face through  wells  and  springs  on  the  land,  some  of  it  flows  under- 
ground to  the  sea  or  to  lakes,  and  issues  as  springs  beneath  them. 
Some  ground-water,  too,  seeps  out  in  such  small  quantities  as  not 
to  appear  to  flow.  In  this  case  it  does  not  constitute  a  spring. 

Ground-water  moves  about  to  some  extent  under  the  influence 
of  forces  other  than  gravity.  Besides  the  movements  resulting 
from  capillarity,  some  of  it  is  taken  up  by  the  roots  of  plants, 
and,  passing  up  through  the  plants,  escapes  through  their  leaves 


88  PHYSIOGRAPHY 

into  the  air.  Still  another  portion  of  the  water  beneath 
the  surface  is  evaporated  directly,  without  the  intervention  of 
plants.  Even  in  regions  where  the  soil  appears  to  be  very  dry 
evaporation  is  constantly  going  on.  If,  for  example,  the  water 
surface  is  500  feet  down,  the  pores  of  the  rock  down  to  the  water 
surface  are  full  of  air.  From  the  water  surface  below,  water  evap- 
orates into  the  air  in  the  rock  and  soil,  and  this  air  may  thus 
become  more  moist  than  the  air  above.  Moisture  is  thus  diffused 
upward,  and,  under  some  circumstances,  it  may  rise  by  convection. 

The  rise  of  invisible  moisture  from  the  ground  may  be  easily 
demonstrated  in  a  very  simple  way.  If  a  rubber  blanket  be  spread 
on  the  ground  on  a  summer  night,  or  if  a  pan  be  inverted  on  the  soil, 
the  under  side  of  the  blanket  or  pan  will  often  be  dripping-wet  in 
the  morning,  before  the  heat  of  the  sun  affects  it.  Had  the  cool 
blanket  or  the  cool  metal  not  been  there  to  stop  it,  the  moisture 
from  below  would  have  escaped  into  the  air  above,  unnoticed. 
It  is  so  escaping  all  day  and  all  night,  and  every  day  and  every 
night,  over  all  land  surfaces  wherever  the  air  in  the  soil  and  below 
it  is  more  moist  than  that  above.  In  this  and  other  ways  the 
supply  of  ground-water  is  being  constantly  drawn  upon.  Constant 
renewal  through  the  descent  of  rain,  or  through  underground 
flowage  from  some  other  region,  is  therefore  necessary  to  maintain 
the  supply. 

It  is  probable  that  nearly  all  of  the  water  which  sinks  beneath 
the  surface  sooner  or  later  comes  up  again  in  some  one  of  these 
various  ways;  but  a  small  amount  of  it  enters  into  combination 
with  the  solid  mineral  matter,  as  in  iron-rust  (p.  71).  So  long  as 
water  remains  in  this  solid  combination,  it  does  not  again  escape 
to  the  surface. 

The  rate  at  which  ground-water  moves  varies  greatly,  and  is 
dependent  chiefly  on  (1)  the  porosity. of  the  rock  or  soil,  and  (2) 
the  pressure  of  the  water.  The  rate  at  which  water  seeps  through 
soils  from  irrigating  ditches  has  been  determined  at  various  points 
in  the  West.  Except  in  very  porous  soils,  it  ranges  from  one  to 
eight  feet  per  day.  In  very  porous  soils  it  is  sometimes  as  much 
as  fifty  feet  per  day.  In  the  Potsdam  standstone,  a  wide-spread 
formation  underlying  southern  Wisconsin  and  its  surroundings  to 
the  south,  and  the  source  of  many  artesian  wells,  the  rate  of  move- 
ment of  ground-water  has  been  estimated  at  half  a  mile  a  year. 
Rain-water  which  enters  this  formation  100  miles  from  Chicago 


THE  WORK  OF  GROUND-WATER 


89 


would  therefore  reach  that  city  in  about  200  years,  if  this  rate  is 
correct.  The  water  which  sinks  to  great  depths  and  into  the  very 
small  pores  and  cracks  moves  with  extraordinary  slowness,  and 
some  of  it  remains  entrapped  within  the  rock  for  very  long  periods 
of  time. 

Springs 

All  water  issuing  from  beneath  the  surface  is  seepage.  Water 
issuing  through  a  natural  opening  in  such  quantity  as  to  make  a 
distinct  current  is  a  spring.  Springs  occur  in  many  sorts  of  situa- 
tions, but  they  are  not  located  by  accident.  They  occur  where 
there  are  natural  passageways  for  the  ground-water  to  escape 
to  the  surface.  Such  passageways  arise  in  various  ways.  Two 


FIG.  81. — Diagram  to  illustrate  two  types  of  springs  as  explained  in  text. 

cases  are  illustrated  by  Fig.  81.  In  one,  the  water  descends  through 
the  porous  bed  e  to  the  layer  d,  which  is  relatively  impervious. 
The  water  flows  along  this  layer  until  the  layer  comes  to  the 
surface  (outcrops)  and  there  the  water  flows  out  as  a  spring,  s'. 
In  the  other,  the  water  moves  underground  through  the  porous 
layer  6,  under  pressure,  until  it  reaches  a  crack  which  leads 
up  to  the  surface.  If  the  crack  is  open  enough  to  afford 
a  passageway,  the  water  may  follow  it  up  to  the  surface, 
as  at  s.  A  spring  may  occur  in  such  a  situation  only  when  the 
opening  is  lower  than  the  water  surface  in  the  rock  which  furnishes 
the  water.  In  the  figure,  the  spring  at  s  is  lower  than  the  .water 
surface  at  w.  This  sort  of  a  spring  is  similar  to  a  flowing  well  in 
principle,  but  in  the  latter  case  the  opening  is  made  by  man. 

Temperature.  The  temperature  of  water  as  it  issues  from 
beneath  the  surface  is  very  variable.  Most  springs  seem  cold  in 
warm  weather.  There  is  indeed  a  popular  impression  that  springs 
are  cooler  in  summer  than  in  winter,  but  this  is  not  the  case.  The 
impression  arises  from  the  fact  that  the  water  is  much  cooler  than 
the  air  in  summer,  and  so  seems  cold,  while  in  winter  the  water 
is  much  warmer  than  the  air,  and  so  seems  less  cold  than  in  sum- 
mer. 


90  PHYSIOGRAPHY 

Springs  which  derive  their  water  from  deep  sources  vary  little 
in  temperature  during  the  year,  while  those  whose  sources  are 
shallow  are  colder  in  the  winter  than  in  summer.  The  reason  is 
that  the  warmth  of  summer  and  the  cold  of  winter  are  most  ex- 
treme at  the  surface,  and  become  less  and  less  with  increasing  depth. 
Below  the  depth  of  50  or  60  feet,  in  middle  latitudes,  the  tempera- 
ture does  not  vary  sensibly  with  the  seasons,  so  that  springs  which 
draw  their  water  from  greater  depths  vary  little  in  temperature, 
while  those  which  draw  their  supply  from  lesser  depths  vary  more. 

Exceptional  springs  are  warm,  and  still  more  exceptional  ones 
are  hot.  Where  spring  water  is  hot  it  is  commonly  because  it 
has  been  in  contact  with  hot  rock.  In  general  the  water  of  warm 
and  hot  springs  probably  rises  from  considerable  depths.  In 
many  cases  the  source  of  the  heat  is  probably  igneous  rock  (lava) 
which  has  not  yet  become  cold.  It  may  be  lava  which  was  forced 
upward  toward  but  not  to  the  surface,  or  it  may  be  the  deeper 
parts  of  lava  which  flowed  out  on  the  surface. 

Mineral  and  medicinal  springs.  All  ground-water  dissolves 
more  or  less  mineral  matter  from  the  rocks,  and  all  springs  there- 
fore contain  more  or  less  mineral  matter  in  solution ;  but  a  spring 
is  not  commonly  called  a  mineral  spring  unless  it  contains  (1) 
much  mineral  matter,  or  (2)  mineral  matter  which  is  conspicuous 
either  by  reason  of  its  color  or  its  odor,  or  (3)  mineral  matter 
which  is  unusual  in  spring  water. 

Many  mineral  springs  are  thought — and  sometimes  justly — 
to  have  healing  properties,  and  so  are  known  as  medicinal  springs. 
Many  of  the  famous  watering-places  and  resorts  for  invalids  are 
located  at  hot  mineral  springs.  The  Hot  Springs  of  Arkansas,  of 
South  Dakota,  and  of  Carlsbad  (Bohemia)  are  examples.  Many 
springs  which  are  charged  with  gases  are  called  mineral  and  me- 
dicinal, especially  if  they  have  an  offensive  odor.  In  the  popular 
mind,  a  spring  is  more  medicinal  the  worse  it  smells  and  tastes. 
Hot  water  is  a  better  solvent  than  cold  water,  so  that  hot  springs 
generally  contain  much  mineral  matter. 

Geysers.  In  some  parts  of  the  world  the  water  of  hot  springs 
is  forced  out  violently  from  time  to  time.  Such  springs  are  called 
geysers.  A  geyser  is  therefore  an  intermittently  eruptive  hot  spring. 
Geysers  are  best  known  in  the  Yellowstone  National  Park,  but 
they  are  well  developed  also  in  Iceland.  They  exist  in  New  Zea- 
land, though  some  of  the  geysers  of  that  island  were  destroyed 


THE  WORK  OF  GROUND-WATER 


91 


by  volcanic  eruptions  in  1886.  In  the  Yellowstone  Park  there 
are  about  100  geysers,  and  more  than  3000  hot  springs  which  are 
not  eruptive.  Some  of  the  geysers  send  up  boiling  water  and 
steam  to  a  height  of  200  feet  or  more  (Fig.  82),  but  this  is  quite 
above  the  average. 

From  some  geysers  the  eruptions  are  frequent,  and  from  others 
infrequent.  From  some  they  occur  at  regular  intervals,  and  from 
others  they  take  place  irregularly.  One  of  the  geysers  in  the 


FIG.  82. — Giant  Geyser,  Yellowstone  National  Park.     (Wineman.) 

Yellowstone  Park  is  named  "Old  Faithful,"  because  it  discharges 
its  waters  at  nearly  regular  intervals  of  about  an  hour.  The 
eruptions  are,  however,  a  little  less  frequent  and  a  little  less  regular 
than  formerly.  In  general,  geysers  which  have  been  known  for 
long  periods  of  time  discharge  their  waters  less  and  less  frequently 
as  time  goes  on. 

The  features  which  may  be  seen  generally  at  a  geyser  are  the 
following : 

(1)  An  opening  in  the  surface  leading  down  to  unknown  depths. 
Though  this  is  sometimes  called  the  geyser  tube,  it  is  probably  not 


92 


PHYSIOGRAPHY 


always  in  the  form  of  a  tube.  (2)  A  shallow  basin  about  the 
opening.  This  is  sometimes,  though  not  always,  in  the  top  of  a 
mound.  In  some  cases  there  is,  instead  of  a  basin,  an  irregular 


FIG.  83. — Cone  (crater)  of  Castle  Geyser,  Yellowstone  National  Park. 
(Detroit  Photo.  Co.) 


FIG.  83a. — The  cone  of  Lone    Star  Geyser,  Yellowstone  National    Park. 

(U.  S.  Geol.  Surv.) 

perforated  mound  (Figs.  83  and  83a).  Both  basins  and  mounds 
are  composed  of  mineral  matter  (commonly  silica)  which  has  been 
deposited  by  the  water  which  has  issued  from  the  geyser.  (3)  At 
the  time  of  discharge  much  steam,  as  well  as  liquid  water,  issues. 


THE  WORK  OF  GROUND-WATER  93 

It  seems  certain  that  steam  is  the  force  which  ejects  the  water. 
It  is  believed  (1)  that  ground-water  enters  the  geyser  tube  much 
as  it  enters  a  well;  (2)  that  the  walls  of  some  part  of  the  tube  are 
of  hot  rock;  (3)  that  the  water  in  the  tube  is  brought  to  the  boiling 
temperature  at  some  point  in  the  tube  below  the  top  of  the  water; 
and  (4)  that  when  this  takes  place,  the  steam  formed  forces  out 
all  the  water  above.  It  forces  it  out  because  water,  when  changed 
to  steam,  expands  about  1700  times. 

This  principle  may  be  illustrated  by  experiment.  If  a  short 
tube  of  water  is  heated,  it  boils  without  violent  discharge,  especially 
if  the  tube  has  a  large  diameter.  But  if  the  tube  is  filled  with 
sand  and  the  sand  then  filled  with  water  and  heated  from  below, 
the  movement  (convection)  of  the  heated  water  in  the  tube  is 
greatly  restricted  by  the  sand.  The  result  is  that  steam  may  be 
formed  abundantly  below  the  surface,  and  a  miniature  eruption 
may  follow. 

Geysers  occur  only  in  regions  of  relatively  recent  volcanic 
activity  f  and  the  heat  which  is  necessary  for  the  geysers  is  probably 
supplied  by  lava  which  has  not  yet  become  cool.  As  it  is  heated, 
the  geyser  water  is  constantly  cooling  the  hot  rock,  and  in  time 
it  will  cease  to  be  hot  enough  to  boil  the  water.  Geyser  action 
will  then  cease,  unless  new  supplies  of  hot  lava  are  forced  up  from 
below.  In  the  Yellowstone  Park  some  geysers  have  died  out 
since  the  region  became  known,  but  little  more  than  thirty  years 
ago.  New  geysers,  on  the  other  hand,  have  been  developed  in 
the  same  region  during  this  period. 

The  reason  why  the  water  in  a  geyser  tube  is  shot  out,  at 
intervals,  while  the  water  in  an  open  kettle  is  not,  is  found  in  the 
difference  in  the  shape  of  the  vessels  holding  the  water.  When 
water  is  heated  it  expands.  When  water  is  heated  in  a  kettle, 
that  at  the  bottom  rises  readily,  by  convection,  to  the  top,  so  that 
there  is  a  nearly  uniform  temperature  throughout.  The  geyser 
tube  is  much  deeper  than  the  kettle,  and  in  places  it  is  probable 
that  it  is  small  in  diameter,  certainly  small  in  proportion  to  its 
length.  The  tube  is  also  more  or  less  crooked.  Both  its  small- 
ness  and  its  crookedness  interfere  with  the  rise  of  the  water 
heated  below,  and  the  result  is  that  water  below  the  surface  is 
brought  to  the  boiling  temperature  before  that  at  the  surface  is. 
Hence  steam  is  formed  below  the  surface,  rather  than  at  the 
surface,  and  blows  out  the  water  above. 


94 


PHYSIOGRAPHY 


If  a  stone  or  a  clod  of  earth,  or  almost  any  other  solid  object, 
be  thrown  into  a  geyser,  its  eruption  may  often  be  hastened  a 
little,  because  such  things  interfere  with  the  convection  of  the 
water  in  the  tube.  They  help  to  hold  the  hot  water  down  where 
it  is  being  heated,  and  so  help  it  to  reach  a  boiling  temperature 
at  some  point  below  the  surface  a  little  sooner  than  it  would  do 
otherwise.  Soap,  especially,  is  supposed  to  hasten  a  geyser's 
eruption.  Its  effect  is  probably  somewhat  less  than  is  usually 
believed.  Anything  which  makes  the  water  more  viscous  hastens 
the  eruption,  because  convection  is  less  free  in  a  thick  fluid  than 
in  a  thin  one. 

Artesian  and  flowing  wells.  When  the  water  in  a  well  rises  so 
as  to  overflow,  the  well  is  said  to  flow.  Flowing  wells  are  not 


FIG.  84. — Artesian  well  at  Woonsocket,  S.  D.     (U.  S.  Geol.  Surv.) 

unlike  springs  whose  waters  spout  up  as  they  issue.  The  chief 
difference  between  them  is  that  the  opening  in  one  case  is  natural, 
while  in  the  other  it  was  made  by  man.  Formerly  artesian  wells 
were  regarded  as  the  same  as  flowing  wells.  The  name  was  derived 
from  Artois,  France,  where  there  was  a  notable  well  of  this  sort. 
Nowadays  the  name  artesian  is  often  applied  to  deep  wells,  whether 
they  flow  or  not. 


THE  WORK  OF  GROUND-WATER 


95 


Fig.  85  illustrates  the  general  conditions  necessary  for  flowing 
wells.  They  are  the  following:  (1)  A  porous  layer  or  bed  of  rock, 
a,  which  underlies  an  impervious  layer,  b,  which  prevents  the  water 
from  escaping  upward  until  it  is  penetrated  by  the  well-hole. 
(2)  The  porous  bed  must  come  to  the  surface  in  a  region  which  is 
somewhat  higher  than  the  site  of  the  well.  (3)  The  rainfall  where 


FIG.  85. — Diagrams  illustrating  the  conditions  favorable  for  artesian  wells. 
In  A  the  porous  bed  a  is  in  the  form  of  a  basin;  in  B  it  merely  dips. 

the  porous  bed  comes  to  the  surface  must  be  sufficient  to  keep 
it  well  filled  with  water.  Under  these  conditions  the  water  beneath 
w,  in  the  stratum  a,  is  under  the  pressure  of  the  water  in  the  same 
stratum  at  higher  levels,  and  if  a  hole  is  made  down  to  it,  it  will 
gush  up  (Fig.  84).  It  is  not  necessary,  as  a  rule,  to  take  much 
account  of  the  layer  below  the  water-bearing  stratum  a.  If  it  is  of 
porous  rock,  it  is  generally  full  of  water,  and  so  prevents  the 
downward  escape  of  that  in  a. 

The  water  at  the  well  will  not  rise  as  high  as  the  water  surface 
in  a,  for  in  flowing  through  the  small  openings  (pores  and  small 
cracks)  in  the  rock,  there  is  loss  of  force  by  friction.  An  allowance 
of  about  a  foot  to  the  mile  must  usually  be  made;  that  is,  if  the 
source  of  supply  is  100  miles  away,  the  water  surface  at  that  point 
should  be  about  100  feet  higher  than  the  top  of  the  well,  in  order 
that  the  water  may  flow  out.  If  the  water-bearing  stratum,  a, 
is  very  porous,  the  allowance  which  must  be  made  for  friction 
is  less;  if  it  is  close-grained,  the  loss  of  force  from  friction  is  more. 

Artesian  wells  vary  much  in  depth.  They  may  be  but  a  few 
feet  deep,  or  they  may  be  thousands  of  feet.  There  is  an  artesian 
well  in  Berlin  more  than  4000  feet  deep,  one  in  St.  Louis  nearly 
4000  feet  deep,  and  one  in  Cincinnati  nearly  2500  feet  deep,  while 
the  deepest  one  in  Chicago  is  some  2700  feet  deep.  There  are 
numerous  artesian  wells  in  New  Jersey  less  than  100  feet  in  depth. 


96  PHYSIOGRAPHY 

The  amount  of  water  flowing  from  artesian  wells  is  often  great. 
At  Belle  Plain,  la.,  the  water  rose  77  feet  above  the  surface  when 
a  certain  well  was  first  drilled.  A  little  later  the  water  rose  from 
another  adjacent  well  with  such  force  as  to  greatly  enlarge  the 
opening  through  which  it  rose,  and  with  force  enough  to  bring  up 
stones  two  to  three  pounds  in  weight.  At  first  the  flow  from  the 
second  well  was  estimated  to  be  more  than  5,000,000  gallons  per 
day,  though  it  soon  became  less.  The  flow  from  many  wells  is 
nearly  constant. 

Many  villages  and  small  cities  get  their  water  from  artesian 
wells.  Charleston ,  S.  C.,  Galveston  and  Fort  Worth,  Texas,  Camden, 
N.  J.,  and  Rockford,  111.,  are  among  the  cities  supplied  partly  or 
wholly  in  this  way.  No  great  city,  however,  such  as  New  York, 
Chicago,  Philadelphia,  etc.,  is  supplied  with  water  from  such 
wells,  and  probably  could  not  be. 

In  the  semi-arid  region  of  the  Great  Plains,  and  at  various  other 
places  in  the  West,  as,  for  example,  in  some  parts  of  California, 
water  from  artesian  (deep)  wells  is  extensively  used  for  irrigating 
the  land. 

THE  WORK  OF  GROUND-WATER 
Chemical  Work 

Solution.  While  rock  seems  to  be  the  symbol  of  all  that  L 
stable,  it  is  nevertheless  dissolved,  to  some  slight  extent,  by  the 
ground-water  which  passes  through  it,  as  has  been  stated  in  con- 
nection with  springs.  Pure  water  does  not  dissolve  mineral  matter 
readily;  but  the  water  beneath  the  surface  is  not  pure.  In  falling 
through  the  atmosphere  it  dissolved  carbonic  acid  gas,  oxygen,  and 
other  gases,  and  in  sinking  through  the  soil  it  took  up  the  products 
of  plant  decay,  so  that  when  it  became  ground-water  it  contained 
numerous  impurities.  With  these  impurities  in  solution,  ground- 
water  dissolves  most  sorts  of  rock  more  readily  than  pure  water 
would.  Pure  water,  for  example,  has  little  effect  on  common 
limestone,  but  water  with  carbonic  acid  gas  in  solution  dissolves 
this  rock  to  some  appreciable  extent.  The  descending  water  often 
changes  the  rocks  and  minerals  through  which  it  passes,  chemically, 
before  it  effects  much  solution;  but  this  is  not  always  the  case. 
Since  solution  is  often  the  result  of  these  chemical  changes,  it  is 
included  under  the  chemical,  rather  than  under  the  mechanical, 
changes  produced  by  ground-water. 


THE  WORK  OF  GROUND-WATER 


97 


That  water  does  dissolve  some  of  the  material  of  rock  is  shown 
by  the  character  of  the  water  which  comes  out  of  the  ground. 
When  the  water  of  wells  or  springs  is  evaporated  it  usually  leaves 
a  little  residue.  This  becomes  noticeable  in  time  in  the  inside 
coating  of  boilers  and  kettles  in  which  water  is  heated.  This 
coating  is  composed  of  mineral  matter  which  was  in  solution  in 
the  water,  and  which  was  left  behind  when  the  water  was  heated 
and  evaporated.  Strictly  speaking,  all  springs  are  mineral  springs 
(see  p.  90),  and  all  wells  are  mineral  wells,  for  all  water  taken 
out  of  the  ground  contains  mineral  matter. 

The  first  work  of  ground-water,  then,  is  solution,  or  the  sub- 
traction of  material  from  the  rocks.  One  result  of  solution  is  to 


FIG.  86. — Diagram  to  illustrate  the  form  and  relations  of  caverns  developed 
by  solution.  The  black  spaces  represent  caverns.  Some  limestone 
sinks  are  represented  at  the  surface  where  the  roofs  of  caves  have  fallen  in. 

make  the  rock  porous.  The  extreme  case  of  porosity  developed 
in  this  way  is  found  in  caverns  and  channels  beneath  the  surface. 
The  great  caves  (Fig.  86),  like  those  of  southern  Indiana  (Wyandotte 
Cave  and  others)  and  Kentucky  (Mammoth  Cave  and  others), 
are  the  work  of  ground-water.  Caves  of  this  sort  occur  chiefly 
in  limestone  regions,  for  limestone  is  the  most  soluble  of  the  com- 
mon rocks.  Even  where  caves  and  caverns  are  not  developed, 
small  pores  and  cavities  are  often  numerous.  The  effect  of  solu- 
tion is,  therefore,  to  weaken  the  rock,  and  finally  to  cause  it  to 
crumble. 

The  roofs  of  underground  caves  sometimes  fall  in,  leaving 
notable  sinks  at  the  surface.  These  are  known  as  limestone  sinks 
(Fig.  87).  Such  sinks  -are  characteristic  of  regions  in  which  there 
are  caves.  They  are  occasionally  so  numerous  that  the  surface 
is  too  much  pitted  to  be  cultivated.  This  is  the  case,  for  example, 


98 


PHYSIOGRAPHY 


in  some  parts  of  Kentucky  and  Tennessee  (Plate  VI).  If  a  part 
of  the  former  cavern  roof  remains  to  span  the  depression,  a 
natural  bridge  is  formed. 

In  Karst,  along  the  east  side  of  the  Adriatic  Sea,  there  is  a 
tract  of  land  underlain  by  white  limestone  which  is  nearly  free 
from  soil.  Its  surface  is  etched  and  eroded  into  fantastic  forms. 
Most  of  the  rainfall  of  the  region  goes  beneath  the  surface,  and 
it  is  the  solvent  action  of  the  water  before  and  after  it  sinks  which 


FIG.  87. — A  sink-hole  of  recent  development  near  Meade.  Kan. 
(Johnson,  U.  S.  Geoi.  Surv.) 

has  developed  the  remarkably  uneven  topography  of  the  region, 
so  bizarre  that  it  has  attracted  wide  attention.  Numerous  short 
gullies,  ravines,  and  valleys  in  the  limestone  terminate  abruptly, 
discharging  their  waters  into  caves  or  subterranean  tunnels. 
Sink-holes  abound,  and  some  of  them  are  several  hundred  feet  deep. 
The  slopes  to  the  depressions,  and  therefore  the  slopes  of  the 
elevations  between  them,  are  very  steep,  so  that  the  surface  is 
extremely  rough.  Topography  similar  to  that  of  this  region 
and  developed  in  the  same  way  is  sometimes  known  as  Karst 
topography. 

The  amount  of  mineral  matter  brought  to  the  surface  through 
wells,  springs,  etc.,  is  very  great.  The  springs  of  Leuk  (Swit- 
zerland) bring  to  the  surface  more  than  2000  tons  of  gypsum 


PLATt   ./I 


Limestone  sinks  due  to  solution  by  ground-water.      The  depression  contours  are 
hachured.     Scale  2—  miles  per  inch.      (Pikeville,  Tenn.,  Sheet,  U.  S.  Geol.  Surv.N, 


THE  WORK  OF  GROUND-WATER  99 

(a  hydrated  sulphate  of  calcium)  in  solution  yearly.  In  the  same 
time  the  springs  of  Bath  (England)  bring  up  enough  mineral 
matter  in  solution  so  that,  if  it  were  taken  out  of  the  water  and 
made  into  a  monument,  it  would  make  a  column  9  feet  in  diam- 
eter and  140  feet  high. 

Much  of  the  water  seeping  out  from  beneath  the  surface  finds 
its  way  to  rivers,  and  the  larger  part  of  the  mineral  matter  in 
solution  in  rivers  has  come  from  the  ground-water,  or  seepage, 
which  has  joined  them.  Rivers  are  estimated  to  carry  nearly 
five  billion  tons  of  mineral  matter  to  the  sea  in  solution  each  year; 
but  even  this  large  amount  does  not  represent  all  the  solvent  work 
of  ground-water,  for  much  of  the  mineral  matter  which  it  dissolves 
is  deposited  without  reaching  either  rivers  or  sea,  for  reasons 
which  will  soon  appear. 

The  transfer  of  this  large  amount  of  mineral  matter  from  the 
land  to  the  sea  each  year  in  solution  must  mean  the  lowering  of 
the  land.  It  has  been  estimated  that  the  land  surface  is  lowered 
in  this  way  about  one  foot  in  13,000  years,  on  the  average.  The 
transfer  of  this  mineral  matter  from  the  land  to  the  sea  does  not 
mean  an  equivalent  building  up  of  the  sea  bottom,  for  some  of;  the 
mineral  matter  remains  in  solution  in  the  sea-water.  Thus  salt  is 
one  of  the  mineral  substances  carried  by  rivers  to  the  sea;  but 
the  larger  part  of  the  salt  which  has  been  carried  to  the  sea  through 
the  ages  probably  remains  there  in  solution  to  this  day. 

On  the  other  hand,  much  of  the  mineral  matter  which  is  carried 
to  the  sea,  especially  the  calcium  carbonate,  is  used  by  the  animals 
and  plants  of  the  sea  in  the  making  of  shells,  tests,  bones,  etc., 
and  these  are  finally  left  on  the  sea  bottom. 

Deposition.  Besides  dissolving  mineral  matter  and  carrying 
much  of  it  away,  ground- water  brings  about  other  changes  in  the 
rocks  of  the  earth.  If,  in  the  chemical  laboratory,  solutions  of 
various  sorts  are  mixed  in  a  test-tube,  some  of  the  materials  in 
solution  are  likely  to  be  precipitated.  The  same  thing  takes  place 
in  the  rocks  beneath  the  surface.  If,  for  example,  waters  from 
different  directions  enter  a  crack  in  the  rock,  and  if  these  waters 
bring  different  mineral  matters  in  solution,  the  mingling  of  the 
waters  may  effect  a  chemical  change  by  which  some  of  the  mate- 
rial comes  out  of  solution,  and  is  deposited  in  the  crack. 

It  follows  that  while  ground-water  tends  to  make  rocks  porous 
when  it  dissolves,  mineral  matter,  it  tends  to  make  them  compact 


100  PHYSIOGRAPHY 

where  it  deposits  the  mineral  matter  which  it  holds  in  solution 
in  pores  and  cracks.  In  some  places  the  first  of  these  processes 
is  the  more  effective,  and  in  others  the  second.  In  general,  ground- 
water  probably  increases  the  porosity  of  rock  near  the  surface 
(especially  above  the  grcund-water  surface)  and  increases  its  com- 
pactness at  greater  depths.  The  effect  of  deposition  is  some- 
times to  cement  the  loose  parts  of  rock  together,  making  the  whole 


FIG.  88. — The   Maryland    Vein,    Nevada    City,    Cal.      The    vein    is    gold 
bearing  quartz.     (U.  S.  Geol.  Surv.) 

more  firm.     Thus  sand  may   be  cemented   into  sandstone,   and 
gravel  into  conglomerate. 

Cracks  in  the  rock  filled  by  mineral  matter  deposited  from 
solution  become  veins  (Fig.  88),  and  many  rocks  are  full  of  veins 
(Fig.  89) .  Ores  occur  in  some  veins,  and  many  mines  are  located 
in  them.  Much  of  the  gold,  silver,  lead,  zinc,  etc.,  is  found  in  such 
positions.  The  ores  of  these  metals  do  not  usually  fill  the  cracks, 
but  they  are  often  associated  with  a  much  larger  amount  of  min- 
eral matter  which  is  not  valuable,  but  which  must  be  worked  over 
in  order  to  get  out  the  ores  which  are.  Mineral  matter  in  solu- 
tion may  be  deposited  in  caves  (Fig.  90).  Many  of  the  most 
attractive  features  of  caves,  such  as  stalactites,  stalagmites,  crys- 
tals on  the  walls,  etc.,  were  formed  by  deposition  from  solution. 


THE  WORK  OF  GROUND-WATER 


101 


FIG.  89. — A  piece  of  rock  showing  many  veins. — the  white  streaks.  The 
vein  filling  is  calcite.  Near  Highgate  Springs,  Vt.  (Walcott,  U.  S. 
Geol.  Surv.) 


FIG.  90. — Deposits   of   calcite    (travertine,  stalactites,  and   stalagmites) 
in  Wyandotte  Cave,  Ind.     (H'ains.) 


102 


PHYSIOGRAPHY 


FIG    91. — Deposit  from  a  hot  spring  in  Yellowstone  Lake.     (Fairbanks.) 


*  - 

. 


FIG.  92. — Hot  Springs  deposit;    terrace  about  Mammoth  Springs.    Yellow- 
stone National  Park. 


THE  WORK  OF  GROUND-WATER 


103 


The  deposition  of  mineral  matter  from  solution  is  determined 
by  several  conditions.     The  more  important  are  the  following: 

(1)  If  water  evaporates^  the  mineral  matter  dissolved  in  it  is  left 
behind.     Surface  gravels  are  sometimes  cemented  in   this  way. 

(2)  If  the  water  which  contains  mineral  matter  is  relieved  of  pres- 
sure, as  when  it  comes  out  to  the  surface,  some  of  the  mineral 
matter  may  be  deposited.     (3)  If  water  contains  much  gas  in  solu- 
tion, and  if  the  gas  escapes,  as  it  is  likely  to  when  pressure  is  re- 


FIG.  93. — Deposits  about  a  hot  spring;  summit  of  Angel  Terrace.    Yellow- 
stone National  Park.     (U.  S.  Geol.  Surv.) 

lieved  or  when  it  is  warmed,  some  of  the  mineral  matter  in  solu- 
tion is  likely  to  be  deposited.  (4)  Some  warm  spring  water  gives 
up  its  mineral  matter  on  cooling.  Some  or  all  of  these  principles 
are  involved  in  the  deposition  of  mineral  matter  about  most  hot 
mineral  springs  (Fig.  91).  (5)  The  mingling  of  solutions  of  dif- 
ferent sorts,  already  referred  to,  is  probably  a  common  cause  of 
deposition.  (6)  In  some  hot  springs,  as  in  the  Yellowstone  Park, 
minute  plants  grow  in  the  hot  waters  which  issue  from  the  springs. 
These  tiny  organisms,  by  some  process  not  well  understood,  ex- 
tract mineral  matter  from  the  water  and  cause  it  to  be  deposited 
(Figs.  92  and  93). 


104 


PHYSIOGRAPHY 


These  are  among  the  simpler  and  more  important  conditions 
under  which  mineral  matter  is  deposited  from  solution  by  ground- 
water,  either  while  it  is  beneath  the  surface  or  after  it  issues. 

Solution  and  deposition  may  be  going  on  at  the  same  time, 
and  perhaps  at  the  same  place;  that  is,  the  water  may  be  dissolv- 
ing certain  substances  at  the  same  time  that  it  is  depositing  others. 
One  sort  of  rock  may  thus  be  changed  to  another.  A  special  phase 


FIG.  94. — Petrified  tree-trunks,  Yellowstone  National  Park. 
(U   S.  Geol.  Surv.) 


of  this  process  results  in  petrifaction.  Thus  the  substance  of  a 
buried  shell,  or  of  coral,  may  be  changed  while  its  form  is  pre- 
served. Another  illustration  is  afforded  by  petrified  wood  (Fig. 
94),  the  substance  of  the  wood  having  been  replaced  by  mineral 
matter.  Such  changes  probably  take  place  slowly,  the  mineral 
matter  which  was  in  solution  replacing  the  woody  matter  as  it 
decays,  molecule  by  molecule. 


THE  WORK  OF  GROUND-WATER  105 

Other  changes.  Besides  the  subtraction  cf  mineral  matter 
from  the  rocks  by  solution,  and  the  addition  cf  material  to  the 
rocks  of  some  places  by  deposition  of  mineral  matter  dissolved 
elsewhere,  the  water  works  still  other  changes  in  the  rocks.  It 
sometimes  enters  into  combination  with  certain  minerals,  chang- 
ing their  character.  This  process  (hydratiori)  has  already  been 
referred  to  in  connection  with  the  work  of  the  air  (see  p.  72). 
The  moisture  beneath  the  surface  affects  minerals  in  much  the  same 
way  as  moisture  in  the  air  or  at  the  surface.  All  changes  of  this 
sort  which  result  in  the  alteration  of  the  composition  of  the  rock, 
or  of  its  constituent  minerals,  are  chemical  changes. 

The  general  result  of  chemical  changes  effected  by  ground- 
water,  like  the  result  of  the  chemical  changes  effected  by  the  ah*, 
is  to  disrupt  the  rock.  Changes  of  this  sort  are  probably  most 
important  near  the  surface,  especially  at  and  above  the  ground- 
water  surface. 

Summary.  From  the  preceding  paragraphs  it  will  be  seen 
that  ground-water  effects  various  changes  in  the  rocks.  The  rocks 
carrying  water  may,  indeed,  be  looked  upon  as  a  sort  of  huge 
chemical  laboratory  in  which  solutions  are  made  and  carried 
from  one  place  to  another,  working  changes  as  they  go.  The  result 
is  a  slow  but  constant  alteration  in  the  character  of  the  rock.  Im- 
pressed by  the  greatness  of  these  changes  in  the  long  course  of  time, 
an  eminent  geologist  has  said  that,  "given  time  enough,  and  nothing 
in  the  world  is  more  changeable  than  the  rocks." 

Mechanical  Work 

Abrasion.  The  mechanical  work  of  ground-water  is  of  rela- 
tively little  importance.  The  water  is  rarely  concentrated  into 
considerable  streams,  but  where  it  is  so  concentrated  the  under- 
ground streams  tend  to  enlarge  their  channels  by  erosion,  somewhat 
as  surface  streams  do.  The  ground-water  which  flows  in  distinct 
channels  transports  and  deposits  the  limited  amount  of  sediment 
which  it  acquires. 

Slumping,  sliding,  etc.  Indirectly,  ground-water  participates 
in  changes  of  another  sort.  When  the  soil  and  earthy  material 
on  a  steep  slope  become  charged  with  water,  their  weight  is  greatly 
increased.  At  the  same  time  the  water  makes  them  more  mobile. 
Under  these  circumstances  the  material  sometimes  slides  down 
slopes.  Such  movements  are  known  as  slumping  or  sliding.  If 


106 


PHYSIOGRAPHY 


the  movement  is  on  a  large  scale,  it  is  called  a  landslide.  Slump- 
ing is  very  common  on  slopes  composed  of  unconsolidated  material, 
such  as  clay  or  accumulations  of  loose  rock  (Figs.  95  and  96). 
Landslides  give  rise  to  a  distinctive  sort  of  topography. 

Many  destructive  landslides  have  been  recorded,  but  a  few 
facts  concerning  a  recent  one  may  serve  to  illustrate  the  phenomena 
of  all.  On  the  29th  of  April,  1903,  there  was  a  slide  on  Turtle 
Mountain,  Province  of  Alberta,  Dominion  of  Canada.  Here  a 
huge  mass  of  material,  nearly  half  a  mile  square  and  probably 


FIG.  95. — South  face  of  Landslip  Mountain,  Colo.     The  protruding  mass  on 
the  right  has  slumped  down.     (U.  S.  Geol.  Surv.) 

400  to  500  feet  deep,  suddenly  broke  loose  from  the  steep  east 
face  of  the  mountain,  and  slid  down  into  the  valley  below.  It 
covered  the  valley,  which  was  half  a  mile  wide,  and  even  rose  a 
few  hundred  feet  on  the  other  side.  When  it  came  to  rest  it  covered 
an  area  of  a  little  more  than  one  square  mile.  The  length  of  the 
slide*  was  about  two  and  a  half  miles,  and  it  is  estimated  that  the 
time  which  it  took  was  not  more  than  100  seconds.  The  heavy 
rainfall  of  the  preceding  year  had  filled  the  rock  with  moisture, 
and  earthquake  tremors,  shortly  before  the  slide,  are  believed  to 
have  also  hastened  the  catastrophe.  Extensive  tunnels,  etc.,  ex- 


THE  WORK  OF  GROUND-WATER 


107 


FIG.  96.— A  slope  affected  by  slumping,  creeping,  etc..  Cascade  Mountains, 
Ore.     (U.  S.  Geol.  Surv.) 


FIG.  97. — The  upper  part  of  a  mountain  valley.  The  loose  material  in  the 
valley  has  slidden,  crept,  and  rolled  down,  making  what  has  been 
called  a  talus  glacier.  Near  Telluride,  Colo.  (Hole.) 


108 


PHYSIOGRAPHY 


cavated  in  mining  at  the  base  of  the  mountain  may  also  have 
played  a  part  by  making  the  under-structure  less  stable.  Many 
lives  were  lost  and  many  buildings  destroyed. 

Instead  of  sliding  down  with  rapid  motion,  the  surface  earth 
sometimes  moves  down  with  extreme  slowness.  This  sort  of 
movement  is  creep.  It  is  often  too  slow  to  be  seen,  but  it  results 


FIG.  98. — Material  settling  off  the  face  of  a  cliff  under  the  influence  of 
gravity.  The  freezing  of  water  in  the  cracks  may  be  a  factor  in  the 
separation  of  the  cliff  face.  (U.  S.  Geol.  Surv.) 

in  the  accumulation  of  mantle  rock,  especially  earthy  matter,  at 
the  bases  of  slopes.  In  one  case  (Rhymney  Valley,  Wales)  the 
rate  of  creep,  where  it  affected  a  railway,  has  been  determined  to 
be  6  to  10  feet  in  fifty  years.  Movement  of  the  same  sort  is  now 
constantly  disturbing  a  railway-track  a  few  miles  from  Golden, 
Colorado. 

The  downward  movement  of  loose  surface  material  on  slopes  is 
very  general.     It  often  causes  the  trees  to  incline  a  little  down 


THE  WORK  OF  GROUND-WATER 


109 


slope  (Fig.  100).  This  is  probably  due  in  part  to  the  fact  that  the 
upper  part  of  the  mantle  rock  in  which  they  are  rooted  creeps 
down  faster  than  the  lower  part. 


FIG.  99. — This  figure  shows  the  same  phenomenon  as  the  last,  but  the 
cliff  in  this  case  is  of  solid  rock  (limestone).  The  open  cracks  are  largely 
the  result  of  solution  and  weathering.  East  Tensleep  Creek,  Bighorn 
Mountains,  Wyo.  (Hole.) 


FIG.  100. — Trees  tipping  down  slope.     North  of  Chicago.     (Coxe.) 

.In  all  cases  of  slumping,  sliding,  and  creeping  the  force  pro- 
ducing the  movement  is  gravity.     Water  only  helps  to  furnish  the 


110  PHYSIOGRAPHY 

conditions  for  the  effective  action  of  gravity,  by  making  the  move- 
ment easier. 

Closely  connected  with  slumping  is  another  phase  of  gravity 
work  which  may  be  mentioned  here,  though  ground-water  has  little 
to  do  with  it.  From  the  faces  of  cliffs,  blocks  and  masses  of  rock 
often  settle  away  (Figs.  98  and  99).  The  freezing  of  ground- 
water  in  the  cracks  may  help  to  pry  off  the  rock  on  the  face  of 
the  cliff,  and  solution  may  help  to  widen  the  cracks.  The  growth 
of  roots  in  the  cracks  acts  much  like  freezing  water,  helping  to  pry 
off  the  loosened  masses  from  the  face  of  the  cliff.  In  such  cases 
the  existence  of  cracks  or  joints  helps  ice,  roots,  solution,  etc., 
to  become  effective. 

WEATHERING 

Some  of  the  processes  of  weathering  have  already  been  men- 
tioned, but  it  may  be  added,  by  way  of  summary,  that  the  chemical 
changes  (oxidation,  carbonation,  etc.)  effected  in  the  rock  by  the 
atmosphere,  the  mechanical  changes  effected  by  variations  of 
temperature  under  the  influence  of  the  atmosphere,  and  the 
chemical  and  mechanical  changes  effected  by  ground-water,  all 
conspire  to  so  alter  the  surface  of  exposed  rock  as  to  cause  it  to 
waste  away.  We  have  already  seen  (p.  74)  that  the  surfaces  of 
the  bowlders  of  the  fields  are  sometimes  scaling  off  or  crumbling,  and 
they  are  often  discolored,  even  when  they  seem  firm.  The  upper 
layers  of  stone  in  a  quarry  are  frequently  broken  or  quite  different 
in  color  from  the  lower  ones.  Inscriptions  on  old  tombstones  are 
often  indistinct,  and  they  have  sometimes  disappeared  completely 
from  stones  which  are  but  a  few  score  years  old.  From  the  walls 
of  stone  buildings,  from  monuments,  and  from  other  stone  struc- 
tures, flakes  of  stone  are  sometimes  seen  to  be  scaling  off. 

In  all  these  cases  some  change  has  taken  place  in  the  rock 
whereby  its  outer  part  is  wasted  away.  All  processes  which  pro- 
duce this  result  are  weathering. 

The  importance  of  rock  weathering  is  great.  Much  soil  is  but 
weathered  rock,  and  without  the  weathering  of  rock  much  of  the 
land  would  be  bare  of  soil  and  so  of  vegetation.  As  we  have  seen, 
the  weathering  of  the  rock  greatly  facilitates  the  work  of  the  wind, 
not  always  to  the  advantage  of  man,  by  preparing  fine  material 
which  may  be  blown  away.  As  we  shall  see  in  the  next  chapter, 
weathering  also  prepares  material  for  ready  removal  by  running 


THE  WORK  OF  GROUND-WATER  111 

water.  Weathering,  conjointly  with  wind  and  water  erosion,  is 
responsible  for  many  striking  bits  of  scenery  (Figs.  153  and  154). 

Conditions  affecting  weathering.  There  are  great  differences 
in  the  durability  of  rocks.  A  coarse-grained  rock  weathers  faster 
than  a  fine-grained  one  of  the  same  composition.  Rock  traversed 
by  fissures  and  cracks  changes  more  rapidly  than  firm,  impervious 
rock.  Some  rocks,  as  limestone,  are  composed  of  relatively  soluble 
material,  and  some,  as  sandstone,  of  material  which  is  compara- 
tively insoluble.  The  former  weather  more  readily  than  the  latter, 
so  far  as  solution  is  a  factor  cf  the  weathering.  A  cold  climate 
favors  the  wedge-work  of  ice,  but  hinders  the  growth  of  vegetation 
and  chemical  changes.  Rock  decay  goes  on  more  rapidly  in  warm, 
moist  regions  than  in  cold  or  dry  ones;  but  rock  breaking  or  splinter- 
ing, due  to  changes  of  temperature,  is  more  effective  in  dry  regions, 
where  daily  changes  of  temperature  are  great.  In  deserts  the 
wear  of  the  rock  by  wind-blown  sand  is  important.  In  general, 
the  weathering  of  bare  rocks  probably  takes  place  more  rapidly  in 
warm,  moist  regions  than  in  cold  or  temperate  ones;  yet  in  warm, 
moist  regions  the  rock  is  usually  covered  and  protected  against 
some  phases  of  weathering  by  a  goodly  layer  of  soil  and  subsoil. 

The  topographic  position  of  the  rocks  also  has  an  important 
bearing  on  the  rate  at  which  they  decay.  On  steep  slopes,  the 
waste  is  commonly  washed  away  as  rapidly  as  formed,  and  the 
bare  rock  is  constantly  exposed,  whereas  on  plains  the  solid  rock 
is  often  deeply  buried  by  mantle  rock,  and  so  protected  from 
some  phases  of  weathering. 

The  thickness  of  the  surface  layer  of  weathered  rock  varies 
greatly.  It  rarely  exceeds  100  feet  and  is  generally  much  less. 
In  many  places  the  depth  of  soil  and  mantle  rock  represents  the 
excess  of  rock  decay  over  the  transportation  of  the  decayed  ma- 
terial. 

Since  over  the  greater  part  of  the  land  area  there  is  a  covering 
of  mantle  rook,  it  follows  that,  in  the  aggregate,  rock  weathering 
exceeds  transportation.  Since  much  material  is  carried  away  in 
solution  or  otherwise,  as  rock  weathers,  a  few  feet  of  rock  waste 
may  represent  the  destruction  of  many  feet  of  rock. 


112  PHYSIOGRAPHY 


MAPS  ILLUSTRATING  TOPOGRAPHIC  EFFECTS  OF 
GROUND-WATER 

Study  the  following  maps  showing  limestone  sinks,  "sinking  creeks  "etc. 
in  preparation  for  conference.1 

1.  Greenville,  Tenn.— N.  C.  3.  Bristol,  Va—  Tenn. 

2.  Standingstone,  Tenn.  4.  Pikeville,  Tenn. 
Greenville  Sheet. 

Note  the  numerous  limestone  sinks  in  the  central  portion  of 
the  area,  and  the  "sinking  creeks"  associated  with  them. 
Sketch  the  probable  history  of  a  "sinking  creek." 

Standingstone  Sheet. 

Note  the  many  limestone  sinks,  particularly  in  the  south- 
western half  of  the  map.  The  rocks  are  here  essentially 
horizontal  and  the  sinks  are  therefore  without  definite 
arrangement.  This  is  in  striking  contrast  to  the  condition 
shown  on  the  Bristol  sheet. 

Bristol  Sheet. 

The  many  depressions  are  sink-holes  in  limestone  rocks.  Note 
the  very  large  depressions  near  Adelphia,  in  the  north- 
western part  of  the  map.  Notice  that  the  sink-holes 
occur  -in  belts,  extending  northeast  and  southwest.  This 
is  because  the  tilted  limestone  beds  of  the  region  outcrop 
(come  to  the  surface)  along  these  lines  (see  Bristol  Folio). 

Pikeville  Sheet. 

Note  the  limestone  sinks,  especially  in  the  northwestern  part 
of  the  map. 

Question.     What  was  probably  the  topography  of  each  region  at 
the  time  the  sinks  developed? 

REFERENCES 

1.  CHAMBERLIN  AND  SALISBURY,  Geologic  Processes,  Chapter  IV.   Henry 
Holt  &  Co.,  1903;  and  all  standard  text-books  en  Geology. 

2.  SHALER,  Chapter  on  Caverns,  in  Aspects  of  the  Earth.  Chas.  Scribner's 
Sons,  1889. 

3.  GEIKIE,  Earth  Sculpture,  Chapter  XIII :  Putnam. 

4.  HOVEY,  Celebrated  American  Caverns:    Rob't  Clarke  Co. 

5.  CHAMBERLIN,  Artesian  Wells:    Geology  of  Wisconsin.  Vol.  I,  1881,  pp. 
689-697;   and  5th  Ann.  Rept.  U.  S.  Geol.  Surv.,  pp.  131-173. 

6.  KING,  Principles  and  Conditions  of  the  Movements  of  Ground-water: 
19th  Ann.  Rept.  U.  S.  Geol.  Surv.,  Pt.  II,  pp.  59-293. 

7.  SLIGHTER,   The  Motions  of   Underground  Water:  Water  Supply  and 
Irrigation  Paper  No.  67,  U.  S.  Geol.  Surv. 

1  See  foot-note,  p.  54. 


THE  WORK  OF  GROUND-WATER  113 

8.  Numerous  Water  Supply  and  Irrigation  Papers,  U.  S.  Geol.  Surv. 

9.  WEED;  Formations  of  Hot  Springs  Deposits:   9th  Ann.  Kept.  U.  S. 
Geol.  Surv.,  pp.  663-676,  and  Amer.  Jour,  of  Sci.,  Vol.  XXXVII,  1889, 
pp.  51-59. 

10.  CHITTENDEN,  Yellowstone  National  Park:   Rob't  Clarke  Co. 

11.  DAVIS  (B.  M.),  The  Vegetation  of  the  Hot  Springs  of  Yellowstone  Park: 
Science,  Vol.  VI,  1897,  pp.  145-167 


CHAPTER  IV 
THE  WORK  OF   RUNNING  WATER 

STREAMS  are  among  the  most  wide-spread  natural  features  of 
the  land.  Only  in  desert  regions,  such  as  the  Sahara,  or  in  areas 
which,  like  Greenland,  are  mainly  covered  with  snow  and  ice,  are 
there  extensive  tracts  without  them.  A  few  streams,  like  the 
Mississippi  and  the  Amazon  rivers,  are  very  large,  but  most  of 
them  are  of  small  size.  Thousands  of  small  creeks  and  brooks, 
some  of  them  having  their  source  in  the  Rocky  Mountains,  some 
in  the  Appalachian  Mountains,  and  some  on  the  plains  and  plateaus 
between  these  mountain  systems,  feed  the  Mississippi.  And  so  it 
is  everywhere.  Every  large  stream  receives  water  from  many 
small  ones. 

The  rivers  of  the  Mississippi  basin  guided  the  early  explorers, 
traders,  and  immigrants,  and  later  became  of  great  commercial  and 
therefore  of  political  importance.  Many  of  the  greater  streams  of 
other  lands  have  played  similar  roles  in  history. 

The  flow  of  some  streams  is  so  gentle  that  they  do  not  appear 
to  work  great  changes  in  their  valleys ;  but  some  of  them  wear  away 
their  banks  so  rapidly  that  the  changes  they  produce  may  be  seen 
from  year  to  year,  or,  when  the  stream  is  in  flood,  from  day  to  day, 
or  even  from  hour  to  hour.  The  force  of  streams  at  such  times 
is  often  disastrous  (Figs.  101  and  102).  Occasionally  they  sweep 
away  bridges  and  dams,  and  sometimes  even  buildings.  The 
strong  beams  and  rods  of  the  bridges,  and  the  steel  rails  of  railways 
are  bent  almost  as  if  they  were  twigs  by  the  force  of  the  occasional 
torrent  which  follows  an  exceptional  rain,  such  as  a  cloud-burst 
(Fig.  103). 

In  the  aggregate,  the  streams  are  estimated  to  send  about 
6500  cubic  miles  of  water  to  the  sea  each  year.  The  average 
height  of  the  land  above  the  sea  is  nearly  half  a  mile.  These  6500 

114 


THE  WORK  OF  RUNNING  WATER 


115 


cubic  miles  of  water,  therefore,  descend,  on  the  average,  nearly  half 
a  mile  before  they  reach  the  sea.     The  energy  of  the  water  in  falling 


FIG.  101.— The  Passaic  River  in  flood.     Little  Falls,  N.  J.    1902. 


FIG.  102. — A  raging  river.     Flood  of  the  Mississippi  River,  breaking  through 

its  levees. 

this  distance  is  very  great.     This  will  be  readily  understood  if  we 
think  of  this  amount  of  water  falling  vertically  from  a  height  of  a 


116  PHYSIOGRAPHY 

little  less  than  half  a  mile.  The  water  in  the  streams  has  the  same 
amount  of  energy  that  it  would  have  if  it  fell  vertically.  This 
energy  is  largely  expended  in  wearing  away  the  materials  of  the 
sides  and  bottoms  of  the  valleys.  Its  force  is  therefore  great,  and 
its  effects  on  the  surface  of  the  land  pronounced. 

Rivers  flow  in  mountains,  in  plateaus,  and  in  plains,  and  wher- 
ever they  flow  they  modify  the  surface  in  their  own  peculiar  way. 


FIG.  103. — Scene  in  the  freight-yards  of  Kansas  City  after  the  flood  of  1903. 
(U.  S.  Weather  Bureau.) 

The  topographic  features  produced  by  running  water  in  mountains, 
plateaus,  and  plains  have  much  in  common,  and  when  we  study 
these  features  for  one  of  these  types  of  regions,  we  really  study 
them,  in  principle,  for  all. 

Sources  of  stream  water.  Most  streams  derive  a  large  part  of 
their  water  from  the  immediate  run-off  and  from  ground-water, 
and  many  of  them  receive  contributions  from  ponds,  lakes,  snow- 
fields,  and  glaciers.  The  Mississippi,  for  example,  receives  water  in 
all  these  ways.  The  immediate  run-off,  the  ground-water,  and  even 
the  water  of  the  lakes  and  glaciers,  all  have  their  sources  in  the 
rain  and  snow,  so  that  rivers  depend  on  atmospheric  precipitation 
for  their  supply  of  water. 


THE  WORK  OF  RUNNING  WATER  117 

The  direct  connection  between  rainfall  and  rivers  may  be  in- 
ferred from  various  familiar  phenomena.  (1)  Streams  are  more 
numerous  in  regions  where  the  rainfall  is  abundant  (Fig.  104) 
than  in  those  where  it  is  scarce  (Fig.  105).  (2)  Multitudes  of  small 
streams  spring  into  being  with  each  heavy  fall  of  rain  and  with  each 
period  of  rapidly  melting  snow.  (3)  Streams  are  notably  swollen 


FIG.   104. — Map  showing  the  many  streams  of  a  humid  region.     Central 
Kentucky.     The  area  is  about  225  square  miles. 

after  rains,  and  most  after  heavy  rains.  (4)  Many  small  streams 
which  flow  during  wet  weather  dry  up  in  times  of  drought,  while 
others  shrink. 

The  water  of  most  streams  which  continue  to  flow  during 
droughts  is  derived  largely  from  springs  and  lakes,  or  from  the 
melting  of  snow  and  ice  about  the  sources  of  the  streams. 

Where  water  flows  on  the  land,  it  is  because  the  surface  has  a 
slope.  If  the  slope  of  a  surface  were  perfectly  even,  the  immediate 


118 


PHYSIOGRAPHY 


run-off  at  any  given  time  would  flow  in  a  sheet.  There  are  slopes 
so  smooth  that  their  water  runs  off  in  this  way;  but  on  most  slopes, 
even  those  which  appear  to  be  regular,  there  are  some  uneven- 
nesses  so  that,  although  the  run-off  may  start  as  a  sheet,  it  is 
soon  concentrated  into  rills  and  streamlets  which  follow  the  de- 
pressions. The  smallest  streamlets  unite  to  form  larger  ones, 


FlG.  105. — Map  showing  the  few  streams  of  an  arid  region.     Northern  Arizona. 
The  area  is  as  great  as  that  shown  in  Fig.  104. 

and  the  little  rills,  after  many  unions  with  one  another,  reach 
valleys  which  have  permanent  streams.  These  may  be  small  (creeks 
or  brooks)  or  large  (rivers).  Streams  which  flow  but  part  of  the 
time,  as  after  a  rain-storm,  during  wet  weather,  or  during  but  a 
part  of  the  year,  are  intermittent  streams. 

Every  permanent  stream  and  many  temporary  ones  flow  in 
depressions  called  valleys  (Fig.  106).  Valleys  are  therefore  about 
as  numerous  as  streams.  The  very  small  depressions  in  which 
water  runs  only  after  smart  showers  are  not  always  called  valleys. 


THE  WORK  OF  RUNNING  WATER 


119 


If  they  are  very  small  they  are  called  gullies  (Fig.  107) ;  or  if  some- 
what larger,  ravines.  Gullies  and  ravines  are  but  small  valleys. 
Just  as  the  tiny  streamlets  unite  with  one  another  to  form  creeks 


FIG.  106. — Map  showing  normal  drainage  relations.  Each  stream  flows  in  a 
depression.  The  largest  stream  has  the  largest  valley.  Streams  of 
smaller  size  have  smaller  valleys,  while  the  valleys  of  the  smallest  streams 
are  very  small.  A  few  miles  southwest  of  Scio,  O.  (U.  S.  Geol.  Surv.) 


and  these  to  form  rivers,  so  the  gullies  in  which  the  smallest  tem- 
porary streams  flow  generally  unite  to  form  wider  and  deeper 
gullies  (Fig.  108).  These,  in  turn,  join  one  another  to  make  ravines, 


120  PHYSIOGRAPHY 

which  are  but  larger  depressions  of  the  same  sort.  Ravines  lead 
to  valleys,  just  as  gullies  lead  to  ravines.  Valleys,  like  streams, 
usually  end  at  the  ocean  or  a  lake;  but  in  some  cases,  especially 
in  arid  regions,  they  end  on  dry  land. 

There  is,  as  a  rule,  some  relation  between  the  size  of  a  valley 
and  the  stream  which  follows  it,  though  this  relation  is  not  one 


FIG.  107. — A  gully  developed  by  a  single  shower.     (Blackwelder.) 

which  can  be  stated  in  mathematical  terms.  The  large  stream 
and  the  large  valley  go  together  so  often,  however,  that  the  com- 
bination cannot  be  accidental,  and  leads  to  the  inquiry  whether 
the  streams  make  the  valleys  in  which  they  flow,  or  whether  the 
streams  flow  where  they  do  because  the  valleys  were  prepared 
for  them  in  advance.  These  are  questions  to  which  we  shall  seek 
an  answer  in  the  following  pages. 


THE  EROSIVE  WORK  OF  STREAMS 

Streams  are  always  carrying  mud,  sand,  etc.,  down  their  valleys. 
This  is  especially  well  seen  when  rivers  are  in  flood,  for  at  such 
times  they  are  usually  muddy.  Besides  the  mud  which  is  sus- 
pended in  the  water,  streams  roll  sand,  gravel,  etc.,  along  their 
bottoms.  The  movement  both  of  the  mud  in  the  water  and  of 


THE  WORK  OF  RUNNING  WATER 


121 


pebbles  and  stones  at  the  bottom  of  the  water  may  be  seen  in 
any  little  stream  that  flows  along  the  roadside  after  a  storm, 
and  the  great  Mississippi  carries  its  load  in  the  same  way.  Even 
when  streams  are  not  in  flood,  they  carry  sediment,  though  its 
amount  is  then  less.  Some  of  them  carry  so  little  that  their 
waters  are  relatively  clear,  while  others,  like  the  Missouri,  are 
always  muddy. 

Since  most  river-water  finally  reaches  the  sea,   much  of  the 


FIG.  108. — Slope  with  numerous  gullies,  the  smaller  ones  joining  the  larger 
ones.     Scott's  Bluff,  Neb.     (U.  S.  Geol.  Surv.) 

sediment  which  they  carry  finally  reaches  the  ocean  and  is  deposited 
there,  chiefly  near  the  shores. 

The  amount  of  material  which  certain  streams  carry  from  the 
land  to  the  sea  has  been  estimated.  The  estimate  for  a  given 
river  is  made  by  calculating  the  volume  of  water  discharged  by 
the  river  each  year  and  then  determining  the  average  amount 
of  sediment  in  each  unit — for  example,  each  gallon,  or  each  cubic 
foot,  of  water.  In  this  way  it  has  been  estimated  that  the  Missis- 
sippi River  carries  to  the  Gulf  more  than  340,500,000  tons  of  sedi- 
ment each  year,  or  nearly  a  million  tons  a  day.  It  would  take 
more  than  750  daily  trains  of  50  cars  each,  each  car  carrying 


122  PHYSIOGRAPHY 

25  tons,  to  carry  an  equal  amount  of  sand  and  mud  to  the  Gulf. 
All  the  rivers  of  the  earth  are  perhaps  carrying  to  the  sea  forty 
times  as  much  as  the  Mississippi. 

We  have  seen  that  ground-water  dissolves  rock  matter,  and 
that  springs  bring  some  of  this  dissolved  matter  to  the  streams. 
Streams,  therefore,  carry  certain  substances  such  as  salt,  carbonate 
of  lime,  etc.,  in  solution.  These  dissolved  substances  are  generally 
invisible,  and,  unlike  mud  and  other  sediment,  remain  in  the 
water  even  after  it  has  become  quiet.  The  presence  of  these 
dissolved  substances  is  sometimes  made  known  by  the  taste; 
but  this  is  rarely  the  case  in  river  waters.  On  evaporating  the 
water,  however,  as  by  boiling,  the  dissolved  substances  are  left 
behind,  as  on  the  inside  of  tea-kettles,  boilers,  etc. 

The  amount  of  matter  carried  to  the  sea  in  solution  each 
year  by  all  the  rivers  of  the  earth  has  been  estimated  at  nearly 
5,000,000,000  tons.  This  is  about  one-third  as  much  as  the  sedi- 
ment carried  by  the  rivers. 

These  general  facts  show  that  the  rivers  are  constantly  shifting 
solid  matter  from  the  land  to  the  sea.  This  is,  indeed,  their  great 
work.  Even  the  water  which  falls  on  the  land,  but  does  not  flow 
directly  to  the  sea,  helps  to  make  the  rock  decay  (p.  105),  and 
so  prepares  it  for  removal  by  running  water.  It  may  therefore 
be  said  that  every  drop  of  water  which  falls  on  the  land  has  for 
its  mission  the  getting  of  the  land  into  the  sea. 

Load  and  loading.  The  sediment  moved  by  a  stream,  whether 
in  suspension  or  at  the  bottom,  is  its  load.  A  stream  is  loaded 
when  it  has  all  the  sediment  it  can  carry;  it  is  but  partially  loaded 
when  it  is  carrying  less  than  it  might. 

How  does  a  stream  get  its  load? 

As  the  rain-water  begins  to  flow  down  the  slopes  of  the  land, 
it  picks  up  and  carries  with  it  particles  of  soil,  subsoil,  etc.;  that 
is,  particles  of  weathered  rock.  The  result  is  that  the  water  which 
runs  down  slopes  after  a  rain  generally  carries  sediment  to  the 
stream  which  it  enters.  This  is  especially  true  if  the  immediate 
run-off  flows  over  cultivated  or  abandoned  fields.  The  water  which 
flows  down  freshly  plowed  slopes,  for  example,  is  usually  very 
muddy,  while  that  which  runs  over  slopes  well  covered  with 
vegetation,  such  as  grass-land  or  forest,  carries  away  little  soil, 
because  the  roots  of  the  vegetation  hold  it.  Gullies  often  develop 
in  plowed  fields  which  lie  on  slopes,  when  adjacent  fields  which  are 


THE  WORK  OF  RUNNING  WATER 


123 


not  cultivated  do  not  suffer  in  the  same  way.  The  loosening  of 
soil  on  hill  and  mountain  slopes  often  leads  to  its  complete  removal, 
and  in  some  parts  of  France,  in  the  southern  part  of  our  own 
country,  and  elsewhere,  slopes  which  were  once  productive  have 
become  barren  by  the  washing  away  of  the  soil. 

The  amount  of  sediment  carried  by  the  immediate  run-off 
from  the  slopes  is  greatest,  other  things  being  equal,  where  the 
water  is  concentrated  into  streamlets,  and  least  where  it  runs  off 
in  sheets.  It  is  under  the  former  condition  that  little  gullies  are 
made  (Fig.  107).  The  gullies  are  themselves  proof  that  erosion  is 
greater  along  their  courses  than  on  either  side,  for  it  was  the  greater 
erosion  along  their  courses  which  made  them. 


FIG.  109. — A  "boiling"  or  eddying  stream.    Woods  Canyon,  Alaska. 
(Spencer,  U.  S.  Geol.  Surv.) 


Much  of  the  sediment  of  streams  is  brought  to  them  by  the 
immediate  run-off  which  flows  down  the  slopes  of  their  valleys. 
But  the  stream  in  the  valley  carries  away  not  only  the  sediment 
which  is  brought  to  it  by  gravity  and  by  wind,  by  sheet-wash 
and  temporary  streamlets  from  the  slopes  above,  but  under  favor- 
able conditions  it  gathers  load  for  itself  from  its  bed  and  from  its 
banks.  This  is  true,  for  example,  wherever  the  bed  of  a  vigorous 


124  PHYSIOGRAPHY 

stream  is  composed  of  clay  or  sand,  for  particles  of  these  materials 
are  easily  loosened  and  hurried  along  in  the  current. 

The  stream  does  not  pick  up  sediment  from  its  bed  merely  by 
the  force  of  the  forward  movement  of  the  water.  We  are  not  to 
think  of  a  stream  as  a  single  straightforward  current.  When 
water  runs  through  an  open  ditch  or  gutter,  some  of  it  may  be 
seen  to  move  from  the  sides  to  the  center,  and  some  from  the 
center  to  the  sides,  while  eddies  are  of  common  occurrence.  These 
subordinate  motions  are  especially  distinct  where  the  current  is 
swift.  A  swift  river,  too,  "boils"  and  eddies  (Fig.  109),  often 
in  a  striking  manner.  In  the  swift  Columbia,  for  example,  eddies 
are  often  so  strong  that  it  is  difficult  to  row  through  them.  In  an 
eddying  current,  objects  are  often  "sucked  under"  and  brought 
up  again.  There  are  similar  movements,  though  often  less  readily 
seen,  in  slower  streams. 

All  these  phenomena  show  that  there  are  numerous  subordinate 
currents  in  the  main  current  of  a  river,  and  that  they  move  in 
various  directions.  Many  of  them  are  caused  by  the  irregularities 


FIG.  110. — Diagram  to  illustrate  the  effect  of  irregularities,  a  and  6,  in  a 
stream's  bed,  on  the  current  striking  them. 

of  the  stream's  bed  (Fig.  110),  from  which  they  diverge  in  various 
directions.  The  subordinate  upward  currents  in  the  main  current 
often  carry  sediment  up  from  the  bottom  of  the  stream;  that  is, 
they  bring  it  into  suspension.  When  these  subordinate  currents 
strike  the  sides  or  bottom  of  a  stream's  channel,  they  are  often 
effective  in  tearing  or  wearing  off  bits  of  loose  matter.  As  we  shall 
soon  see,  these  subordinate  currents  not  only  help  to  get  fine 
sediment  into  suspension,  but  they  help  to  keep  it  there. 

There  are  two  reasons  why  a  stream  which  is  clear  or  nearly 
so  at  the  usual  stage  of  water,  becomes  muddy  when  it  is  swollen. 
One  is  that  in  time  of  flood  there  is  more  immediate  run-off  entering 
the  stream,  and  this  usually  brings  abundant  sediment:  the  other 
is  that  the  stream  when  flooded  flows  much  more  swiftly  than 


THE  WORK  OF  RUNNING  WATER 


125 


at  other  times,  and  so  has  power  to  rub  off  and  pick  up  much  more 
sediment  for  itself. 

It  might  seem  from  these  statements  that  swift  streams  should 
always  be  muddy  and  slow  ones  always  clear,  but  this  is  not  the 
case.  Many  a  swift  stream,  especially  in  the  mountains,  is  re- 
markably clear,  while  some  sluggish  ones  are  always  muddy. 
The  reason  is  not  far  to  seek.  Even  a  swift  stream  is  clear  (1)  if 
immediate  run-off  (slope-wash)  and  tributaries  bring  it  no  sediment, 
and  (2)  if  the  materials  -of  its  own  bed  are  so  coarse  that  it  cannot 
pick  them  up.  The  clearness  of  many  swift  mountain  streams  is 
due  to  the  fact  that  there  is  no  mud  or  sand  or  fine  material  of 
any  sort  in  their  beds  or  banks,  while  the  muddiness  of  many 
sluggish  streams  in  plains,  such  as  the  Lower  Missouri  and  the 
Platte,  is  due  to  the  fact  that  their  bottoms  and  banks  are  of  such 
fine  material  that  even  their  slow  currents  can  get  and  carry  it. 


FIG.  111. — Tools  with  which  a  river  works.  These  cobblestones  and  small 
bowlders  were  brought  down  by  the  stream  in  flood,  and  left  where  they 
now  appear.  Other  similar  materials  now  in  transit  cause  the  riffles 
in  the  current.  Chelan  River,  Wash.,  just  above  its  junction  with  the 
Columbia.  (Willis,  U.  S.  Geol.  Surv.) 

Again,  the  stream  by  friction  with  its  bed  tends  to  drag  the 
loose  sediment  at  its  bottom  along  with  it,  somewhat  as  a  weight 
of  any  sort  pulled  over  a  surface  of  mud  drags  some  of  the  mud 
beneath  along  with  it.  Every  stream,  therefore,  which  is  not 
already  loaded  wears  its  bed,  if  it  is  of  soft  material  such  as 
mud,  by  (1)  friction  of  the  main  current,  (2)  impact  of  the  sub- 


126 


PHYSIOGRAPHY 


ordinate  currents,  and  (3)  by  urging  or  dragging  along  the  fine 
material  of  its  bed. 

But  some  river  valleys  are  in  solid  rock,  even  in  rock  which  is 
very  hard  (Fig.  26).  How  are  such  valleys  made? 

In  the  first  place,  rock  exposed  to  the  water,  as  in  a  stream's 
channel,  or  to  the  atmosphere,  decays.  As  it  decays  it  crumbles, 
and  the  crumbled  part  is  readily  swept  away.  Again,  the  sand 
and  gravel  rolled  along  by  a  stream  (Fig.  Ill)  wear  its  bed,  even 


FIG.  Ill  A. — Tools  with  which  a  river  works.  Bowlders  left  by  the  Dela- 
ware River  on  its  flood  plain  in  times  of  flood,  near  the  Water  Gap. 
(N.  J.  Geol.  Surv.) 

if  it  is  of  hard  rock.  Even  the  fine  sediment  which  a  stream 
carries  helps  to  wear  its  channel.  The  sediment  which  a  stream 
carries,  therefore,  becomes  the  tool,  or,  better,  a  collection  of 
tools,  with  which  the  running  water  works,  and  with  these  tools 
even  hard  rock  is  worn  away. 

Clear  water  flowing  over  a  bed  of  firm,  hard  rock  effects 
little  or  no  mechanical  wear.  This  is  well  shown  in  the  case  of 
relatively  clear  streams  like  the  Niagara.  Tiny  plants,  like  those 
which  make  moist  stone  walls  green,  may  often  be  seen  growing 
on  the  limestone  of  its  bed  where  the  water  is  shallow  enough  to 
allow  the  bed  to  be  seen.  This  is  the  case  even  at  the  brink  of 
the  falls,  where  the  current  is  very  swift,  and  all  the  force  of  the 


THE  WORK  OF  RUNNING  WATER 


127 


mighty  torrent  is  unable  to  sweep  these  tiny  plants  from  their 
moorings.  If  the  stream  had  a  moderate  load  of  sand  or  mud 
there  can  be  no  doubt  that  these  plants  would  be  swept  away 
with  great  despatch.  The  sediment  carried  by  a  stream  is  there- 
fore a  factor  which  influences  its  rate  of  erosion,  especially  where 
the  bed  is  of  solid  rock. 


FIG.  112. — A  stream  channel  clogged  with  bowlders  too  big  for  the  stream 
to  move,  except  in  times  of  flood. 


Carrying.  It  has  already  been  stated  that  streams  move 
their  load  (sediment)  (1)  by  rolling  it  along  their  bottoms,  and  (2) 
by  carrying  it  in  suspension  above  the  bottoms.  Coarse  materials, 
such  as  pebbles,  are  generally  rolled,,  while  fine  materials,  such  as 
particles  of  mud,  are  often  suspended. 

The  material  rolled  on  the  bottom  is  moved  directly  by  the 
force  of  the  water.  Each  pebble  which  is  moved  is  pushed  or 
rolled  along  by  the  water  which  strikes  against  it.  The  principle 
is  the  same  as  that  involved  in  the  movement  of  pebbles  on  a 
beach,  except  that  the  stream  always  carries  them  down  the  valley, 
instead  of  rolling  them  back  and  forth. 

Mud  is  composed  chiefly  of  fine  particles  of  rock,  which  are 
nearly  three  times  as  heavy  as  water.  In  spite  of  this,  they  remain 
in  suspension,  often  for  long  periods  of  time.  The  mud  is  kept 
in  suspension  much  as  dust  is  kept  in  suspension  in  the  air.  Since 


128  PHYSIOGRAPHY 

its  particles  are  heavier  than  the  water,  they  tend  to  sink  all  the 
time.  They  do  in  fact  sink;  but  as  they  sink  under  the  influence 
of  gravity  they  may  be  caught  by  minor  upward  currents  and 
carried  upward  in  spite  of  gravity.  It  is  chiefly  by  means  of  these 
minor  upward  currents  in  the  main  current  that  sediment  is  kept 
in  suspension.  Because  of  the  manner  in  which  fine  sediment 
is  carried  in  suspension,  it  helps  to  deepen  and  widen  the  valley 
of  the  carrying  stream.  As  the  minor  upward  currents  of  a  stream 
carry  sediment  upward  through  the  water,  so  minor  downward 
currents  drive  it  against  the  bottom,  and  minor  sideward  currents 
against  the  sides  of  the  channel.  In  these  ways  even  the  fine 
sediment  helps  the  strea-m  to  enlarge  its  valley. 

The  particles  of  sediment  suspended  in  a  stream  are  dropped 
and  picked  up  again  repeatedly.  A  particle  may  make  a  long 
journey,  but  the  long  journey  may  be  made  up  of  many  short 
ones.  Pai  tides  of  mud  carried  from  Dakota  to  the  Gulf  of  Mexico 
ordinarily  make  many  stops  in  every  state  along  the  route,  and 
the  time  consumed  in  their  journey  is  generally  many  times  as 
long  as  that  consumed  by  the  water  which  started  them. 

Amount  of  load.  The  amount  of  sediment  a  stream  carries 
depends  on  (1)  its  velocity,  (2)  its  volume,  and  (3)  the  amount 
and  kind  of  sediment  available.  A  swift,  large  stream  can  carry 
more  than  a  slow,  small  one. 

The  effect  of  velocity  on  the  carrying  power  of  streams  may 
be  seen  in  most  creeks  and  rivers  whose  width  varies  notably. 
At  the  narrow  places,  the  swift  water  is  likely  to  carry  away  all 
fine  material,  permitting  only  coarse  pebbles  and  stones  to  remain 
upon  the  bottom,  while  in  the  broader  places  the  bottom  may  be 
covered  with  mud.  By  artificially  narrowing  (by  jetties)  the 
Mississippi  near  its  debouchure  (1875),  James  B.  Eads  not  only 
prevented  further  deposition  of  sediment  there,  but  forced  the 
river  to  clear  out  its  channel.  This  change  permitted  the  larger 
ocean  vessels  to  reach  New  Orleans,  and  insured  the  commercial 
prosperity  of  that  city. 

That  fine  sediment  is  picked  up  and  carried  more  readily  than 
coarse  is  illustrated  by  the  familiar  fact  that  a  stone  a  pound  in 
weight  thrown  into  any  common  stream  would  sink  to  the  bottom 
promptly,  while  if  a  pound  of  fine  dust  were  thrown  into  the  same 
stream,  its  particles  would  be  carried  forward  some  distance  before 
sinking  to  the  bottom. 


THE  WORK  OF  RUNNING  WATER  129 

A  stream  can  carry  a  much  greater  weight  of  fine  sediment 
than  of  coarse,  both  because  each  pound  of  fine  material  carried 
taxes  the  stream's  energy  less  than  a  pound  of  coarse,  and  because 
a  larger  part  of  a  stream's  energy  can  be  used  in  carrying  the 
former  than  in  carrying  the  latter. 

Erosion  defined.  The  wearing  away  of  the  land  surface  is 
erosion.  In  general,  erosion  consists  of  three  more  or  less  distinct 
processes.  These  are  (1)  weathering,  (2)  common,  or  the  picking 
up  of  the  rock  material  loosened  by  weathering  or  by  any  other 
process,  and  (3)  transportation.  The  solution  of  rock  material  by 
water  is  often  included  under  corrasion.  It  would  perhaps  be  well 
to  call  it  corrosion  instead.  When  the  running  water  is  no  longer 
able  to  carry  away  sediment,  it  ceases  to  degrade  its  bed. 

Deposition  a  necessary  consequence  of  erosion.  The  sedi- 
ment carried  by  rivers  is  deposited  whenever  they  are  unable  to 
carry  it  farther.  The  cause  of  deposition  is  most  commonly  loss  of 
velocity.  Some  of  the  sediment  is  left  in  the  valleys,  especially  in 
their  lower  courses;  and  some  of  it  is  carried  to  the  sea,  or  to  the 
lake  or  other  basin  to  which  the  river  flows.  Deposits  of  sediment 
in  valleys  build  up  or  aggrade  their  bottoms.  Thus  the  Mississippi 
is  spreading  sediment  over  the  bottom  of  its  valley  for  hundreds 
of  miles  north  of  the  Gulf  of  Mexico,  and  many  other  large  streams, 
like  the  Nile,  the  Hoang-Ho,  and  the  Ganges,  are  doing  the  same 
thing.  The  total  amount  of  aggradation  accomplished  by  running 
water  on  land  is,  however,  far  less  than  the  amount  of  degradation. 
The  deposition  of  sediment  by  streams  will  be  more  fully  considered 
in  its  appropriate  place. 

Changes  Made  by  Rivers  in  their  Valleys 

A  valley  has  three  dimensions,  depth,  width,  and  length,  and 
each  dimension  is  subject  to  change. 

The  deepening  of  valleys.  Eroding  streams  make  their  valleys 
deeper  and  wider.  Where  streams  are  depositing,  that  is,  where 
they  leave  more  than  they  take  away,  they  are  making  their  valleys 
shallower.  In  general,  swift  streams  deepen  their  valleys,  while 
slow  ones  often  make  their  valleys  shallower.  Many  valleys  are 
being  deepened  in  their  upper  courses  where  the  streams  are 
swifter,  and  made  shallow  in  their  lower  courses  where  the 
streams  are  more  sluggish. 


130 


PHYSIOGRAPHY 


Swift  streams  are  swift  because  they  flow  in  channels  which 
have  relatively  steep  slopes;  but  as  such  streams  deepen  their 
valleys,  the  slopes  or  gradients  of  the  valley  bottoms  become  less , 
and  the  streams  flow  more  slowly.  In  time  every  swift  stream  mil 
cut  its  channel  so  low  that  its  current  will  become  sluggish. 

There  is  no  fixed  relation  between  the  depth  of  a  valley  on  the 
one  hand  and  erosion  or  deposition  in  its  bottom  on  the  other. 
Some  deep  valleys,  like  the  canyon  of  the  Colorado  (Fig.  27),  are 
becoming  deeper  by  erosion,  while  others  which  are  shallow  are 


FIG.  113. — A  shallow  valley  becoming  shallower  by  deposition.     North  Platte 
River  near  the  Nebraska- Wyoming  line.     (U.  S.  Geol.  Surv.) 

becoming  shallower  by  deposition  (Fig.  113).  Some  deep  valleys, 
on  the  other  hand,  are  being  aggraded,  and  some  shallow  ones  are 
being  degraded. 

The  depth  which  a  valley  may  attain  depends  primarily  on  the 
height  of  the  land  in  which  it  is  cut.  The  higher  the  land,  the  deeper 
the  valley  may  become.  Such  valleys  as  the  canyons  of  the  Colo- 
rado (Fig.  27)  and  the  Yellowstone  (Fig.  152)  are  never  found  in 
plains  (compare  Figs.  140  and  169).  Valleys  of  great  depth  are 
characteristic  of  plateaus  and  mountains.  With  land  of  a  given 
height,  the  depth  which  a  valley  may  attain  depends  on  its  dis- 
tance from  the  sea  by  the  route  which  the  water  follows.  Thus, 
if  a  stream  flows  by  a  direct  course  from  a  plateau  2000  feet  above 


THE  WORK  OF  RUNNING  WATER  131 

the  sea  and  200  miles  from  it,  it  has  an  average  fall  of  10  feet 
per  mile;  but  if  it  runs  off  a  plateau  of  equal  height  2000  miles 
from  the  sea  by  the  course  which  the  water  follows,  the  stream 
has  an  average  fall  of  but  one  foot  per  mile.  If  the  volume  of  the 
stream  is  the  same  in  the  two  cases,  the  valley  in  the  plateau 
nearer  the  sea  will  become  much  deeper  than  the  other.  In  other 
words,  the  depth  which  a  valley  may  attain  depends  primarily 
on  the  fall  (or  gradient)  of  the  water  which  flows  through  it. 
Valleys  near  the  borders  of  continents  are  therefore  likely  to  be 
deeper  than  those  in  lands  of  the  same  elevation  in  the  interiors  of 
continents. 

Depth-limit.  At  its  lower  end,  a  stream  usually  cuts  its  channel 
down  to,  or  even  a  little  below,  the  level  of  the  lake,  sea,  or  other 
river  into  which  it  flows.  The  body  of  water  into  which  a  river 
flows  therefore  determines  the  depth-limit  of  its  valley;  but  the  valley 
reaches  this  limit  only  at  its  lower  end.  The  upper  end  of  a  river 
valley  is  always  above  sea-level. 

The  lowest  level  to  which  a  stream  can  bring  its  valley  bottom 
by  mechanical  wear  is  called  base-level.  It  is  to  be  noted,  how- 
ever, that  a  stream's  channel  may  be  below  sea-level  at  and  near 
its  lower  end.  Thus  the  channel  of  the  Mississippi  is  below  sea- 
level  for  some  distance  above  the  mouth  of  the  stream  and  locally 
as  much  as  100  feet  below.  The  broad  valley  plain  of  the 
Mississippi,  on  the  other  hand,  is  just  above  sea-level  in  the  same 
region. 

Many  conditions  affect  the  rate  at  which  a  stream  erodes,  and 
everything  which  affects  the  rate  of  erosion  affects  the  length 
of  time  which  it  will  take  a  stream  to  bring  the  bottom  of  its  valley 
to  base-level.  Other  things  being  equal,  a  large  stream  will  bring 
its  valley  to  base-level  sooner  than  a  small  one,  and  any  stream 
will  bring  its  channel  to  base-level  in  weak  rock  sooner  than  in 
resistant  rock. 


FIG.  114. — Diagram  of  a  valley,  the  top  of  which  is  ten  times  the  width  of 

the  stream. 

The  widening  of  valleys.     If  the  growth  of  a  valley  were  due 
merely  to  the  down-cutting  of  the  stream,  the  valley  would  be  no 


132 


PHYSIOGRAPHY 


wider  than  the  stream  which  flows  through  it  (Fig.  114,  see  also 
Figs.  27  and  27a).  Since  most  valleys  are  very  much  wider 
than  their  streams,  other  factors  besides  down-cutting  must  be 
involved  in  their  development. 

Most  valleys  are  much  wider  at  their  tops  than  at  their  bottoms, 
and  all  valleys  are  being  made  wider  all  the  time.  The  widening 
is  brought  about  in  many  ways.  Some  of  them  are  the  following: 

(1)  Sometimes  a  stream  flows  against  one  side  of  its  channel 
with  such  force  as  to  under-cut  the  slope  above  (PI.  VIII  and 


FIG.  115.— River  under-cutting  its  bank  and  widening  its  valley  by  planation 
where  the  material  is  unconsolidated  sand,  gravel,  etc. 

Figs.  115  and  116).  The  material  under-cut  is  likely  to  fall  or 
slide  down,  and  this  makes  the  valley  wider  than  before.  Slow 
streams  widen  their  valleys  more  rapidly  than  swift  ones,  partly 
because  they  are  more  easily  turned  against  their  banks  by  any 
sort  of  obstacle  in  the  channel. 

(2)  Again,  some  of  the  rain  falling  on  the  slopes  of  a  valley 
runs  down  to  the  bottom  and  is  likely  to  carry  mud,  sand,  and 
coarser  materials  with  it.     This  also  widens  the  valley  by  slowly 
wearing  back  its  slopes. 

(3)  The  loose  earthy  matter  which  lies  on  the  slopes  of  a  valley 
is  slowly  creeping  downward.     The  movement  is  brought  about  in 
various  ways,     (a)  If  the  material  is  clay,  it  contracts  when  dry, 


PLATE  VI i 


Streams  disappearing  in  the  sand,  gravel,  etc.,  at  the 
base  of  mountains  in  an  arid  region.  Scale  4— 
miles  per  inch.  (Paradise,  Nev.,  Sheet,  U.  3, 
Geol.  Surv.) 


PLATE  VIII 


IS  MILES  SOUTHWEST  OF  ST.  LOUIS.  MISSOURI. 


A  stream  widening  its  valley  by  lateral  planation.     Scale  1—  miles  per  inch. 
(TJ.  S.  Geol.  Surv.) 


THE  WORK  OF  RUNNING  WATER 


133 


and  as  it  contracts,  it  cracks.  The  gaping  of  the  crack  is  due 
chiefly  to  the  downward  movement  of  the  clay  on  the  down-slope 
side  (A,  Fig.  117).  Cracking  of  the  same  sort  (Fig.  118)  may  be 
seen  where  a  pool  or  pond  has  dried  up,  though  creep  is  not  in- 
volved. When  the  cracked  clay  on  a  slope  becomes  wet  again, 
as  by  rain,  the  clay  swells  and  the  cracks  are  closed;  but  the 
swelling  takes  place  in  such  a  way  that  the  cracks  are  closed  chiefly 
by  the  moving  down  of  the  clay  on  the  upper  side  of  the  crack 
(C,  Fig.  117),  not  by  the  moving  up  of  the  clay  on  the  lower  side. 


FIG.  116. — The  Green  River,  Wyo.,  cutting  against  its  bank  and  widening  iu- 
valley  by  planation  where  the  material  is  indurated.     (Fairbanks.) 

This  is  because  gravity  helps  to  pull  the  clay  down,  while  upward 
movement,  if  it  took  place,  would  have  to  take  place  against 
gravity.  (6)  Again,  clayey  material  tends  to  become  a  viscous 
fluid  when  wet,  and  in  so  far  as  it  takes  on  fluidity,  it  tends  to 
creep  or  flow  down-slope  (p.  108).  Friction,  the  roots  of  plants, 
etc.,  on  the  other  hand,  tend  to  restrain  its  descent.  All  down- 
ward movement  of  this  sort  tends  to  widen  the  valley,  for  much 
or  all  the  material  descending  in  this  way  is  carried  off  by  the 
stream  when  it  reaches  the  bottom  of  the  valley. 

(4)  When  the  loose  material  of  the  steep  valley  slope  is  thor- 
oughly filled   with  water,  as   after  a  long  rain  or  when  snow  is 


134 


PHYSIOGRAPHY 


melting,  it  may  slide  or  slump  from  higher  to  lower  levels  (Fig.  119). 

Slumping  is  common  on  steep  valley  slopes  composed  of  uncon- 
solidated  material  like  clay.  Slumping 
widens  the  valley  at  the  point  whence  it 
starts.  Material  descending  the  slopes  in 
this  as  in  other  ways  is  sooner  or  later 
carried  away  by  the  rivers. 

(5)  Every  animal  which  walks  over  the 
slope  of  a  valley  is  likely  to  loosen  more 
or  less  material  if  the  slope  is  steep,  and 
if  this  material  is  moved  at  all,  it  is  likely 
to  be  moved  downward.     Burrowing  ani- 
mals of  all  sorts  loosen  the  surface  ma- 
terial and  prepare  it  to  be  worked  down 
the  slope  readily.     All  these  processes  help 
to  widen  the  valley. 

(6)  Trees  which  grow  on  the  sides  of 
valleys  are  sometimes  overturned.    When- 
ever they  fall,  they  disturb  more  or  less 
earthy  matter,  and  some  of  it  is  likely  to 
roll  down  if  the  slopes  are  steep.     If  they 
are  not,  the  material  loosened  may  be  car- 
ried down  by  slope-wash  or  by  other  means. 

(7)  Fine  material  on  the  slopes  of  val- 
FIG.  117. — Diagram  to  il- 
lustrate the   effects  of    leys  may  be  blown  away. 

Various  other  processes  are  also  in  oper- 
ation, helping  to  loosen  rock  or  soil  on  the 

cracking  open.     Tn  B    siOpes    ancj  &[\  processes  which  loosen  the 

the    process    has   gone  . 

b  material  m  this  position  prepare  it  for  de- 
scent, and  the  descent  or  removal  of  mat- 
ter from  the  slopes  of  a  valley  always 
increases  its  width.  All  valleys,  therefore, 
are  being  widened  all  the  time.  In  most 

by  the  moving  down  of    processes  of  widening,  the  stream  itself  is 

a  rather  than  the  mov-  ... 

ing  up  of  b.  an  important  factor,  for   it   carries   away 

much  of  the  material  which  descends  the 

slopes.     Along  the  bases  of  the  slopes  of  many  valleys  there  is 
much  debris  (talus)  waiting  to  be  carried  away  (Fig.  121). 

Width-limit.     As  a  result  of  all  the  processes  which  wear  back 
their  slopes,  adjacent  valleys  may  be  widened  until  the  divide 


drying  and  wetting  on 
a  clay  slope.  In  A  the 
clay  is  drying  and 


further,  and  it  is 
which  has  moved  down, 
while  a  remains  where 
it  was  in  A.  C  repre- 
sents the  same  after  it 
has  been  wet  again  and 
the  crack  closed,  chiefly 


THE  WORK  OF  RUNNING  WATER 


135 


between  them  is  worn  away  (Figs.  122  and  122a).  More  commonly, 
however,  the  divide  between  valleys  becomes  low  without  dis- 
appearing altogether  (Fig.  123). 


FIG.  118. — Sun-cracks  in  the  flood  plain  of  the  Missouri.     (Chamber lin.) 

Valley  flats.     As  already  implied,  streams,  after  they  have 
cut  their  channels  down  to  low  gradients,  develop  flats,  or  flood 


1 


FIG.  119. — Slumping  in  the  side  of  a  valley,  two  miles  southeast  of  Trout 
Lake,  near  Telluride,  Colo.     (Hole.) 

plains,  in  the  bottoms  of  their  valleys.   These  flats  are  always  below 
the  level  of  the  surface  in  which  the  valley  lies.     Thus  the  Missis- 


136 


PHYSIOGRAPHY 


sippi  River  at  Dubuque  has  a  flat  between  one  and  two  miles  wide, 
about  300  feet  below  its  surroundings,  and  about  600  feet  above 
sea-level.  Near  St.  Louis  the  flat  is  10  miles  wide,  about  150 


FIG.  120. — Slumping  on  the  slope  of  Monte  Crist  o  Cieek,  Alaska. 
(U.  S.  Geol.  Surv.) 


FIG.  121. — Talus  at  base  of  valley  slope,  ready  to  be  can  led  off  by  the 
stream.  Little  Canyon — looking  south  into  Snake  River.  (U.  S. 
Geol.  Surv.) 

feet  below  its  surroundings,  and  about  400  feet  above  sea-level. 
At  Memphis  it  is  about  35  miles  wide  and  but  220  feet  above  sea- 
level.  At  Vicksburg  it  has  a  similar  width  and  a  height  of  but 


THE  WORK  OF  RUNNING  WATER 


137 


90  feet.     Though  increasing  width  of  flat  down-stream  is  char- 
acteristic of  valleys  in  general,  it  must  not  be  understood  that  the 


FIG 


.  122. — Diagram  showing  streams  in  adjacent  valleys,  under-cutting  the 
divide  between  them.  They  may,  in  time,  destroy  the  divide  by  lateral 
planation. 


m 


A 


FIG.  122a. — Diagram  to  show  how  the  divide  between  streams  may  be 
done  away  with  by  lateral  planation.  In  A  the  stream  at  the  left  is 
represented  as  under-cutting  the  divide  between  the  two  valleys.  Later, 
by  shifting  of  its  channel,  the  stream  in  the  other  valley  might  under- 
cut the  other  slope  of  the  divide,  as  shown  in  B.  In  C  both  streams  are 
represented  as  under-cutting  the  divide  between  them,  and  in  D  the 
divide  has  been  done  away  with. 

increase  of  width  is  uniform.  Narrower  portions  (often  where  the 
rock  is  more  resistant)  often  alternate  with  wider  ones  (often 
where  the  rock  is  less  resistant). 


138 


PHYSIOGRAPHY 


Combining  these  facts  with  a  generalization  previously  made, 
we  may  say  (1)  that  rivers  tend  constantly  to  get  the  material  of 
the  land  into  the  sea;  (2)  that  in  working  to  this  end  they  develop 


FIG.  123. — Diagram  to  illustrate  the  leveling  of  the  surface  by  valley  erosion. 
The  ground  profile  represented  at  the  top  shows  two  young  valleys, 
1  and  1,  in  an  otherwise  flat  surface.  In  time  these  valleys  will  develop 
the  cross-sections  represented  by  2,  2,  and  later  those  represented  by  3, 3, 
4,4,  etc.  The  divide  between  them  may  finally  reach  5,  when  the 
surface  is  nearly  flat. 

flats  below  the  general  level  of  the  surface  in  which  the  valleys  lie; 
and  (3)  that  these  flats  are,  in  general,  wider  and  lower  near  the 
sea,  and  narrower  and  higher  far  from  it.  Plates  VIII  to  X  and 
Figs.  124  and  125  show  valley  flats  in  various  sorts  of  regions. 


FIG.  124. — A  valley  flat  in  an  early  stage  of  development.     Monte  Cristo 
Creek,  Alaska.     (U.  S.  Geol.  Surv.) 

Most  valley  flats  are  developed  chiefly  by  the  side-cutting  of 
the  streams  (PI.  VIII)  after  they  have  become  sluggish.  The 
streams  which  flow  through  flats  generally  meander,  that  is,  they 
have  very  winding  courses  (Pis.  IX,  X,  and  XI). 

The  valley  flat  is  a  sort  of  base-level,  though  the  first  flat  developed 
by  a  stream  is  not  necessarily  the  lowest  level  to  which  it  may 


PLATE  IX 


FIG.  1. — A  meandering  stream.  The  Mis- 
souri River.  Scale  2—  miles  per  inch. 
(Marshall,  Mo.,  Sheet,  U.  S.  Geol.  Surv.) 


FIG.  2. — A  further  stage  in  the 
development  of  a  meander. 
The  Schell  River,  Missouri. 
Scale  2—  miles  per  inch. 
(Butler,  Mo.,  Sheet,  U.  S. 
Geol.  Surv.) 


FIG.  3. — A  plain  in  old  age.     Scale  2—  miles  per  inch.     (Abilene,  Kan.,  Sheet, 

U.  S.  Geol.  Surv.) 


PLATE  X 


A  well-developed  river  flat.  Valley  of  the  Mississippi,  near  Prairie  du  Chien, 
Wis.  Scale  2—  miles  per  inch.  (Waukon,  la.  Wis.  Sheet,  U.  S.  Geol 
Surv.) 


PLATE  XI 


Stream  flats.     The  Missouri  and  Big  Sioux  rivers.     Scale  2—  miles  per  inch 
(Elk  Point,  S.  Dak. -la.— Neb.  Sheet,  U.  S.  Geol.  Surv.) 


THE  WORK  OF  RUNNING  WATER 


139 


bring  its  valley  bottom.  It  is  the  lowest  level  to  which  the  stream 
can  bring  its  valley  under  the  conditions  which  exist  when  the  flat 
is  developed.  It  is  therefore  a  temporary  base-level,  and  serves  as 


FIG.  125. — A  wide  valley  flat.     Milk  River  near  Pendant  d'Oreille,  Canada. 

(U.  S.  Geol.  Surv.) 


FIG.  126.— Trout  Creek,  Yellowstone  Park.     (U.  S.  Geol.  Surv.) 

the  limit  below  which  tributary  streams  may  not  cut.  Later, 
under  changed  conditions,  the  stream  may  sink  its  channel  well 
below  its  first  flat,  and  when  this  is  done  by  a  main  stream,  all  its 
tributaries  may  do  the  same. 


140 


PHYSIOGRAPHY 


The  lengthening  of  valleys.  Valleys  are  lengthened,  too, 
in  various  ways.  Illustration  of  one  way  in  which  they  are  made 
longer  is  furnished  by  the  gullies  developed  on  hillsides  during 
heavy  rains.  The  gully  made  during  one  rain-storm  is  often 


FIG.  12V. 


FIG.  128. 


FIG.  127. — Two  young  valleys  heading  toward  each  other. 

FIG.   128. — Valleys  of  Fig.   127  developed  headward  until  their  respective 

heads  have  met  and  the  divide  has  been  lowered  a  little  at  the  point 

of  meeting. 

lengthened  at  its  upper  end  (headward)  during  the  next,  by  the 
water  which  flows  in  at  its  head.  The  process  of  lengthening 
may  sometimes  be  seen  even  during  the  progress  of  a  single  storm. 
The  heads  of  valleys  often  have  the  characteristics  of  ravines  or 
gullies.  Valleys  are,  indeed,  in  some  cases  no  more  than  growing 
ravines  which  are  working  their  heads  inland,  after  the  manner 
of  hillside  gullies. 

By  this  process  the  head  of  a  valley  may  advance  until  a 
permanent  divide  is  established.  Thus  in  Fig.  127  the  heads  of 
the  valleys,  a  and  6,  may  be  worn  back  farther  into  the  upland; 
but  when  the  heads  of  the  valleys  reach  the  points  shown  in  Fig. 


FIG.  129. — Diagram  to  illustrate  the  lowering  of  a  divide  without  shifting 
it.  The  crest  of  the  divide  is  at  a,  b,  and  c  successively.  If  the  erosion 
was  unequal  on  the  two  sides,  the  divide  would  be  shifted. 

128,  neither  can  advance  farther,  if  the  rates  of  erosion  are  the 
same  on  both  sides  of  the  divide.  The  divide  is  then  permanent, 
for  though  continued  rainfall  may  lower  it,  it  cannot  shift  its  posi- 
tion (Fig.  129). 


THE  WORK  OF  RUNNING  WATER 


141 


It  is  not  to  be  understood  that  all  valleys  are  being  lengthened 
at  their  heads  in  this  way.     Thus  the  head  of  the  St.  Lawrence 
River  is  at  the  foot  of  Lake  Ontario,  and  will  remain  there  as  long 
as  the  lake  shore  remains  where 
it  now  is. 

In  its  growth  in  length,  the 
head  of  one  valley  may  reach 
another  valley,  when  the  two 
become  one.  This  is  illustrated 
by  Fig.  130.  Streams  are 
sometimes  lengthened  at  their 
lower  ends.  This  is  the  case 
where  the  sediment  which  they 
deposit  at  their  debouchures 
(lower  ends)  builds  the  land 
out  into  the  sea.  The  streams 
then  find  their  way  across  the 
new-made  land.  Across  such 
lands  the  streams  have  chan- 
nels, but  never  valleys  of  much 
depth.  There  are  various  other 
ways  in  which  valleys  become 
longer,  but  they  will  not  be 
'  considered  at  this  point. 


FIG.  130. —  Diagram  to  illustrate  one 
mode  of  valley  lengthening.  In  A 
there  are  two  small  valleys,  a  and 
b,  and  the  former  ends  at  the  base 
of  the  steep  slope.  In  B  the  valley 
b  is  represented  as  having  been 
lengthened  so  as  to  join  a,  and  the 
two  have  become  one. 


Summary.  All  valleys  are 
being  made  deeper  in  at  least 
some  part  of  their  courses  all 
the  time;  all  valleys  are  being 

made  wider  all  the  time;  and  some  valleys  are  growing  longer.  All 
streams  sooner  or  later  develop  flats  in  their  valleys,  and  these  flats 
may  increase  in  width  till  the  divides  between  them  are  worn  away. 
Where  the  divides  between  streams  are  not  worn  away  by  the 
lateral  planation  of  the  streams,  they  may  become  so  low  as  to 
be  inconspicuous.  In  either  case  the  area  affected  becomes  nearly 
flat,  at  a  level  as  low  as  running  water  can  cut  it.  The  land  is 
then  base-leveled. 

The  History  of  a  River  System 

Since  valleys  grow  deeper,  wider,  and  longer  year  by  year, 
they  must  formerly  have  been  smaller  than  now.     If,  in  imagination, 


142 


PHYSIOGRAPHY 


we  trace  them  backward  in  their  history,  we  may  think  of  a  time 
when  the  large  valleys  of  the  present  day  were  small,  when  the 
small  valleys  were  only  ravines,  when  the  ravines  were  only  gullies, 
and  when  the  present  gullies  did  not  exist.  Or,  going  still  fur- 
ther back,  we  may  imagine  a  time  when  even  the  large  valleys 
had  a  beginning. 

A  principal  method  of  valley  birth  and  growth  is  illustrated 
by  the  development  of  a  gully.    The  rain-water  which  falls  on  the 


FIG.  131. — Gullies  developing  on  easily  eroded  soil.     Clear  Lake,  Cal.    Every 
shower  will  cause  them  to  grow  headward.     (Fairbanks.) 

surface  tends  to  gather  in  such  depressions  as  exist,  and  to  flow 
through  them  down  the  slopes.  The  water  concentrated  in  the 
depressions  flows  faster  than  that  not  so  concentrated,  and  wears 
the  surface  there  more  than  elsewhere,  and  so  starts  a  gully.  The 
gully  started  during  one  shower  is  made  deeper,  wider,  and  longer 
by  the  next.  Year  by  year,  as  the  result  of  repeated  showrers  and 
repeated  meltings  of  snows,  the  gully  may  grow  to  be  a  ravine,  and 
still  later,  by  the  same  processes,  it  may  become  a  valley.  A 
hillside  gully  is  essentially  like  a  river  valley  except  in  size,  and 
many  valleys  are  but  gullies  grown  big. 

Not  all  gullies,  however,  become  valleys,  and  not  all  valleys 


THE  WORK  OF  RUNNING  WATER 


143 


start  as  gullies.  On  a  steep  slope  numerous  gullies  may  start 
(Figs.  107,  131,  132,  and  133);  but  as  they  grow,  some  are  so 
widened  as  to  take  in  others  (Fig.  134),  and  the  number  is  reduced. 


FIG.  132. — Gullies  on  slope  above  a  valley  flat.     (Montana.) 

Relatively  few  gullies  become  even  ravines,  fewer  still  become 
small  valleys,  and  very  few  ever  attain  great  size.     As  valleys 


FIG.  133. — Surface  much  furrowed  by  the  development  of  erosion  gullies. 
Montana.     (George. ) 

develop  from  gullies,  the  heads  of  some  work  back  faster  than  others, 
with  the  result  that  many  valleys  are  arrested  in  their  develop- 
ment early,  and  so  are  dwarfed  (Fig.  135).  For  example,  g, 


144 


PHYSIOGRAPHY 


Fig.    135A,   will    grow  in   length  little  more,  because  the  water 
which   falls    on    the    land    above    its    head   flows    off    by    some 

other  route  to  the  sea.  Later 
stages  in  the  development  of 
these  valleys  are  illustrated 
by  Fig.  135,  B  and  C.  The 


FIG.  134. — Diagram  illustrating  how  one  contest  among  gullies  and  val- 
gully  takes  another  as  a  result  of  leys  resulting  in  the  survival 
lateral  erosion.  J  ' 

of  a  few  and  the  exceptional 

development  of  a  very  small  number,  may  be  called  a  struggle  for 
existence. 


FIG.  135. — Diagrams  illustrating  successive  stages  in  the  struggle  for  existence 
and  dominion  among  streams. 

The  courses  of  valleys.  The  headward  growth  of  a  gully  is 
due  chiefly  to  the  erosion  of  the  water  which  flows  into  its  upper 
end.  If  the  material  about  the  upper  end  of  a  gully  is  of  uniform 
hardness,  the  head  of  the  gully  works  back  in  the  direction  from 


THE  WORK  OF  RUNNING  WATER  145 

which  the  greatest  volume  of  water  enters.  Unevenness  of  the 
surface  about  the  head  of  the  gully  may  concentrate  the  inflo\ving 
water  now  at  this  point  and  now  at  that,  as  the  head  of  the  gully 
advances.  Consequently  the  head  of  the  gully  is  rarely  worn  back 
in  a  straight  line.  It  turns  to  the  right  (bf,  Fig.  136)  where  more 


FIG.  136. — Diagram  to  illustrate  the  direction  of  lengthening  of  a  valley. 
At  1  the  valley  is  straight.  If  at  this  stage  more  water  comes  in  from 
the  direction  b  than  from  the  direction  a,  the  wear  is  greater  toward  b 
than  toward  a,  and  the  head  turns  as  shown  in  2.  If  at  this  stage  more 
water  comes  in  from  the  direction  c  than  from  any  other  direction,  the 
head  turns  in  this  direction,  as  shown  in  3. 

water  comes  in  from  that  side,  and  to  the  left,  6",  where  there  is 
more  inflow  and  wear  on  that  side. 

If  the  material  about  the  head  of  the  gully  is  less  hard  at  one 
point  than  at  another,  the  head  of  the  gully  will  work  back  on 
the  material  which  is  most  easily  worn,  even  though  the  amount 
of  water  flowing  in  from  that  direction  is  no  greater  than  elsewhere. 
Inequalities  of  slope  or  material,  therefore,  cause  the  gully's  head 
to  turn  now  to  one  side  and  now  to  the  other,  and  where  the  gully's 
head  goes,  there  the  valley  which  develops  from  it  follows,  if  the 
gully  reaches  valley-hood.  The  crookedness  of  many  valleys  is 
thus  explained. 

The  permanent  stream.  Water  commonly  flows  in  a  gully 
only  when  it  rains  and  when  the  snow  is  melting,  and  for  a  short 
time  afterward;  but  many  valleys  were  developed  from  gullies, 
and  sooner  or  later  most  valleys  have  permanent  streams.  Where 
does  the  water  for  the  permanent  stream  come  from? 

The  answer  to  this  question  may  be  readily  inferred.  When 
a  valley  has  been  deepened  so  that  its  bottom  is  well  below  the 
ground-water  surface,  the  ground-water  seeps  or  flows  out  into 
the  valley,  and  once  in  the  valley  in  sufficient  quantity,  it  becomes 


146  PHYSIOGRAPHY 

a  stream  (Fig.  137).  The  valley  whose  cross-section  is  shown  by 
1,  Fig.  137,  would  not  have  a  stream;  the  valley  whose  cross-section 
is  represented  by  2  would  have  a  stream  in  wet  weather,  when 
the  ground- water  level  is  at  a;  while  the  valley  3  would  have  a 
permanent  stream  because  it  is  well  below  the  ground-water  level, 
b,  of  dry  times.  In  regions  where  the  ground-water  surface  is  deep, 


\2> 


FIG.  137. — Diagram  showing  ground-water  surface:  a  the  ground-water  sur- 
face at  ordinary  times,  and  b  in  times  of  drought.  When  a  valley  has 
been  cut  below  a  there  will  be  a  stream  in  wet  weather,  but  it  will  go 
dry  in  time  of  drought.  When  the  valley  is  down  to  3  below  the  ground- 
water  surface  of  dry  weather  the  stream  will  be  permanent. 

the  valley  must  be  deep  to  get  a  stream.  In  regions  where  the 
ground- water  surface  is  near  the  land  surface,  even  shallow  valleys 
may  have  permanent  streams. 

Streams  which  are  fed  by  lakes  and  streams  which  have  their 
sources  in  snow-  and  ice-fields  which  persist  from  year  to  year, 
are  not  immediately  dependent  on  ground-water,  though  they 
often  receive  it. 

Not  all  valleys  are  grown-up  gullies.  Not  all  valleys  were 
formed  by  the  growth  of  gullies.  A  great  area  in  the  northern 
part  of  North  America,  for  example,  was  once  covered  by  a  great 
sheet  of  snow  and  ice.  When  it  finally  melted,  large  parts  of 
the  surface  were  left  without  well-defined  valleys,  but  with 
numerous  lakes.  The  rainfall  of  the  region  was  enough  to  make 
many  of  these  lakes  overflow.  When  a  lake  overflows,  the  out- 
going water  follows  the  lowest  line  accessible  to  it,  so  long  as  there 
is  a  line  of  descent.  In  this  case,  the  running  water  will  start  to 
cut  a  valley  all  the  way  from  the  lake  which  furnishes  the  water, 
to  the  end  of  the  stream,  at  the  same  time.  No  part  of  such  a 
valley  is  much  older  than  another.  Valleys  developed  in  this  way 
may  have  permanent  streams  at  the  outset,  since  they  are  not 
dependent  on  ground-water.  The  course  of  a  valley  developed 
in  this  way  was  not  determined  by  the  direction  in  which  the 
head  of  the  valley  grew,  but  by  the  direction  which  the  water  took 
at  the  outset,  that  is,  by  the  course  of  the  lowest  descending  slope. 


THE  WORK  OF  RUNNING  WATER 


147 


Growth  of  tributaries.  Most  valleys  are  joined  by  many 
smaller  tributary  valleys.  The  reason  may  be  easily  understood 
by  the  study  of  a  gully. 

If  the  slopes  of  a  gully  were  worn  back  everywhere  at  the  same 
rate,  tributaries  would  not  develop;  but  the  sides  are  rarely  or 


FIG.  138. — Diagram  showing  tributaries  in  an  early  stage  of  development. 

never  worn  back  equally.  Either  the  material  is  softer  at  some 
places  than  at  others,  or  the  water  flowing  down  the  slopes  is 
concentrated  along  some  lines  more  than  along  others.  In  either 
case  the  erosion  of  the  side  sbpes  is  greater  at  some  points  than 
at  others,  and  where  the  erosion  on  the  slope  of  a  main  gully  is 
greater  than  at  adjacent  points,  a  tributary  gully  is  started  (Fig. 
138).  Tributary  gullies  are  therefore  developed  in  the  same  way, 
and  for  the  same  reason,  as  the  larger  ones  from  which  they  grow. 
The  tributary  gully  grows  in  length,  width,  and  depth  as  its  main 


FIG.  139. — Diagrammatic  representation  of  a  surface  much  dissected  by  the 
development  of  numerous  tributaries. 

did,  and  in  time  it  may  become  a  valley  and  acquire  a  permanent 
stream.  Tributaries  to  the  tributaries  are  developed  in  turn,  until 
a  network  of  watercourses  affects  the  surface.  Figs.  139,  140,  and 
141  show  a  surface  in  this  condition.  A  valley  developed  by 
outflow  from  a  lake  develops  tributaries  in  the  same  way  as  one 


148 


PHYSIOGRAPHY 


developed  from  a  gully.     Such  a  valley  might  also  get  tributaries 
by  the  inflow  of  water  from  other  lakes. 

A  valley  and  its  tributaries  constitute  a  valley  system.  A 
stream  and  its  tributaries  constitute  a  drainage  system,  and  the 
area  drained  by  a  river  system  through  a  valley  system  is  a 


•     '-  -  • 
^^ 


FIG.  140. — Photograph  of  the  model  of  an  area  in  northwestern  Connecticut, 
showing  a  surface  much  dissscted  by  erosion.     (Model  by  Howell.) 

drainage  basin.  From  the  conditions  under  which  a  valley  system 
develops,  the  outline  of  a  drainage  basin  often  comes  to  be  rudely 
pear-shaped  (Fig.  142). 

Stages  in  the  history  of  a  valley.  We  have  seen  that  valleys 
normally  grow  as  they  advance  in  years.  When  a  valley  is  young, 
it  is  narrow,  and  its  slopes  are  steep.  If  the  land  is  high,  it  has 
a  high  gradient  (unless  far  from  the  sea)  and  soon  becomes  deep. 


THE  WORK  OF  RUNNING  WATER 


149 


Its  cross-section  is  then  somewhat  V-shapsd  (Fig.  143),  and  its 
tributaries  are  short.  The  mature  valley  is  wider  (Fig.  144),  its 
slopes  are  often  gentler,  and  its  tributaries  are  longer  and  older. 


FIG  141. — Contour  map  of  the  area  shown  in  Fig.  104,  representing  the  same 
type  of  surface  shown  in  Figs.  139  and  140. 


An  old  valley  is  wide,  has  a  broad  flat  or  flood  plain  and  a  low 
gradient. 

A  stream  also,  as  well  as  its  valley,  passes  from  youth  to  ma- 


150  PHYSIOGRAPHY 

turity,  and  from  maturity  to  old  age.  In  its  youth  it  is  likely 
to  be  swift  and  impetuous,  unless  it  flows  through  low  land. 
In  maturity  it  is  much  steadier  in  its  flow,  and  when  it  reaches 
old  age  it  meanders  through  its  wide  plain.  Even  an  old 
stream,  however,  may  take  on  the  vigor  of  youth  when  it  is 
flooded. 

The  terms  youth,  maturity,  and  old  age  are  also  applied  to 
river  systems.     Every  river  system,   aided  by   weathering,  has 


FIG.  142. — Map  of  the  principal  streams  of  southern  New  Jersey,  and  outlines 
of  their  basins,  shown  in  dotted  lines. 

entered  upon  the  task  of  carrying  to  the  sea  all  the  land  of  its  basin 
which  is  above  base-level.  So  long  as  the  river  system  has  the 
larger  part  of  its  task  before  it,  it  is  young  (Fig.  1,  PL  XII).  In 
youth  the  land  is  often  ill  drained  and  may  have  many  ponds 
and  lakes  (PL  III).  When  the  main  valleys  have  become  wide 
and  deep,  and  the  areas  of  upland  have  been  well  cut  up  (dissected) 


PLATE  XII 


.  1. — Youthful  valleys.     Shore  of  Lake  Michigan  just  north  of  Chicago.    Scale 
1  —  mile  per  inch.     (Highwood  Sheet,  U.  S.  Geol.  Surv.) 


FIG.  2. — A  region  in  a  mature  stage  of  erosion.     Scale  2—  miles  per  inch. 
(Kentucky,  U.  S.  Geol.  Surv.) 


The  Niagara  Gorge.     Scale  1  —  mile  per  inch.     (Niagara  Falls  Sheet,  U.  S.  Geol.  Surv.) 


THE  WORK  OF  RUNNING  WATER 


151 


FIG.  143. — A  young  V-shaped  valley,  the  Stehekin  River,  Wash. 
(U.  S.  Geol.  Surv.) 


FIG.  144. — A  valley  much  older  than  that  shown  in  Fig.  143,  Gray  Copper 
Gulch,  southwestern  Colorado.     (U.  S.  Geol.  Surv.) 


152  PHYSIOGRAPHY 

by  valleys,  the  river  system  is  said  to  have  reached  maturity  (PI. 
XII,  Fig.  2).  The  land  is  then  well  drained.  When  the  task  of 
base-leveling  its  drainage  basin  is  nearing  completion,  the  river 
system  has  reached  old  age,  Fig.  3  (PL  IX).  The  master  stream 
of  a  drainage  system  attains  the  characteristics  of  maturity  and 
age  sooner  than  its  tributaries,  and  in  its  lower  course  sooner 
than  in  its  upper. 

The  topography  of  a  drainage  basin  is  youthful  when  its  river 
system  is  youthful,  mature  when  its  river  system  is  mature,  and 
old  when  its  drainage  is  old.  In  an  area  of  youthful  topography 
much  of  the  surface  has  not  yet  been  much  affected  by  erosion 
(Fig.  1,  PL  XII);  in  an  area  of  mature  topography  the  surface 
has  been  largely  reduced  to  slopes  by  erosion  (Fig.  2,  PL  XII); 
while  an  area  of  old  topography  is  one  which  has  been  brought 
down  to  general  flatness  by  erosion  (Fig.  3,  PL  IX).  Some 
parts  of  a  drainage  basin,  especially  those  parts  near  the 
master  stream,  may  take  on  the  characteristics  of  age,  while 
other  parts  farther  from  the  trunk  stream  may  not  be  advanced 
beyond  maturity  or  even  youth. 

MAP  EXERCISE 

Topographic  Maps  Showing  Erosion  Topography  in  Various  Stages  of 

Development 

I.  Study  the  following  maps  in  preparation  for  the  conference : 

1.  Emporia,  Kan.  7.  Fredonia,  Kan. 

2.  Kanawha,  W.  Va.  8.  Mt.  Guyot,  Tenn.— N.  C. 

3.  Prince  Frederick,  Md.  9.  Casselton,  N.  D. 

4.  Ridgeway,  N.  Y.  10.  Princeton,  Ind.— 111. 

5.  Canyon,  Wyo.  11.  Bright  Angel,  Ariz. 

6.  Yosemite,  Cal. 

II.  Suggestions  for  the  study  of  each  map : 

1.  Does  the  map  represent  a  plain,  plateau,  or  mountain  region? 

If  more  than  one  of  these  great  types  appears,  note  location 
of  each. 

2.  What  is  the  age  of  the  topography,  in  terms  of  erosion,  and  how 

is  it  shown?     (If  different  parts  of  an  area  are  in  different 
stages,  note  the  fact.) 


THE  WORK  OF  RUNNING  WATER  153 

3.  What  inferences  may  be  made  from  the  map  as  to  the  climate  of 

the  region  represented?    The  evidence  on  which  the  inference 
is  based? 

Note.  Does  the  evidence  (1)  merely  suggest  the  inference, 
or  (2)  make  the  inference  probable,  or  (3)  make  the  inference 
altogether  trustworthy? 

4.  Are  there  topographic  features  which  cannot  be  accounted  for 

by  the  erosion  of  running  water?     If  so,  where? 

Cycle  of  erosion.  The  time  necessary  for  the  development  of 
a  base-level  throughout  a  drainage  basin  is  a  cycle  of  erosion. 
This  period  of  time  is  very  long.  While  land  is  high  and  the 
streams  swift,  erosion  is  rapid;  but  the  nearer  the  land  approaches 
base-level,  the  slower  the  processes  of  erosion.  The  last  part 
of  the  process  of  base-leveling  is  therefore  the  slowest  of  all. 

Peneplains.  It  is  doubtful  whether  any  extensive  land  area 
was  ever  worn  down  to  a  perfect  base-level ;  but  great  areas  have 
been  worn  down  almost  to  that  level.  In  such  cases  low  hills  or 


FIG.  145. — A  peneplain  near  Camp  Douglas,  Wis.     (Atwood.) 

ridges  remain  between  the  valleys,  and  hard  bodies  of  rock  may 
rise  abruptly  above  the  general  level  of  the  plain  of  degradation. 
A  region  in  this  condition  is  called  a  peneplain  (an  almost-plain, 
Fig.  145).  It  has  a  surface  which  has  been  brought  nearly,  but 
not  quite,  to  base-level.  If  conspicuous  elevations  of  slight  extent 
remain  above  it,  they  are  monadnocks.  The  name  was  derived 
from  Mount  Monadnock  (N.  H.),  because  that  mountain  was  formed 
in  this  way. 


154  PHYSIOGRAPHY 

Rate  of  Land  Degradation 

Since  all  lands  are  being  cut  down  by  running  water,  it  is  a 
matter  of  interest  to  know  how  fast  they  are  being  brought  low. 
It  is  also  of  interest  to  know  whether  the  lands  are  to  be  destroyed 
altogether,  and  if  so,  how  long  they  are  to  last. 

It  has  been  estimated  recently  by  the  United  States  Geological 
Survey  that  the  Mississippi  River  carries  to  the  sea  yearly  about 
.340,500,000  tons  of  sediment  in  suspension,  and  about  136,400,000 
tons  in  solution.  Of  these  amounts,  the  Ohio  and  the  Missouri 
contribute  more  than  half,  and  the  Missouri  more  than  twice  as 
much  as  the  Ohio.  The  Colorado  River  is  estimated  to  remove 
387  tons,  on  the  average,  from  each  square  mile  of  its  basin.  The 
same  Survey  estimates  that  the  total  amount  of  sediment  carried 
from  the  United  States  to  the  sea  in  a  year  is  about  513,000,000 
tons,  and  the  amount  in  solution  about  270,000,000  tons.  On  the 
basis  of  these  figures,  it  is  estimated  that  the  land  is  being  degrad- 
ed at  the  average  rate  of  about  1  foot  in  9,120  years.  It  is  clear 
that  some  parts  of  the  country  are  being  cut  down  much  faster 
than  this,  and  other  parts  much  more  slowly. 

If  this  rate  were  to  be  continued  without  interruption,  and 
if  nothing  occurred  to  counteract  it,  the  North  American  conti- 
nent would  be  reduced  to  sea-level  in  about  18,000,000  years,  for 
its  average  height  is  about  2000  feet.  But  as  already  pointed 
out,  however,  the  present  rate  of  down-cutting  cannot  continue, 
for  as  the  land  becomes  lower  the  rate  of  erosion  must  diminish, 
since  the  water  must  then  move  more  slowly.  As  a  matter  of  fact, 
mechanical  erosion  by  running  water  would  cease  when  the  sur- 
face was  brought  to  base-level,  though  solution  would  still  go  on. 

Other  changes,  to  be  discussed  later,  are  likely  to  occur  to  pre- 
vent the  land  from  being  worn  down  to  base-level.  The  continent 
is  therefore  likely  to  endure  not  only  much  longer  than  18,000,000 
years,  but  probably  indefinitely.  Nevertheless,  these  figures  serve 
a  useful  purpose  in  indicating  the  rate  of  change  which  the  land 
is  undergoing  as  the  result  of  the  fall  of  rain  and  snow  upon  it. 

Conditions  affecting  the  rate  of  erosion.  Some  of  the  condi- 
tions affecting  the  rate  of  erosion  by  running  water  have  been 
stated  or  implied  in  the  preceding  pages.  By  way  of  summary 
they  may  be  brought  together  at  this  point. 

The  rate  at  which  running  water  wears  down  the  surface  over 


THE  WORK  OF  RUNNING  WATER 


155 


which  it  flows  depends  largely  on  (1)  the  volume  of  the  water, 
(2)  its  velocity,  (3)  the  character  of  the  material  over  which  it 
flows,  and  (4)  the  amount  and  character  of  the  load  it  carries. 

(1)  The  volume  of  water  flowing  over  the  land  outside  of  streams 
depends  chiefly  on  the  rainfall.     The  volume  of  a  stream  depends 
chiefly  on  (a)  the  area  which  it  drains,  and  (6)  the  amount  of  pre- 
cipitation within  its  basin.     The  larger  the  area  and  the  greater 
the  amount  of  the  precipitation  the  larger  the  stream. 

(2)  The  velocity  of  running  water  depends  on  (a)  its  gradient  or 
slope,  (6)  its  volume,  especially  its  depth,  (c)  its  load,  and  (d)  the 
shape  and  configuration  of  its  channel.     The  higher  the  gradient, 
the  greater  the  volume,  the  less  the  load,  and  the  smoother  and 
narrower  its  channel,  the  faster  the  flow. 

The  effect  of  slope  on  velocity  needs  no  explanation.  That 
increase  of  volume  increases  the  rate  of  flow  is  shown  by  the 
familiar  fact  that  a  stream  in  flood  runs  faster  than  at  other  times. 
The  erosive  force  of  a  flooded  stream  has  already  been  referred 
to  (Figs.  101  and  102).  The  carrying  of  sediment,  in  whatever 
form,  is  a  tax  on  the  stream's  energy,  and  the  more  the  load  the 
greater  the  tax.  The  energy  used  in  carrying  is  taken  from  the 
energy  which  would  otherwise  be  available  for  flowing.  A  smooth 
channel  offers  less  friction  than  a  rough  one,  and  so  favors  high 
velocity.  But,  apart  from  smoothness,  the  channel  which  favors 

great  velocity  is  that  which  offers 
least  area  of  contact  with  the  water. 
Thus    a    broad    shallow    channel 
(Fig.  146)  has  a  greater  surface  of 
contact   with   the    water   than    a 
deeper,    narrower     channel    (Fig. 
147).     The  water  in  the  former  has 
nore  friction  with  its  bed,  and  fric- 
tion retards  the  current.     Nearly 
FIG.  147.— A  deeper  and  nanower    all  streams  which  flow   now  in  a 
channel  than  that  shown  in  Fig.    narrow  channel  and  now  in  a  wide 
146,  with  the  same  gradient.     A  ,  ,      .  , 

stream  in  a  channel  such  as  is    one,  have  greater  velocity  where 


FIG.  146.- 


-A  broad,  shallow  river 
channel. 


represented  in  Fig.  147  will 
flow  faster  than  one  in  such  a 
channel  as  that  shown  in  Fig. 
146. 


their  channels  are  narrowed. 

(3)  The  character  of  the  surface 

of  its  basin,  and  especially  the 
character  of  the  material  in  its  channel,  also  influences  the  rate  of 
a  stream's  erosion.  If  the  surface  of  the  land  on  which  the  rain 


156 


PHYSIOGRAPHY 


FIG.  148. — Oneonta  Gorge,  Canyon  of  the  Columbia,  Ore.     (Fairbanks.) 


FIG.  149.— Grand  Canyon  of  the  Colorado.      (Peabody.) 


THE  WORK  OF  RUNNING  WATER  157 

falls  is  bare  solid  rock,  the  immediate  run-off  brings  little  sedi- 
ment to  the  stream,  and  if  the  bed  of  the  stream  is  bare  solid  rock, 
the  stream  wears  it  less  than  if  it  is  of  mud  or  sand. 

(4)  To  work  most  effectively,  the  stream  must  carry  load  (tools) 
enough  to  enable  it  to  cut  rapidly,  but  not  so  much  as  to  make 
it  flow  so  slowly  that  it  cannot  use  its  tools  effectively. 


Exceptional  Features  Developed  by  Erosion 

Canyons  and  gorges.  When  valleys  are  so  narrow  and  deep 
as  to  be  striking,  they  are  called  gorges  or  canyons.  In  general 
canyons  are  larger  than  gorges,  though  there  is  no  sharp  distinction 
between  them.  The  sides  of  small  gorges  and  young  canyons 
are  sometimes  nearly  vertical  (Fig.  148),  but  the  sides  of  the  large 
canyons  are  rarely  so  (Fig.  149).  The  distinction  between  a  canyon 
and  a  valley  which  is  not  a  canyon  is  not  a  very  sharp  one,  and,  in 
regions  where  canyons  abound,  the  term  is  often  applied  to  all 
valleys. 

The  Colorado  canyon  (Figs.  27,  27a,  and  149)  is  the  greatest 
canyon  known.  Its  maximum  depth  is  about  a  mile,  but  where 
it  has  this  depth,  it  is  often  8  to  10  miles  wide  from  rim  to  rim, 
though  very  narrow  at  the  bottom.  With  a  depth  of  one  mile  and 
a  width  of  8  miles,  the  slope,  if  uniform,  would  have  an  angle  of 
less  than  15°.  The  cross-section  of  such  a  valley  is  shown  in  Fig. 

150.  But  the  slopes  of  the 
canyon  are  not  uniform,  as 
shown  by  Fig.  151.  The  ir- 

FIG.  150.— Diagram  showing  the  proper-  regularities  of  slope  are  caused 
tions  of  a  valley  the  width  of  which  .  •       i      i 

is  eight  times  the  depth.  These  are  by  inequalities  in  the  hardness 
approximately  the  proportions  of  the  of  the  rock  of  the  canyon 
Colorado  Canyon. 

walls. 

The  Yellowstone  River  also  has  a  notable  canyon  1000  feet  or 
so  deep  (Fig.  152  and  Fig.  1  cf  PI.  IV).  Its  wiith  is  less  in  pro- 
portion to  its  depth  than  that  of  the  canyon  of  the  Colorado. 

Narrow  valleys  mean  that  the  processes  of  valley  deepening 
have  outrun  the  processes  of  valley  widening.  This  in  turn  means 
that  the  stream  which  made  the  gorge  or  the  canyon  was  swift, 
or  that  the  processes  of  valley  widening  (p.  131)  were  slow,  or 
both. 


158  PHYSIOGRAPHY 

Valleys  are  deepened  rapidly  when  their  gradients  are  high 
and  the  streams  strong.  They  are  widened  slowly  when  the  climate 
is  arid  so  that  there  is  little  slope-wash,  when  the  stream  is  so 
swift  that  it  does  not  meander,  and  when  the  material  of  the  sides 
is  such  that  it  will  stand  with  steep  slopes.  Solid  rock,  for  ex- 
ample, will  stand  with  steeper  slopes  than  loose  sand.  We  con- 
clude that  (1)  great  altitude,  (2)  arid  climate,  (3)  strong  streams, 
and  (4)  a  rock  structure  which  will  stand  in  steep  slopes,  favor 
the  development  of  valleys  of  the  canyon  type.  In  other  words, 
youthful  valleys  in  plateaus  and  mountains  are  likely  to  be  canyons, 


^^!M&'^^'-'^^ 

FIG.  151. — Cross-section  of  the  Colorado  Canyon. 
(After  Gilbert  and  Brigham.) 

if  climate  and  rock  structure  favor.  The  plateaus  of  the  western 
part  of  the  United  States  furnish  these  conditions,  and  canyons 
are  there  common.  This  is  true  not  only  of  main  streams  but 
of  their  tributaries  as  well. 

A  strong  stream  in  an  arid  region  is  possible  when  the  valley 
is  supplied  with  abundant  water  from  a  humid  region  above. 
The  Colorado  River  is  an  example. 

Since  gorges  often  occur  in  humid  regions,  it  is  clear  that  all 
the  conditions  favoring  the  development  of  canyons  need  not 
be  present  in  order  to  develop  gorges.  Thus  the  Niagara  River 
has  a  gorge  or  canyon  below  the  falls  (PI.  XIII).  Here  the 
down-cutting  is  so  rapid  that  the  processes  of  valley  widening 
have  not  kept  pace  with  it,  in  spite  of  the  fact  that  the  region  is 
humid. 

The  deeper  canyons  of  the  west  constitute  well-nigh  impassable 
barriers  to  travel  athwart  their  courses,  while  their  rivers  rarely 
serve  the  needs  of  commerce  or  irrigation.  Considerations  of 
defence  doubtless  led  the  cliff-dwellers  to  make  their  homes  in  the 
almost  inaccessible  canyon  walls. 

Canyons  must  ultimately  develop  into  valleys  of  another 
type,  for  the  stream  of  the  canyon  will  ultimately  cut  to 
base-level.  The  valley  will  then  cease  to  become  deeper,  but 


THE  WORK  OF  RUNNING  WATER 


159 


the  processes  of  valley  widening  will  still  go  on,  and  the 
narrow  valley  will  become  wider  and  wider  until  it  ceases  to 
be  a  canyon. 


FIG.  152. — The  canyon  of  the  Yellowstone  below  the  falls. 
Yellowstone  National  Park. 

Bad  lands.  To  a  type  of  topography  developed  in  early 
maturity  in  certain  high  regions  where  the  rock  is  but  slightly, 
though  unequally,  resistant,  a  special  name,  bad  land,  is  some- 
times given.  Some  idea  of  bad-land  topography  is  gained  from 


160 


PHYSIOGRAPHY 


FIG.  153. — Bad-land  topography  north  of  Scott's  Bluff,  Neb. 
(U.  S.  Geol.  Surv.) 


FIG.  154. — Bad-land  topography  southwest  foot  of  Mesa  Verde,  Colo. 
(U.  S.  Geol.  Surv.) 


THE  WORK  OF  RUNNING  WATER 


161 


Figs.  153  and  154.  Bad-land  topography  is  found  in  various 
localities  in  the  West,  conspicuously  in  western  Nebraska,  in 
Wyoming  and  the  western  parts  of  the  Dakotas.  The  formations 
here  are  often  beds  of  sandstone  or  shale,  alternating  with  un- 
indurated  beds  of  clay.  Climatic  factors  are  also  concerned  in 
the  development  of  bad-land  topography.  A  semi-arid  climate, 
where  the  precipitation  is  much  concentrated,  seems  to  be  most 
favorable  for  its  development. 

Natural  bridges.      If  a  stream  flowing  over  jointed  rock  has 
falls,  the  conditions  are  sometimes  afforded  for  the  development  of 


fe^=^ 

-J-eq 

_^  — 
1 

~r 

rH 

( 

^r 

^3 

?  ?'  <'v\ 

^T^ 

2 

; 

5d 

'.r^.A/j 

—U^-U 

FIG.  155. — Diagram  to  illustrate  the  initial  stage  in  the  development  of  a 
natural  bridge.  Longitudinal  section  at  the  left,  cross-section  at  the 
right. 

an  exceptional  and  striking  scenic  feature.  If  above  a  waterfall 
there  were  an  open  joint  in  the  bed  of  the  stream  (as  at  6,  Fig.  155), 
some  portion  of  the  water  would  descend  through  it.  After  reach- 
ing a  lower  level  it  might  find  or  make  a  passage  through 
the  rock  to  the  river  below  the  falls.  If  even  a  little  water 


FIG.  156. — A  stage  later  than  that  shown  in  Fig.  155. 


takes  such  a  course,  the  flow  will  enlarge  its  channel,  making  a 
passageway  from  the  joint  through  which  the  water  descends  to 
the  valley  below  the  fall  (bcde,  Fig.  155).  This  passageway 
may  become  large  enough  to  accommodate  all  the  water  of  the 
river.  In  this  case,  the  entire  fall  would  be  transferred  from  the 
position  which  it  previously  occupied  (/)  to  the  position  of  the 
enlarged  joint  (&).  The  fall  would  then  recede.  The  under- 
ground channel  between  the  old  falls  and  the  new  would  be  bridged 
by  rock  (&/"  and  /",  Fig.  156),  making  a  natural  bridge.  A  bridge 
of  this  sort  is  now  in  process  of  development  in  Two  Medicine 


162 


PHYSIOGRAPHY 


FIG.  157. — A  partially  developed  natural  bridge  in  Two  Medicine  River, 
Mont.     (Whitney.) 


FIG.  158.— The  Natural  Bridge  of  Virginia.     (U.  S.  Geol.  Surv.) 


THE  WORK  OF  RUNNING  WATER  163 

River  in  northwestern  Montana  (Fig.  157).  Once  in  existence, 
a  natural  bridge  will  slowly  weather  away.  The  natural  bridge 
near  Lexington,  Va.  (Fig.  158),  almost  200  feet  above  the  stream 
which  flows  beneath  it,  is  believed  to  have  been  developed  in  this 
way.  It  is  not  to  be  understood,  however,  that  all  natural  bridges 
have  had  this  history  (see  p.  98). 

Effects  of  Inequalities  of  Hardness  of  Rock 

Rapids  and  falls.  The  bed  of  a  stream  is  often  steeper  at 
some  point  than  at  others  (Fig.  159),  and  there  the  stream  flows 
more  rapidly.  In  such  a  case  as  that  illustrated  by  Fig.  159  the 
quickened  flow  constitutes  a  rapid.  If  the  water  in  a  stream's 


FIG.  159. — Chandlar  Rapids  in  river  of  the  same  name  in  Alaska. 
(U.  S.  Geol.  Surv.) 

bed  drops  over  a  cliff,  it  makes  a  waterfall  (Figs.  160,  161,  and  162). 
Between  a  waterfall  and  a  rapid  there  are  all  gradations  (Fig.  163). 
Steep  rapids  are  often  called  falls,  and  both  are  sometimes  called 
cascades. 

Falls  and  rapids  occur  in  many  places  and  under  many  con- 
ditions, but  they  are  most  common  where  the  material  of  the  valley 


164 


PHYSIOGRAPHY 


FIG.  160.— Niagara  Falls.     (U.  S.  Geol.  Surv.) 


FIG.  161. — The  lower  falls  of  the  Yellowstone. 


THE  WORK  OF  RUNNING  WATER 


165 


FIG.  162.— Twin  Falls,  Snake  River.     (U.  S.  Geol.  Surv.) 


FIG.  163. — Rustic  Falls.     A  succession  of  slight  falls  in  the  Yellow- 
stone Park.      (U.  S.  Geol.  Surv.) 


166 


PHYSIOGRAPHY 


bottom  is  of  unequal  hardness.  They  are  commonly  located  where 
the  river  passes  from  more  resistant  rock  to  that  which  is  less 
resistant. 

The  falls  and  the  rapids  of  many  rivers  add  greatly  to  their 
beauty,  and  sometimes  enhance  their  value  to  mankind  by  afford- 
ing abundant  water-power.  Niagara  Falls  affords  about  4,000,000 
horse-power,  so  much  of  which  has  been  or  seems  likely  to  be 
granted  to  manufacturing  companies,  that  a  movement  has  been 
begun  to  "save  the  falls."  The  Falls  of  St.  Anthony  did  much 
to  make  Minneapolis  the  greatest  flour  manufacturing  city  of  the 
world.  Some  of  the  great  manufacturing  cities  of  New  England 
also  grew  up  about  low  falls  and  rapids.  Advantageous  as  falls 
are  as  a  source  of  power,  they  are  enemies  of  navigation.  The 
falls  (really  rapids)  of  the  Ohio  necessitated  the  breaking  of  bulk 
at  that  point,  and  so  determined  the  location  and  early  growth 
of  Louisville.  A  canal  designed  to  evade  the  obstacle  was  com- 
pleted in  1830. 

Falls  and  rapids  are  undergoing  constant  change,  although 
the  change  is  usually  very  slow.  The  falls  of  the  Niagara  are 
receding  up-stream,  because  the  falling  water  undermines  the 
hard  layer  of  rock  over  which  it  is  precipitated  (Fig.  164).  As  a 
fall  recedes,  it  generally  becomes  lower.  In  such  cases  it  is  clear 

that  the  fall  will  disappear  if  it 
recedes  far  enough.  If  the  hard 
rock  over  wrhich  the  water  drops 
be  in  the  position  shown  in  Fig. 
165,  the  fall  will  not  recede, 
though  it  will  become  lower  and 
will  disappear  when  the  stream 
cuts  down  to  base-level  where 
the  fall  is.  Rapids  and  falls  are 
therefore  temporary  features  of 
Like  canyons,  they 
are  marks  of  youth,  for  they 
show  that  the  stream  is  well  above  base-level.  In  time,  all  existing 
rapids  and  waterfalls  will  disappear,  for  they  can  no  longer  exist 
after  rivers  have  reached  base-level,  the  goal  of  every  stream. 

From  a  waterfall  we  may  reason  backward  in  time  as  well 
as  forward.  If  existing  falls  are  to  disappear,  was  there  a  time 
before  thev  existed? 


FIG.    164. — Diagram    illustrating    the   streams 
conditions  at  Niagara.     (Gilbert.) 

"  marks 


THE  WORK  OF  RUNNING  WATER 


167 


Suppose  the   material   along   the   line  followed  by   vigorous 
drainage  to  be  of  unequal  hardness.    The  less  resistant  part  will  be 


FIG.  165. — Diagram  illustrating  a  condition  where  a  fall  will  not  recede. 

worn  more  rapidly  than  the  more  resistant  part  farther  up  the 
stream,  with  the  result  shown  in  Fig.  166.     The  continued  wear 


FIG.  166. — Diagram  illustrating  the  development  of  a  fall  where  the  hard 
layer  dips  up-stream. 

of  the  water  in  such  a  case  will  cause  the  rapids  at  a  to  become 
more  rapid,  and  the  process  of  steepening  the  bed  of  the  descend- 
ing water  will  go  on  until  the  rapids  become  a  fall.  In  this  case, 
the  rapid.s  and  falls  depend  on  inequalities  of  hardness  discovered 
by  the  stream  in  the  excavation  of  its  valley.  This  is  perhaps  the 
commonest  way  in  which  falls  and  rapids  originate.  Falls  originat- 
ing in  this  way  are  developed  gradually.  Such  falls  may  be  called 
subsequent  falls,  since  they  do  not  depend  on  the  original  shape 
of  the  surface. 

In  other  cases  the  surface  run-off,  in  following  its  course  to 
the  sea,  may  reach  a  cliff  and  plunge  over  it.  In  this  case,  the 
steep  descent  of  surface  existed  before  the  stream  found  it,  and  the 
falls  began  when  the  river  came.  Since  such  falls  result  from  the 
irregularities  of  surface  over  which  the  river  began  to  flow,  they 
may  be  called  consequent  falls.  A  good  example  of  such  a  fall  is 
that  of  the  Niagara,  formed  when  the  outflow  from  Lake  Erie 
reached  and  fell  over  a  cliff  on  its  way  to  Lake  Ontario.  Since 
the  fall  began  it  has  receded  some  seven  miles. 

Falls  are  formed  in  still  other  ways.  A  landslide  or  a  lava- 
flow  may  form  a  dam,  over  which  the  water  falls  or  flows  in  rapids. 
In  such  cases,  especially  the  former,  the  dams,  and  therefore  the 
rapids  and  falls,  are  often  temporary. 


168 


PHYSIOGRAPHY 


At  the  bottoms  of  falls  pot-holes  (Fig.  167)  are  sometimes  de- 
veloped.    The  start  is  made  as  a  result  of  slight  inequalities  in  the 


FIG.  167. — Pot-holes  in  granite.     Upper  Tuolumne  River,  Cal. 

surface  of  the  rock.  The  holes  reach  their  conspicuous  size  as 
the  result  of  wear  by  stones  kept  in  motion  in  them  by  the  eddies  of 
the  falling  water. 


FIG.  168. — Diagram  showing  a  narrow  place  in  a  valley  where  the  stream 
crosses  a  hard  layer  of  rock. 


Narrows.  When  a  stream  cuts  through  a  bed  of  hard  rock, 
it  not  only  develops  rapids  and  falls,  but  the  hard  rock  affects  the 
valley  in  other  ways.  The  resistant  rock  weathers  less  rapidly 


THE  WORK  OF  RUNNING  WATER 


169 


than  the  weak  rock,  and  hence  the  valley  is  narrower  where  the 
rock  is  resistant  than  where  it  is  weak.  Such  a  constriction  of 
the  valley  is  a  narrows  (Fig.  168)  or  a  water-gap.  The  Delaware 
Water  Gap  through  the  Kittatinny  Mountain  is  a  well-known 


FIG.  169. — The  Lower  Narrows  of  the  Baraboo  River,  Wisconsin. 
(At  wood.) 


example.  The  narrows  of  the  Baraboo  River  in  Wisconsin  (Fig.  169) 
is  another  good  example.  Unlike  falls,  narrows  are  not  most 
conspicuous  in  the  youth  of  the  stream,  but  at  a  later  time,  after 
the  valley  has  been  much  widened  in  the  weak  rock  adjacent  to 


FIG.  170. — Rock  terraces,  due  to  resistant  layers  of  rock. 

that  which  is  resistant.  Falls  are  common  in  horizontal  or  nearly 
horizontal  beds,  but  narrows  are  commonly  developed  in  stratified 
rock  only  where  the  beds  are  tilted. 

Narrows  sometimes  serve  as  gateways  through  mountains, 
and  so  control  lines  of  travel  and  transportation.  The  narrows 
of  Wills  Creek  in  Wills  Mountain,  Maryland,  may  serve  as  an 
example.  From  Fort  Cumberland  (site  of  Cumberland),  built  by 
the  Ohio  Company  to  guard  the  important  passageway,  Nemacolin's 
Path  and  Washington's  and  Braddock's  roads  ran  west  through  it, 


170 


PHYSIOGRAPHY 


and  the  Cumberland  National  Road  and   an  important  railway 
now  pass  through  it. 


FIG.  171. — A  monadnock:  a  mass  of  igneous  rock  isolated  by  erosion  and 
remaining  because  of  its  superior  hardness.  Matteo  Tepee,  Wyo. 
(Detroit  Photo.  Co.) 


FIG.  172. — Hogbacks,  due  to  the  erosion  of  tilted  beds  of  unequal  resistance. 
The  harder  layers  stand  up  as  ridges  and  constitute  the  "hogbacks." 
(Powell.) 

Rock    terraces.     Again,  if  the  hard   layer  through  which  a 
stream  cuts  is  horizontal,  the  resistant  rock  weathers  less  rapidly 


THE  WORK  OF  RUNNING  WATER 


171 


than  the  weaker  rock  above  and  below,  giving  rise  to  rock  terraces, 
as  shown  in  Fig.  170. 

Monadnocks,    rock    ridges,    etc.     Elsewhere  than   in  valleys, 


FIG.  173. — A  butte.     A  characteristic  feature  of  the  arid  plateau  region  of 
the  West.     The  butte  is  really  a  monadnock.     (U.  S.  Geol.  Surv.) 

too,  rock  of  more  than  average  resistance  makes  itself  felt  in  the 
topography,  for  rain-wash,  wind,  and  most  phases  of  weathering 


FIG.  174.— The  Encnanted  Mesa.  A  striking  butte  in  New  Mexico.  The 
name  mesa  is  not  commonly  applied  to  elevations  of  such  small  summit 
area.  (R.  T.  Chamberlin.) 

affect  resistant  rock  less  than  weak  rock.     The  result  is  that  hard 
rock  often  remains  as  hills,  or  even  as  mountains  (monadnocks, 


172 


PHYSIOGRAPHY 


p.  153),  after  the  weaker  rock  about  it  has  been  reduced  nearly 
to  base-level.     Fig.  171  is  an  example.     An  elongate  narrow  ridge, 


FIG.  175. — A  canoe-shaped  valley  bordered  by  a  >  ridge  formed  by 
the  outcrop  of  a  hard  layer.     (Willis.) 

due  to  the  isolation  of  a  tilted  layer  of  resistant  rock,  is  sometimes 
called  a  "  hogback"  (Fig.  172).  * 


FIG.  176. — Diagram  to  illustrate  the  effects  of  erosion  on  a  fold  or  anticline, 
both  ends  of  which  dip  down  (or  plunge).     (Willis.) 


In  the  West  similar  elevations  are  often  called  buttes  (Figs.  171 
and  174).     A  hard  stratum  of  rock,  such  as  a  lava-bed,  overlying 


THE  WORK  OF  RUNNING  WATER 


173 


less  resistant  formations,  such  as  clay  or  soft  shale,  often  gives 
rise  to  buttes.  If  such  an  elevation  has  a  considerable  expanse  of 
surface  at  its  top,  it  is  a  mesa  (Fig.  28),  though  this  term  is  also 
applied  to  wide  terraces,  especially  if  high. 

The  elevations  due  to  the  isolation  of  outcrops  of  hard  rock 
by  the  removal  of  their  less  resistant  surroundings  often  take  on 
peculiar  forms,  dependent  on  the  structure  of  the  rock.  Elongate 
ridges  are  common  where  the  strata  are  folded.  Where  the  tops 
of  the  original  folds  were  not  horizontal,  erosion  gives  the  ridges 
which  result  from  the  isolating  of  the  outcrops  of  hard  rock 
peculiar  forms,  as  shown  in  Figs.  175  and  176.  Such  forms  are 
not  uncommon  in  the  Appalachian  Mountains. 

Accidents  to  Streams 

Drowning.  Streams  are  subject  to  many  accidents.  If  the  land 
through  which  they  flow  sinks  so  as  to  decrease  their  gradients, 


FIG.  177. 


FIG.  178. 


FIG.  177. — Chesapeake  Bay  and  its  surroundings.  The  bay  is  a  drowned 
river  valley,  and  the  lower  ends  of  its  tributary  valleys  are  also  drowned. 

FIG.  178. — The  drainage  of  the  region  about  Chesapeake  Bay  as  it  would 
.have  been  but  for  drowning. 

they  flow  less  rapidly,  or  may  even  cease  to  flow.     If  the  lower 
end  of  a  valley  sinks  below  sea-level,  sea-water  enters  and  forms 


174 


PHYSIOGRAPHY 


an  estuary.  In  such  cases  the  lower  end  of  the  river  and  its  valley 
are  drowned.  If  along  a  coast  the  streams  end  in  bays,  the  inference 
is  that  the  coast  has  sunk,  and  that  the  rivers  and  valleys  have  been 
drowned.  The  Atlantic  coast  between  New  York  and  the  Carolinas  is 
a  good  example  (Fig.  177).  Delaware  Bay,  Chesapeake  Bay,  and 
numerous  other  smaller  bays  mark  the  sites  of  drowned  rivers.  With- 
out the  drowning,  the  drainage  of  this  region  would  be  somewhat 
as  shown  in  Fig.  178.  By  comparison  of  these  figures,  it  is  apparent 
that  drowning  has  the  effect  of  isolating  the  parts  of  a  river  system. 
Rejuvenation.  If  the  basin  of  an  old  stream  is  raised,  so  that 
the  gradient  of  the  stream  is  increased,  its  velocity  is  increased, 
and  it  again  takes  on  the  characters  of  youth.  Such  a  stream  is 
said  to  be  rejuvenated  (Fig.  179).  The  rejuvenation  of  a  stream 
means  the  beginning  of  a  new  cycle  of  erosion,  even  though  the 
preceding  one  was  incomplete. 

If  the  old  stream  was  meandering  in  its  valley,  as  old  streams 
often  do,  the  quickened  stream  cuts  its  meanders  deeper.  The 
meanders  are  thus  entrenched.  Where  a  stream  has  entrenched 
meanders,  there  is  a  strong  presumption  that  it  has  been  reju- 
venated. Entrenched  meanders  are 
:;hown  by  many  streams.  Plate 
XIV  shows  the  entrenched  meanders 
of  the  Conodoguinet  River  in  Penn- 
sylvania. Young  valleys  in  the  bot- 
toms of  old  ones,  and  entrenched 
meanders,  are  among  the  more  com- 
mon marks  of  a  second  cycle  of 
erosion. 

Ponding.     If    a  portion    of    the 
stream's  bed  is  warped  upward,  the 
gradient  above  the  point  of  up-warp 
is  lessened,  the  flow  is  checked,  and 
the  stream  widened.    Streams  above 
such  an  obstruction  are  ponded,  that 
!is,  the  waters  accumulate  in  a  pond 
FIG.  179.— Diagram  to  illustrate    or   lake.     If   the   up-warp    is   great 
an  ideal  case  of  rejuvenation    enough   ft  may  completely  dam  the 
as  the  result  of  uplift.     The 
black  area  at  the  bottom  repre-    stream.     Streams     are     also     some- 

sents  the  sea.  times  ponded  by  lava-flows,  by  land- 

slides, etc.,  and  by  darm  made  by  man.      The  mill-ponds  along 


PLATE  XIV 

"7" 


Entrenched  meanders.      Scale  1—  mile  per  inch.      (Harrisburg,  Pa.,  Sheet,  U.  S. 

Geol.  Surv.) 


THE  WORK  OF  RUNNING   WATER 


175 


numerous  creeks  are  illustrations  of  streams  ponded  in  the  last  of 
these  ways.  If  the  ponded  waters  flow  out  over  the  dam,  they  will 
ultimately  cut  it  down.  If  the  dam  is  sufficiently  high,  the  water 


FIG.  180. — A  rejuvenated  stream,  the  Wisconsin  River.  The  stream  is  large 
and  swift,  and  flows  through  a  young,  narrow  gorge,  cut  in  the  plain 
shown  in  Fig.  145. 

may  be  forced  out  of  its  valley  altogether  and  find  a  new  course. 

Piracy.     One  stream  may  steal  another.     One  way  in  which 
this  is  done  is  suggested  by  Figs.  181  and  182.     The  head  of  a 


FIG. 181. 


FIG.  182. 


Diagrams  to  illustrate  a  phase  of  piracy.  By  the  headward  growth  of  a, 
Fig.  181,  it  reaches  6,  and  carries  off  its  upper  waters,  a,  Fig.  182,  is 
a  pirate;  b,  Fig.  182,  has  been  diverted,  and  c  has  been  beheaded. 

valley  as  at  a,  Fig.  181,  may  work  back  until  it  reaches  the  channel 
of  another  stream,  such  as  b.  It  then  carries  off  the  water  coming 
down  to  b  (Fig.  182).  The  stealing  of  one  stream  by  another  is 
stream  piracy.  The  stream  which  steals  is  a  pirate.  The  stream 


176 


PHYSIOGRAPHY 


stolen  is  diverted,  and  the  stream  which  has  lost  its  upper  waters 
is  beheaded.  When  a  stream  is  diverted  from  a  narrows  or  water- 
gap,  the  latter  becomes  a  wind-gap.  Such  gaps  are  common  in  most 
mountain  regions. 

The  numerous  wind-gaps  of  the  Blue  Ridge  Mountains  figured 
prominently  in  the  westward  movement  and  in  the  strategy  of  the 
Virginia  campaigns  of  the  Civil  War.  Farther  south,  the  Cumber- 
land Gap  afforded  the  early  emigrant  the  most  available  route 
across  the  mountains,  and  during  the  last  quarter  of  the  eighteenth 
century  probably  more  than  300,000  people  passed  through  it  to 
settle  in  Kentucky  and  Tennessee. 


THE 
KITTATINNY 

PLAIN 


THE 

5HENANDOAH 
PLAIN 


FIG.  183.  FIG.  184. 

The  capture  of  the  head  of  Beaverdam  Creek  by  the  Shenandoah  River. 
Virginia-West  Virginia.     (After  Willis.) 

Piracy  has  been  much  more  common  among  rivers  than  is 
generally  known.  In  the  Appalachian  region,  for  example,  where 
the  conditions  for  piracy  have  been  favorable,  there  are  few  large 
streams  which  have  not  either  increased  their  waters  by  piracy, 
or  suffered  loss  by  the  piracy  of  others.  Figs.  183  and  184  afford 
one  illustration.  Piracy  is  favored  by  inequalities  of  hardness, 
for  the  streams  which  do  not  cross  hard  rock  deepen  their  channels 
more  readily  than  those  which  do. 


THE  WORK  OF  RUNNING  WATER 


177 


Consequent  and  Antecedent  Streams 

When  streams  develop  on  a  land  surface  in  harmony  with  its 
slope,  they  are  said  to  be  consequent  (Fig.  185),  that  is,  consequent 
on  the  slope  of  the  surface  in  which  they  developed.  After  streams 


FIG.  185. — A  consequent  stream  whose  course  is  in  harmony  with  that  oi 
the  slope  of  the  area  it  drains. 

have  established  their  courses,  the  land  surface  which  they  drain 
may  be  warped  or  deformed,  but  the  deformation  may  go  on  so 
slowly  that  the  streams  are  able  to  hold  their  courses  established 
before  it  began  (Fig.  186).  The  streams  then  have  courses  which 
they  would  not  have  taken  had  the  deformation  taken  place 
before  they  were  established.  Such  streams,  with  courses  ante- 
dating the  present  general  slope  of  the  surface  and  out  of  harmony 


FIG.  186. — An  antecedent  stream.  The  stream  and  its  valley  are  conceived 
to  have  developed  as  consequent  stream  and  valley.  An  up-warp  athwart 
the  valley  then  took  place,  but  so  slowly  that  the  stream  cut  down  its  bed 
as  fast  as  the  up-warp  raised  it,  so  that  the  stream  held  its  old  course, 
not  now  in  harmony  with  the  slope  of  the  area  drained. 

with  it,  are  antecedent  streams.  They  may  at  the  outset  have 
been  consequent,  but  they  ceased  to  be  consequent  when  the 
deformation  took  place. 


178  PHYSIOGRAPHY 

MAP  EXERCISE 

Maps  Showing  the  Topographic  Effects  of  (1)  Inequalities  of  Hardness, 
(2)  Piracy,  (3)  Cycles  of  Erosion,  etc. 

I.  Study  the  following  maps  in  preparation  for  the  conference: 

1.  Niagara,  N.  Y.  7.  Harpers  Ferry,  Va. — W.  Va. — Md 

2.  London,  Ky.  8.  Monterey,  Va.— W.  Va. 

3.  Charleston,  W.  Va.  9.  Kaaterskill,  X.  Y. 

4.  Tuscumbia,  Mo.  10.  Pawpaw,  Md. — Pa. 

5.  Lancaster,  Wis. — la. — 111.  11.  Relay,  Md. 

6.  Independence,  Kan.  12.  Fredericksburg,  Va. — Md. 

II.  Apply  each  of  the  following  questions  to  each  of  the  above  maps: 

1.  The  age  of  the  topography  in  terms  of  erosion?     Reasons? 

Are  different  parts  of  the  area  represented  in  different  stages 
of  topographic  development? 

2.  Is  more  than  one  cycle  of  erosion  shown?     If  so,  the  evidence? 

3.  Is  there  any  indication  of  inequalities  of  hardness  in  the  under- 

lying rock?     If  so,  what? 

4.  Is  there  anything  in  the  topography  to  indicate  the  position  of 

the  strata  beneath  the  region?    If  so,  what? 

5.  What  inferences  can  be  made  from  the  map  as  to  (1)  the 

climate,  (2)  the  density  of  the  population,  and  (3)  the  occu- 
pations of  the  people? 

Note.  In  making  inferences  from  the  topographic  map, 
the  measure  of  certainty  or  uncertainty  should  be  carefully 
recognized.  Some  inferences  may  be  certain,  some  almost 
certain,  some  probable,  some  possible  but  not  very  probable, 
etc.  The  student  should  note  to  which  of  these  classes  each 
inference  belongs. 

III.  Questions  on  individual  maps: 
Monterey  and  Charleston  Sheets. 

Compare  and  contrast  (1)  the  topography  and  (2)  the  drainage 

of  the  two  areas  represented  by  these  sheets. 
Lancaster  Sheet. 

1.  Account  for  the  depression  which  extends  between  Sageville 

and  Dubuque,  in  the  southern  part  of  the  map. 

2.  What  is  the  probable  explanation  of  Sinsinawa  and  Sherrill 

mounds,  in  the  southern  part  of  the  region? 
Kaaterskill  Sheet. 

1.  Find  a  case  of  piracy  shown  on  the  map. 

2.  Is  there  any  chance  for  future  piracy  in  this  region? 


THE   WORK  OF  RUNNING   WATER  179 

3.  The  possible  explanations  of  the  steep  slope  in  the  eastern 

part  of  the  area?    The  probable  explanation? 
Pawpaw  Sheet. 

1.  How  are  the  great  meanders  of  the  Potomac  to  be  accounted 

for,  in  view  of  the  fact  that  the  valley  is  narrow? 
Harpers  Ferry  Sheet. 

1.  What  is  the  origin  of  Snickers  Gap  in  the  Blue  Ridge? 
London  Sheet. 

1.  Account  for  the  large  depressions  near  Lincoln,  in  the  south- 
west, and  elsewhere. 
Relay  Sheet. 

1.  What  indication  is  there  of  change  of  level  in  this  region? 
Fredericksburg  Sheet. 

1.  Account  for  the  peculiar  character  of  the  lower  portions  of 
Aquia  Creek,  Potomac  Creek,  Rappahannock  River,  etc. 

DEPOSITION  BY  RUNNING  WATEK 

We  have  seen  that  rivers  carry  mud,  sand,  gravel,  etc.,  from 
land  to  sea,  and  that  their  goal  is  the  degradation  of  the  land 
nearly  to  the  level  of  the  sea.  We  have  also  seen  that  rivers  do 
not  always  carry  the  sediment  derived  from  the  land  directly 
to  the  sea.  It  is  often  dropped  for  a  time  on  the  land,  perhaps 
to  be  picked  up  and  carried  on  again  when  the  conditions  for  its 
transportation  are  more  favorable.  We  have  now  to  inquire  into 
(1)  the  causes  which  make  running  water  drop  some  of  its  load, 
temporarily,  at  least;  (2)  the  places  where  the  material  is  aban- 
doned; (3)  the  topographic  features  developed  by  the  deposition 
of  sediment;  (4)  the  effect  of  deposition  on  the  stream  depositing 
it;  and  (5)  the  advantages  and  disadvantages  of  stream  deposition 
to  mankind. 

Causes  of  Deposition 

When  running  water  drops  its  load,  or  any  part  of  it,  it  is 
generally  because  the  current  has  lost  something  of  its  velocity. 
We  have  already  seen  (p.  155)  that  gradient  and  volume  are  the 
most  important  factors  in  determining  the  velocity  of  a  small 
stream. 

1.  Loss  of  velocity.  The  commonest  cause  of  loss  of  velocity 
is  decrease  of  slope  or  gradient.  Running  water  may  lose  velocity 
(1)  suddenly,  as  when  it  passes  from  a  steep  slope,  whether  of  hill 


180 


PHYSIOGRAPHY 


or  mountain,  to  a  gentle  one,  or  to  a  body  of  standing  water,  or 
(2)  slowly,  as  in  descending  a  valley  the  gradient  of  which  becomes 
gradually  less.  We  therefore  look  to  the  places  where  these  changes 
in  velocity  occur  for  the  principal  deposits  of  running  water. 
Streams  also  become  slower  wherever  their  channels  become  wider, 
if  volumes  and  gradient  remain  constant. 

A  less  common  cause  of  decrease  of  velocity  in  a  stream  is 


FIG.  187  — The  lower  end  of  the  Mississippi,  showing  its  distributaries. 

(C.  &  G.  Surv.) 

decrease  of  volume.  Streams  generally  increase  in  size  with  in- 
creasing distance  from  their  sources,  but  to  this  general  rule  there  are 
exceptions.  (1)  If  a  stream  flows  through  a  very  dry  region,  it 
may  receive  few  tributaries  and  few  springs.  Evaporation,  on 
the  other  hand,  is  great,  and  some  of  the  water  may  be  absorbed 
by  the  thirsty  soil  and  rock  through  which  it  flows.  This  is 


THE  WORK  OF  RUNNING  WATER  181 

especially  the  case  if  the  ground-water  surface  (p.  85)  of  the 
region  is  below  the  level  of  the  stream.  In  a  dry  region  there- 
fore a  stream  may  diminish  as  it  flows,  and  may  even  disappear 
altogether  (Pis.  VII  and  XV).  (2)  A  stream  sometimes  breaks 
up  into  several  streams  (Fig.  187).  The  volume  of  each  is  less 
than  that  of  the  original  stream.  (3)  Still  again,  many  streams, 
especially  in  semi-arid  regions,  have  much  of  their  water  withdrawn 
for  purposes  of  irrigation.  Many  streams  in  the  West  are  made 
smaller  in  this  way.  (4)  Streams  decrease  in  volume  as  their  floods 
decline. 

Increase  of  load  makes  running  water  flow  more  slowly;  but 
a  stream  which  is  increasing  its  load  by  its  own  action  is  an  eroding 
not  a  depositing  stream.  A  stream  may  deposit  coarse  sediment 
and  pick  up  fine  in  its  stead,  but  in  this  case  the  amount  of  fine 
material  wrhich  it  picks  up  is  usually  greater  than  the  amount 
of  coarse  which  it  leaves.  Erosion  is  therefore  greater  than  depo- 
sition, and  a  stream  which  erodes  more  than  it  deposits  is  not  a 
depositing  stream,  as  the  term  is  commonly  used. 

2.  Excess  of  load  from  tributaries.  Tributary  streams  with 
high  gradients  may  bring  to  their  mains  more  sediment  than  the 
latter  can  carry  away.  This  is  an  occasional  cause  of  deposition 
in  the  channel  of  the  main  stream,  especially  where  mountain 
torrents  with  high  gradients  join  older  streams  which  have  reduced 
their  channels  to  much  lower  gradients. 

Location  of  Alluvial  Deposits  and  their  Topographic  Forms 

The  deposits  made  by  running  water  are  found  principally 
in  those  situations  where  the  flow  of  the  water  is  checked  or 
stopped. 

i.  At  the  bases  of  steep  slopes.  Every  shower  washes  fine 
sediment  down  the  slopes  of  the  hills,  and  much  of  it  is  left  at  their 
bases.  Fences  in  such  situations  are  often  buried,  little  by  little, 
by  the  mud  thus  lodged.  Temporary  streams,  bred  of  showers, 
sometimes  flow  down  steep  slopes,  and  are  suddenly  checked  at 
their  bases.  Such  streams  gather  much  debris  in  their  headlong 
courses  down  the  slopes,  but  abandon  it  where  their  velocity  is 
suddenly  checked.  Thus,  at  the  lower  end  of  every  new-made 
gully  on  the  hillside  there  is  a  mass  of  debris  which  was  washed 
out  of  the  gully  itself  (Figs.  107  and  188).  Material  in  such 


182  PHYSIOGRAPHY 

positions  accur  ulates  in  the  form  of  a  partial  cone,  known  as  an 
alluvial  cone.  Alluvial  cones  have  much  in  common  with  cones 
of  talus;  but  in  the  former,  gravity  brings  the  material  down  by 
the  help  of  water,  while  in  the  latter  gravity  brings  the  material 
down  without  the  aid  of  water,  or.  with  but  little  help  from  it. 
Between  talus  cones  and  alluvial  cones  there  are  however  all  gra- 
dations. 

Conspicuous  alluvial  cones  are  rather  more  common  in  semi- 
arid  regions  than  elsewhere,  if  steep  slopes  are  present;  for  in 
such  regions  the  rainfall  is  fitful,  and  the  occasional  heavy  showers, 
which  give  rise  to  temporary  and  powerful  torrents,  favor  the  de- 


FIG.  188.— An  alluvial  cone.     (U.  S.  Geol.  Surv.) 

veloprnent  of  cones  of  great  size.  Talus  cones  often  have  great 
development  in  the  same  regions.  At  the  bases  of  the  mountain 
ranges  in  the  Great  Basin  the  talus  and  alluvial  cones  from  the 
mountains  are  sometimes  2000  or  3000  feet  high. 

An  alluvial  fan  is  the  same  as  an  alluvial  cone,  except  that  it 
has  a  lower  angle  of  slope.  The  term  fan  is  indeed  more  appro- 
priate than  cone  for  most  alluvial  accumulations  at  the  bases 
of  slopes.  The  lower  angle  of  the  fan  may  be  due  to  the  less 
abrupt  change  of  slope  where  it  is  developed,  to  the  larger 
quantity  of  water  concerned  in  its  deposition,  to  the  smaller 
amount  of  detritus,  or  to  its  greater  fineness.  Less  change 
of  slope,  more  water,  and  less  and  finer  material,  all  favor  the 
wider  distribution  of  the  sediment,  and  so  the  development  of 
fans  rather  than  cones.  Nearly  all  young  rivers  descending 


THE  WORK  OF  RUNNING  WATER  183 

from  mountains  build  fans  where  they  leave  the  mountains. 
Thus,  the  rivers  descending  from  the  Sierras  to  the  great 
valley  of  California  build  great  fans  at  the  base  of  the  mountain 
range.  Most  of  the  rivers  descending  from  the  Rockies  to  the 
plains  to  the  east  do  the  same  thing.  The  fans  of  streams  descend- 
ing from  the  mountains  are  often  many  miles  across.  The  fan  of 
the  Merced  River  in  California,  for  example,  has  a  radius  of  about 
40  miles. 

The  fans  made  by  neighboring  streams  often  grow  laterally  until 
they  merge.  The  union  of  several  such  fans  makes  a  compound 
alluvial  fan,  or  a  piedmont  alluvial  plain  (PI.  XV).  Such  plains 
exist  at  the  bases  of  most  considerable  mountain  ranges.  The 
depth  of  alluvial  material  in  such  situations  is  often  scores  and 
sometimes  hundreds  of  feet. 

Alluvial  cones  and  fans  react  on  the  course  of  the  water  which 
makes  them.  The  loose  debris  of  the  cones  and  fans  is  capable  of 
absorbing  much  water,  and  the  water  of  even  a  considerable  stream 
may  sink  into  its  fan  (PI.  XV).  Before  it  disappears,  the  stream 
is  often  divided  into  several  smaller  ones.  This  is  because  the 
sediment  deposited  by  the  stream  in  its  channel  makes  the  channel 
too  small  to  hold  all  the  water.  Some  of  it  therefore  runs  over 
(out  of  the  channel)  and  makes  a  new  channel  for  itself.  The 
deposits  which  clog  the  channel  may  be  the  result  of  (1)  dimin- 
ished slope,  ana  so  diminished  activity,  or  (2)  diminished  volume, 
due  to  absorption  of  water.  The  distributaries  thus  formed, 
being  small,  are  likely  to  be  slower  than  the  stream  from  which 
they  sprang,  and  so  more  likely  to  choke  themselves.  They  there- 
fore give  rise  to  other  and  smaller  distributaries.  Thus  the  water 
of  the  main  stream  is  likely  to  be  spread  about  over  its  cone  or  fan, 
and  the  stream  sometimes  disappears. 

Aside  from  well-developed  fans  and  cones  there  is  much  sedi- 
ment at  the  bases  of  slopes  which  are  not  steep.  In  such  positions, 
however,  the  alluvium  is  often  without  distinct  topographic  form. 
Such  accumulations  at  the  bases  of  slopes  are  almost  as  widespread 
as  the  bases  of  slopes  themselves. 

Alluvial  fans  and  piedmont  alluvial  plains  are  often  valuable 
for  agricultural  purposes.  In  some  parts  of  California,  for  ex- 
ample, the  alluvial  lands  are  so  valuable  that  holdings  are  generally 
small  and  highly  improved.  Even  in  semi-arid  regions  they  are 
often  extensively  cultivated,  the  water  being  supplied  (1)  by  wells, 


184 


PHYSIOGRAPHY 


through  which  the  debris  of  the  fan  is  made  to  yield  up  the  wate. 
it  has  absorbed,  or  (2)  by  irrigation  ditches,  which  connect 


FIG.  189. — A  branching  stream.      Junction  of  the  Cooper  and  Yukon  rivers, 
Alaska.     Shows  also  bars,  etc,     (U.  S.  Geol.  Surv.) 

the  stream  farther  up  the  valley,  and  lead  the  water  out  of  its 
natural  channel  over  the  fan  or  plain  (Fig.  200). 


Scale 


FIG.  190.— A  braided  river,  Dawson  Co..  Neb.     (U.  S.  Geol.  Surv.) 

2.    In  valley  bottoms.     A  stream  which  makes  deposits  in  its 
channel  reduces  the  size  of  the  channel.      In  time  it  may  become 


PLATE  XV 


./.  / 

fe* 


~x  \ 


i         I 


FARTS  OF  LOS  ANCELES  AND 


SAN  BERHASOINO  COUNTIES,  CALIFORNIA. 


A.  piedmont  alluvial  plain  or  compound  alluvial  fan  in  Southern  California.     Scale  1  — 
mile  per  inch.     (Cucamonga  Sheet,  U.  S.  Geol.  Surv.) 


PLATE  XVI 


The  alluvial  plain  of  the  Platte  rivers  in  Nebraska.  The  South  Platte 
is  braided  and  the  North  Platte  shows  bars.  The  map  also  shows 
irrigating  canals  leading  out  from  the  river.  Scale  2—  miles  per 
inch.  (Paxton  Sheet,  U.  S.  Geol.  Surv.) 


THE  WORK  OF  RUNNL\7G  WATER 


185 


too  small  to  hold  all  the  water.  A  part  then  breaks  out,  and 
follows  a  new  course  in  the  valley  flat.  This  process  may  be  re- 
peated again  and  again  (Figs.  189  and  190).  The  diverging  stream 
may  or  may  not  return  to  the  main.  Those  which  do  not  return 
are  called  distributary  streams.  This  term  is  sometimes  applied 
also  to  all  diverging  streams,  without  reference  to  their  return. 
The  breaking  up  of  a  stream  into  parts  may  go  so  far,  especially 
when  the  water  is  low,  that  there  can  hardly  be  said  to  be  a  main 
channel.  The  stream  then  becomes  a  network  of  minor  streams, 
or  a  braided  stream.  The  Platte  River  in  Nebraska  is  an  excellent 
example  (Fig.  190).  This  condition  exists  only  at  low  water. 


FIG.    191. —  Bars    in   river.      The    Yellowstone    River,    34    miles   south    of 

Livingston,  Mont. 

At  high  water  the  entire  flat  through  which  the  minor  streams 
shown  in  Fig.  190  flow  is  covered  by  water,  and  becomes  the  bed 
of  a  single  river  (PI.  XVI). 

Streams  sometimes  deposit  sand-bars  in  their  channels  (Figs. 
189  and  191),  especially  in  low  water,  even  when  they  do  not  be- 
come braided.  These  bars  are  obstacles  to  navigation,  and  are  a 
constant  source  of  embarrassment  to  river  traffic  in  the  low  stages 
of  many  navigable  streams.  The  bars  deposited  in  low  water 
are  often  swept  away  in  times  of  flood,  when  the  velocity  of  the 


186 


PHYSIOGRAPHY 


stream  is  greatly  increased.  Occasionally  bars  become  more  or 
less  permanent  islands.  If  they  become  covered  with  forests 
they  are  less  easily  eroded  by  the  swift  waters  of  floods,  since  the 
roots  have  a  strong  protective  influence. 

The  profiles  of  most  valleys  are  curves,  the  curvature  becom- 
ing less  and  less  steep  as  the  lower  end  of  the  stream  is  approached 
(Fig.  192).  It  therefore  happens  that  as  a  stream  descends  its 


FIG.  192. — Profile  of  a  normal  valley. 

valley  it  generally  reaches  a  point  where  its  reduced  gradient  so 
diminishes  its  velocity  that  it  must  abandon  some  of  its  load.  In 
this  way  sediment  is  distributed  for  long  distances  along  valley 
bottoms.  It  is  left  in  the  channels  of  the  streams  and  spread  over 
their  flood  plains,  aggrading  them  and  making  them  alluvial 
plains.  Deposition  in  a  valley  which  has  no  flat  tends  to  develop 
one  (Fig.  193). 

Deposition  on  valley  flats  has  but  little  effect  on  their  topog- 


V      ::':/T 


FIG.  193. — Flat  developed  by  aggradation — diagrammatic. 

raphy;  but  a  few  minor  features  deserve  mention.  Among  them 
are  natural  levees.  This  term  is  applied  to  the  low  ridges  on  stream 
flats  along  the  banks  of  the  channel  (Fig.  194).  They  are  built 


FIG.  194. — Levees  of  the  Mississippi  in  cross-section,  four  miles  north  of 
Donaldsonville,  La.  Vertical  scale  X  50.  The  horizontal  line  repre- 
sents sea-level.  The  bottom  of  the  channel  is  far  below  sea-level  at  this 
point. 

in  times  of  flood.  At  such  times  the  current  in  the  main  channel 
is  swift;  but  as  the  water  escapes  its  channel  and  spreads  over  the 
adjacent  flat,  its  velocity  is  checked  promptly,  because  its  depth 


THE  WORK  OF  RUNNING  WATER 


187 


suddenly  becomes  less.  It  must  therefore  abandon  much  of  its 
load  then  and  there.  Repeated  deposition  in  this  position  gives 
rise  to  the  levees.  Natural  levees  are  sometimes  high  enough 
and  continuous  enough  to  turn  the  courses  of  tributary  streams. 
This  is  well  illustrated  by  the  Yazoo  River  of  Mississippi,  which 
flows  some  200  miles  in  the  flat  of  the  Mississippi  before  being 
able  to  join  it.  Near  Vicksburg  the  Mississippi  swings  over 
to  the  east  side  of  the  valley,  and  thus  receives  its  tributary, 
which  the  levees  have  shut  off.  The  early  population  of  Louisiana 


FIG.  195. 


FIG.  196. 


FIG.  195. — Diagram  illustrating  an  early  stage  in  the  development  of  river 
meanders.  The  dotted  area  represents  the  area  over  which  the  stream 
has' worked. 

FIG.  196. — A  later  stage  in  the  development  of  meanders. 

and  Mississippi  was  largely  distributed  in  narrow  belts  along  the 
levees  of  the  Mississippi  and  its  tributaries  and  distributaries. 
Here  was  the  highest,  driest  land,  of  great  fertility,  fronting  ready- 
made  highways. 

Flood-plain  meanders.  A  stream  with  an  alluvial  plain  is 
likely  to  meander  widely  (Pis.  IX,  X,  and  XI).  In  general  terms 
this  may  be  said  to  be  the  result  of  low  velocity,  which  allows 
it  to  be  turned  aside  easily.  Were  the  course  of  such  a  stream 
made  straight,  it  would  soon  become  crooked  again.  The  manner 
of  change  is  illustrated  by  Figs.  195  and  196.  If  the  banks  be 


188 


PHYSIOGRAPHY 


less  resistant  at  some  points  than  at  others,  as  is  always  the  case, 
the  stream  will  cut  in  at  those  points.  If  the  configuration  of  the 
channel  is  such  as  to  direct  a  current  against  a  given  point,  6  (Fig. 
195),  the  result  is  the  same,  even  without  inequality  of  material. 
Once  a  curve  in  the  bank  is  started,  it  is  increased  by  the  current 
which  is  directed  into  it.  Furthermore,  as  the  current  issues  from 
the  curve,  it  impinges  against  the  opposite  bank  and  develops  a 

curve  at  that  point.  The 
water  issuing  from  this  curve 
develops  another,  and  so  on. 
Once  started,  the  curves 
or  meanders  tend  to  become 
more  and  more  pronounced 
(Fig.  196).  In  the  case  rep- 
resented by  Fig.  1,  PI.  IX, 
the  narrow  neck  of  land  be- 
tween curves  is  almost  cut 
through.  When  this  is  ac- 
complished, the  stream  will 
abandon  its  wide  curve.  A 
later  stage  in  the  process 
is  shown  in  Fig.  2,  PI.  IX 
(the  Osage  River  near  Schell, 
Mo.). 

When  the  stream  has  cut 
off  a  meander,  the  abandoned 
part  of  the  channel  often 
remains  unfilled  with  sedi- 
ment. If  it  contains  standing 

.,      ,  ,,  water,  as  it  often  does,  it  be- 

FIG.  197. — Meanders  and  cut-offs  in  the 

Mississippi     Valley    below  Vicksburg.  comes  the  Site  of  a  lake  (Fig. 
The  figure  shows  the  migration  of  the  197)>     Such  lakes  sometimes 
meanders  down-stream  and  their  ten- 
dency to  increase  in  size.  have  the  form  of  an  ox-bow. 

and  so  are  called  ox-bow  lakes 
(Pis.  IX  and  X).  They  are  also  known  as  bayous. 

In  meandering,  a  stream  sometimes  reaches  and  undermines  the 
valley  bluff,  thus  widening  its  valley. 

By  the  shifting  of  their  courses,  as  the  result  of  deposition 
and  meandering,  streams  have  affected  human  interests  in  many 
ways.  Villages  built  on  the  banks  of  a  stream  because  of  the 


THE  WORK  OF  RUNNING  WATER 


189 


river  traffic  which  the  situation  favored  have  sometimes  been 
abandoned  by  changes  in  the  stream's  course.  Such  villages 
usually  decay  when  the  stream  has  withdrawn  its  patronage. 
Some  have  been  destroyed,  while  others  have  been  preserved  at 
great  expense.  Kaskaskia,  the  capital  of  Illinois  until  1819,  was 
situated  on  the  flood  plain  of  the  Mississippi.  In  1881  a  change  in 
the  channel  of  the  river  converted  the  larger  part  of  the  village  site 
into  an  island,  the  last  vestige  of  which  was  washed  away  in  1899. 
Large  sums  have  been  expended  by  the  National  Government  and 
by  the  Chicago  and  Alton  Railway  to  keep  the  Missouri  in  its  course 


FIG.  198. — A  cement-lined  canal  prepared  for  irrigation.  Truckee-Carson 
project,  Nev.  The  cement  lining  prevents  free  seepage.  (U.  S.  Geol. 
Surv.) 


at  Glasgow,  Mo.  Again,  streams  are  sometimes  the  boundaries 
between  counties,  and  even  states.  In  such  cases  the  shifting  of 
the  stream  would  transfer  territory  from  one  state  to  another.  To 
prevent  this,  complicated  legal  devices  and  complicated  definitions 
of  boundaries  are  sometimes  resorted  to.  The  case  is  still  more 
serious  where  a  river  forms  an  international  boundary.  Thus  the 
shifting  of  the  Rio  Grande  makes  that  river  an  unsatisfactory 
boundary  between  the  United  States  and  Mexico. 


190 


PHYSIOGRAPHY 


FlG.  199. — An  irrigating  canal  not  cemented,  before  the  water  is  turned  in. 
Salt  River  Valley,  Ariz.     (U.  S.  Geol.  Surv.) 


FIG.  200. — A  diversion  dam  where  the  water  of  the  stream  is  raised  and 
turned  into  the  canal.     Truckee-Carson  project,  Nev.     (U.  S.  Geol.  Surv.) 


THE  WORK  OF  RUNNING  WATER 


191 


Fertility  of  alluvial  plains.  Alluvial  plains  are  often  very 
fertile  and  are  among  the  tracts  most  prized  for  agricultural  pur- 
poses. This  was  as  true  in  ancient  times  as  now,  for  the  valleys 
of  the  Nile,  the  Po,  and  of  several  of  the  rivers  of  southern  Asia 
were  the  garden  spots  of  ancient  civilizations.  The  frequent  de- 
posits of  silt  and  mud  on  such  plains  continually  renew  the  soil 
and  render  it  fertile.  So  strictly  were  the  earlier  civilizations 
confined  to  valley  plains  that  the  period  antedating  800  B.C.  has 
been  called,  with  some  propriety,  the  "fluvial  period"  of  history. 
In  America,  valleys  have  been  sought  out  for  habitation  from  the 


FIG.  201. — An  irrigating  canal  filled  with  water.     Salt  River  Valley,  Ariz. 

(U.  S.  Geol.  Surv.) 

earliest  times.  In  Virginia  and  Maryland  early  settlements  were 
made  in  the  valleys  of  the  James  and  the  Potomac ;  and  in  Pennsyl- 
vania, in  the  valleys  of  the  Delaware,  the  Schuylkill,  and  the  Sus- 
quehanna.  In  New  York  the  principal  settlements  were  long 
confined  to  the  valleys  of  the  Hudson  and  the  Mohawk;  and  when 
the  early  settlements  of  Massachusetts  began  to  spread  beyond 
the  coast,  they  occupied  the  Connecticut  Valley. 

Valley  flats,  as  well  as  alluvial  fans,  are  favorably  situated  for 
irrigation.  Figs.  198  and  199  show  irrigation  canals  or  large  ditches, 
Fig.  200  the  beginning  (head)  of  a  canal,  and  Fig.  201  a  canal  filled 


192 


PHYSIOGRAPHY 


FIG.  202. — Fields  prepared  for  irrigation  by  methods  of  squares.     Las  Cruces, 
N.  M.     (Photograph  by  Fairbanks.) 


n,.  '^?n—' 
/!5^A/'^?^^^r— 

—•--f^*'  -  o  „      -~^r * « *  T  „ 
>"7  ° 4     « AK  *'  i D A K ° ••  * 


'-^^^ 


\   r* 

\  <• 


t^fi  i 


rv 


X  /rr-ftt/e  area 

—.    Projects  untltr 
<D  cor.strvcfion. 

(J)   Projects  approved 


FIG.  203. — Map  showing  irrigation  projects  completed  and  under  construction; 
spring,  1906.     (Blanchard.) 


THE  WORK  OF  RUNNING  WATER 


193 


FIG.  204. — A  type  of  the  arid  lands  of  the  West  before  irrigation. 
(U.  S.  Geol.  Surv.) 


FIG.  205. — The  same  type  of  land  shown  in  Fig.  204,  after  irrigation. 
River  Valley,  Ariz.     (U.  S.  Geol.  Surv.) 


Salt 


194 


PHYSIOGRAPHY 


with  water.  Fig.  202  shows  a  field  prepared  by  ditching  for  irrigation. 
Water  is  drawn,  as  required,  from  the  canals  into  the  small  ditches  of 
the  field.  Great  progress  has  already  been  made  in  the  utilization 
of  the  arid  lands  in  the  western  part  of  the  United  States.  The 
lands  thus  utilized  are  largely  in  valleys  and  on  plains  adja- 
cent to  mountains.  The  general  distribution  of  the  irrigated  and 
irrigable  lands  is  shown  in  Fig.  203.  The  Government  has  under- 
taken the  construction  of  many  reservoirs  in  favorable  sites  in 
the  mountains  to  hold  the  waters  of  the  wet  seasons,  so  that  they 
may  be  drawn  out  and  used  on  the  lands  below  during  the  growing 
season.  The  sites  selected  for  dams  are  usually  narrow  places  in 
the  valleys  (Fig.  206). 


FIG.  206. — Roosevelt  dam  site.     Salt  River  project,  Ariz. 
(U.  S.  Geol.  Surv.) 


River  floods.  Alluvial  plains  are,  however,  not  without  their 
drawbacks  as  agricultural  regions,  for  the  floods  to  which  they 
are  subject  are  often  disastrous  both  to  life  and  to  property. 

Terrible  illustrations  are  afforded  by  the  valleys  of  many  great 
rivers.  Thus  in  the  spring  of  1897  many  thousand  square  miles 
of  the  flood  plain  of  the  Lower  Mississippi  were  covered  with 
water.  It  was  estimated  that  50,000  to  60,000  people  suffered 
serious  loss.  In  1881  and  1882  the  floods  of  the  same  stream  and 


THE  WORK  OF  RUNNING  WATER 


195 


of  the  Ohio  are  estimated  to  have  caused  a  loss  of  $15,000,000  and 
138  lives.  The  losses  occasioned  by  the  floods  of  the  Ohio  alone 
were  estimated  at  $10,000,000  in  1884,  and  at  $40,000,000  in  1903. 
There  was  a  disastrous  flood  in  the  valley  of  the  W abash  and 
another  in  the  valley  of  the  Susquehanna  in  1904,  each  causing 
the  destruction  of  property  to  the  extent  of  nearly  $10,000,000. 


FIG.  207. — Diagram  illustrating  changes  in  the  course  of  the  Yellow  River. 
The  shaded  area  represents  the  area  subject  to  flooding  by  the  main 
stream  and  its  tributaries.  (Richthofen.) 

Cities  built  on  flood  plains  are  also  subject  to  great  injury  from 
floods.  An  exceptional  flood  of  the  Passaic  River  (N.  J.)  in  1902 
is  estimated  to  have  destroyed  millions  of  dollars'  worth  of  prop- 
erty in  the  city  of  Paterson  alone. 

Disastrous  floods  occur  from  time  to  time  in  most  great  valleys. 
In  1885  a  heavy  rainfall  of  about  24  inches  of  water  over  an  area 
of  about  1000  square  miles  in  the  valley  of  the  Ganges  caused  a 
disastrous  flood.  The  volume  of  the  river  was  greatly  swollen,  and 


196 


PHYSIOGRAPHY 


the  water  rushed  down  the  valley  with  terrible  velocity,  under- 
mining the  banks,  cutting  new  channels  in  the  valley  plain,  sweeping 


FIG.  208.— Delta  of  Lake  St   Clair.     (Lake  Survey  Chart.) 


FIG.  209. — A  general  view  of  the  lower  part  of  the  delta  of  the  Mississippi. 

away  roads,  ditches,  bridges,  aqueducts,  retaining-walls,  and  even 
villages. 


THE  WORK  OF  RUNNING  WATER  197 

The  most  disastrous  river  floods  recorded  are  those  of  the 
Hoang-ho  or  Yellow  River  of  China.  Previous  to  1892,  this  river 
flowed  into  the  Yellow  Sea  south  of  the  Shan-tung  promontory.  In 
that  year  it  shifted  its  course  when  in  flood,  and  formed  a  new 
channel  leading  northwest  into  the  Gulf  of  Pechili,  300  miles  to 
the  north  (Fig.  207).  Such  changes  in  a  stream's  course  are  of 
much  consequence  to  commerce. 

The  alluvial  plains  of  some  valleys  are  protected  against  flood  by 
levees,  or  dykes.  In  such  cases  the  natural  levees  are  built  higher  by 
man,  and  the  gaps  in  them  are  filled.  They  then  protect  the  flat 
outside  in  ordinary  floods;  but  extraordinary  floods  sometimes 
burst  through  the  dykes,  working  great  disaster.  Some  parts 
of  the  rich  flood  plain  of  the  Mississippi  which  are  used  for  agri- 
culture are  so  subject  to  flood  that  all  buildings  connected  with 
the  farms  are  placed  above  the  flat. 

3.  At  debouchures.  Where  a  swift  stream  flows  into  the  sea  or 
a  lake,  its  current  is  promptly  checked  and  soon  destroyed  alto- 


FIG.  210. — Diagrammatic  profile  and  section  of  an  alluvial  fan. 

gether.  Its  load  is  accordingly  dropped.  If  not  washed  away 
by  waves,  etc.,  the  deposits  of  river-borne  sediment  in  such  places 
make  deZtas  (Figs.  208  and  209). 

The  delta  has  some  points  in  common  with  the  alluvial  fan. 
In  both  cases  the  principal  deposit  is  concentrated  at  the  point 
where  the  velocity  is  suddenly  checked.  In  the  case  of  the  delta, 
however,  the  current  is  checked  more  completely,  and  the  debris 
accumulates  (at  the  outset)  below  the  surface  of  the  standing 
water.  In  form,  the  delta  differs  from  the  alluvial  fan  in  that  its 
edge  has  a  steep  slope  (compare  Figs.  210  and  211). 

Once  a  delta  is  started  below  water,  deposition  takes  place 
upon  its  surface,  which  may  be  built  up  to,  and  even  above,  the 
water-level.  That  part  of  the  delta  above  the  surface  of  the  water 
in  which  it  is  built  is  like  a  flat  alluvial  fan. 


198 


PHYSIOGRAPHY 


Waves,  currents,  etc.,  may  prevent  the  building  of  a  delta,  but 
otherwise  all  sediment-bearing  streams  make  deltas  at  their  de- 
bouchures. Deltas  are  sometimes  built  where  one  stream  flows  into 


FIG.  211. — Diagrammatic  profile  and  section  of  a  delta. 

another  (Fig.  212).  This  is  especially  the  case  where  a  swift, 
debris-laden  stream  joins  a  slow  one.  Deltas  built  into  rivers 
are  usually  of  slight  extent. 

Much  land  has  been  made  by  delta-building.    Thus  the  Colorado 
River  has  built  a  great  delta  many  square  miles  (above  water)  in 


FIG.  212. — Delta  of  the  Chelan  River  built  into  the  Columbia  River.  Wash. 
(Willis,  U.  S.  Geol.  Surv.) 

area  at  the  head  of  the  Gulf  of  California  (Fig.  213).  The  delta 
has  been  built  quite  across  the  gulf  near  its  upper  end,  shutting 
off  the  head.  In  the  arid  climate  of  the  region  this  shut-off  head 
has  become  a  nearly  dry  basin,  the  lowest  part  of  which  is  about 
300  feet  below  sea-level.  The  Skagit  River,  in  Washington,  has 
built  out  its  delta  so  as  to  surround  what  were  high  islands  in  Puget 
Sound,  thus  joining  them  to  the  mainland.  The  deltas  of  the 


THE  WORK  OF  RUNNING  WATER 


199 


Mississippi  (Fig.  209),  the  Nile  (Fig.  214),  and  the  Hoang-ho  rivers 
are  among  the  large  and  well-known  deltas.  The  united  delta  of  the 
Ganges  and  Brahmaputra  is  also  a  great  one,  having  an  area  (above 
water)  of  some  50,000  square  miles.  The  Po  has  built  a  delta  14 


FIG.  213. — Relief  map  of  an  area  about  the  head  of  the  Gulf  of  California, 
showing  the  delta  of  the  Colorado  River,  outlined  in  a  general  way  by 
dotted  lines.  (U.  S.  Reclamation  Service.) 

miles  beyond  the  former  port  of  Adria,  which  gave  its  name  to  the 
Adriatic  Sea.  The  Rhone  River  (France)  has  advanced  its  delta 
(Fig.  215)  some  15  miles  in  as  many  centuries. 

The  borders  of  a  delta  are  often  difficult  of  determination. 
A  delta  is  sometimes  said  to  be  limited  up-stream  by  the  point 
where  the  distributaries  begin  to  be  given  off.  This  definition  is 


200 


PHYSIOGRAPHY 


convenient,  but  arbitrary.     It  is  less  definite,  but  perhaps  truer,  to 
regard  the  up-stream  border  of  the  land  reclaimed  from  the  sea 


Jfemjim 
FIG.  214.— The  delta  of  the  Nile.     (Prestwich.) 

or  lake  by  the  river  deposits,  as  the  head  of  the  delta.  This  defini- 
tion would  in  many  cases  make  the  areas  of  deltas  much  greater 
than  the  other.  On  this  basis,  the  head  of  the  delta  of  the  Missis- 
sippi, for  example,  would  be  near  the  mouth  of  the  Ohio. 


FIG.  215.— Delta  of  the  Rhone  River.     (Prestwich.) 

The  effect  of  delta-building  is  to  increase  the  area  of  the  land; 
but  it  is  to  be  noted  that  the  processes  which  lead  to  delta-building 


THE  WORK  OF  RUNNING  WATER 


201 


reduce  the  volume  of  the  land-masses,  even  though  they  increase 
their  area. 

The  outline  of  some  deltas  is  determined  by  the  surroundings 
in  which  they  are  built.  When,  for  example,  a  delta  is  built  into  a 
bay,  the  form  of  the  bay-head  determines  the  shape  of  the  delta. 
The  normal  form  of  a  delta  built  on  an  open  coast  is  somewhat 
semicircular,  though  there  is  often  a  fringe  of  delta  fingers  which 
together  have  some  resemblance  to  the  Greek  letter  J,  which 
gave  these  terminal  deposits  of  streams  their  names. 

The  silting  up  of  river  mouths  is  sometimes  disastrous  to  cities 
whose  commerce  is  based  on  river  trade.  Thus  the  silting  up 


FIG.  216. — Delta  of  the  Danube.     (Prestwich.) 

of  the  Tapti  River  was  largely  responsible  for  the  decline  of  Surat, 
once  the  leading  commercial  centre  of  India.  Between  1797  and 
1847  its  population  declined  from  800,000  to  80,000. 

The  surfaces  of  deltas  are  usually  nearly  plane,  and  the  streams 
which  cross  them  often  give  off  distributaries,  as  the  preceding 
figures  show.  These  distributaries  are  subject  to  great  and  sudden 
changes  of  course,  as  well  as  to  minor  shiftings  which  are  in  prog- 
ress all  the  time.  These  changes  sometimes  affect  commerce  in  a 
vital  way.  Thus  the  site  of  Kasimbazar,  in  India,  described  as 
the  "chief  emporium  of  the  Gangetic  trade"  early  in  the  eighteenth 
century,  is  now  a  swamp  as  a  result  of  a  sudden  change  in  the 
course  of  the  Bhagirathi  River  (a  distributary  of  the  Ganges),  on 
the  banks  of  which  it  stood. 


202  PHYSIOGRAPHY 

Many  deltas  are  cultivated,  and  some  of  them,  like  that  of  the 
Hoang-ho,  support  dense  populations.     Delta  lands  are,  however, 


FIG.  217. — Terraces  on  the  Fraser  River  at  Lilloet,  B.  C. 
(Photograph  by  Calvin.) 

subject  to  disastrous  floods.  It  is  estimated  that  the  flood  of  the 
Hoang-ho  River  in  September,  1887,  drowned  at  least  a  million 
people  who  lived  upon  its  delta,  and  caused  the  death  of  many 


FIG.  218. — Diagram  to  illustrate  the  development  of  river  terraces. 

more  by  disease  and  famine  afterward.     Many  villages  were  com- 
pletely destroyed,  and  hundreds  more  were  temDorarilv  submerged. 


THE  WORK  OF  RUNNING  WATER  203 

Ill-defined  alluvium.  Alluvial  deposits  as  a  whole  are  wide- 
spread. A  large  part  of  the  surface  of  the  land  is  covered  with  a 
little  alluvial  material,  though  relatively  small  areas  are  deeply 
covered.  Alluvial  material  is  so  disposed  as  to  tend  to  even  up 
slopes.  Thus  alluvial  fans  and  cones  tend  to  bring  the  steep  slope 
above  and  the  gentle  slope  below  into  harmony. 

Alluvial  Terraces 

When  a  river  which  has  an  alluvial  flat  is  rejuvenated,  the 
stream  sinks  its  channel  below  the  level  of  the  flat  (Fig.  179). 
The  remnants  of  the  old  flood  plain  then  constitute  alluvial  terraces 
(Figs.  217  and  218).  Such  terraces  are  also  formed  in  other  ways. 
Thus  if  a  stream  is  temporarily  supplied  with  an  excess  of  load, 
it  aggrades  its  valley  (Fig.  193).  If,  later,  the  source  of  the  ex- 
cessive load  is  removed,  the  stream  sets  to  work  to  remove  that 
which  was  temporarily  laid  aside  in  its  flood  plain,  even  without 
rejuvenation.  The  more  conspicuous  alluvial  terraces  arise  in  some 
such  ways.  Many  cities,  such  as  Dubuque,  la.,  Peoria,  111.,  and 
Harrisburg,  Pa.,  were  begun  on  stream  terraces,  though  they  have 
now  spread  above  them. 

MAP  EXERCISES 

Topographic  Maps  and  Coast  Survey  Charts  Showing  Stream  Deposition, 

Terraces,  etc. 

List  of  Maps 

1.  Cucamonga,  Cal.  6.  Sacramento,  Cal. 

2.  Marshall,  Mo.  7.  Savanna,  la. — 111. 

3.  Marseilles,  111.  8.  Mississippi  River  Chart  No.  14.1 

4.  Donaldsonville,  La.  9.  Coast  Survey  Chart  No.  19. 2 

5.  Tacoma,  Wash. 

Cucamonga  Sheet. 

1.  Of  what  kind  of  material  are  the  uplands  probably  composed? 

2.  Of  what  kind  of  material  are  the  lowlands  composed? 

'.  The  Mississippi  River  charts  can  be  had  of  the  Mississippi  River  Commis- 
sion, St.  Louis.  Catalogs  and  prices  furnished  on  application  to  the  Com- 
mission. 

2  The  Coast  Survey  charts  are  published  by  the  U.  S.  Coast  and  Geodetic 
Survey,  Washington,  D.  C.  Catalogs  and  prices  furnished  on  application  to 
the  Director  of  that  Survey. 


204  PHYSIOGRAPHY 

3.  Why  do  the  contour  lines  in  general  curve  out  from  the  upland 

along  the  drainage  lines? 

4.  Why  are  the  outward  curves  of  the  contours  in  some  instances 

notched  backward  toward  the  mountains  along  the  immediate 
drainage  lines? 

5.  Why  are  the  streams  intermittent  on  the  lowlands? 

6.  Explain  the  peculiar  manner  in  which  the  streams  of  waste 

divide. 

7.  What  possible  water-supply  is  there  for  the  dense  population  of 

the  lowland? 
Marshall  Sheet. 

1.  How  is  the  extent  to  which  the  Missouri  River  has  shifted  its 

course  in  recent  times  shown? 

2.  What  are  the  evidences  that  deposition  is  now  in  progress? 

3.  What  probably  determined  the  immediate  location  of  Miami, 

Dewitt,  and  Brunswick? 
Marseilles  Sheet. 

1.  Interpret  the  flats  upon  which  the  town  of  Seneca  and  the  Black 

Ash  Swamp  are  situated. 
Donaldsonville  Sheet. 

1.  Explain  the  general  distribution  of  the  higher  land. 

2.  What  is  the  explanation  of  the  alluvial  deposits  northeast  of 

Colomb  Park? 

3.  Note  the  relation  of  the  minor  streams  to  the  main  stream. 

4.  Note  the  location  of  common  roads,  railroads,  and  settlements. 
Tacoma  Sheet. 

1.  What  phase  of  river  work  is  the  Puyallup  River  now  performing? 

2.  What  was  the  probable  origin  of  the  low  marshy  ground  at  the 

head  of  Commencement  Bay? 

3.  Possible  reasons  why  Wapato  and  Hylebo  creeks  flow  independ- 

ently   to    Commencement    Bay    instead    of    into    Puyallup 

River? 
Sacramento  Sheet. 

1.  What  was  the  probable  origin  of  the  plain  covering  the  western 

half  of  this  area? 
Savanna  Sheet. 

1.  Explain  the  lakes  and  swamps  in  the  lowland. 

2.  What  is  the  probable  meaning  of  the  abrupt  slope  east  of  Dyson 

Lake? 

3.  Is  there  any  discrepancy  (in  stage  of  development)  between  the 

valley  of  the  Mississippi  and  its  tributaries? 

4.  Note  the  distribution  of  the  common  roads  in  the  northern  part 

of  the  area,  and  suggest  the  explanation. 


THE  WORK  OF  RUNNING  WATER  205 

Mississippi  River  Chart  No.  14. 

(Read  carefully  the  note  printed  in  red.) 

1.  What  was  the  origin  of  Lake  Chi  cot? 

2.  What  changes  have  taken  place  at  Bachelor's  Bend  since  1883? 

3.  Can  you  find  evidence  on  the  map  that  the  meanders  of  the  Mis- 

sissippi are  working  down  the  valley? 


REFERENCES 

"  1.  CHAMBERLIN  AND  SALISBURY,  Geologic  Processes,  Chapter  III:  Henry 
Holt  &  Co.,  1903,  and  other  standard  text-books  on  Geology. 

2.  RUSSELL,  Rivers  of  North  America:   G.  P.  Putnam's  Sons,  1898. 

3.  SHALER,  Aspects  of  the  Earth,  Chapters  III  and  IV.    Chas.  Scribner's 
Sons,  1889. 

4.  GILBERT,  Geology  of  the  Henry  Mountains,  Chapter  on  Land  Sculpture: 
Government  Publication,  1877;    Chapter  on  Niagara  Falls,  in  Physiography 
of  the  United  States:   Am.  Bk.  Co.,  1896;    and  Physical  History  of  Niagara 
River:    Am.  Geol.,  Vol.  XXV11,  pp.  375-377. 

5.  CAMPBELL,  Drainage  Modifications  and  their  Interpretation:    Jour,  of 
Geol.,  Vol.  IV,  1896,  pp.  567-581  and  657-678. 

6.  WILLIS,   The  Northern  Appalachians,  in  Physiography  of  the   United 
States:  Am.  Bk.  Co.,  1896. 

7.  HAYES,   The  Southern  Appalachians,  in  Physiography  of  the   United 
States:    Am.  Bk.  Co.,  1896;    and  Physiography  of  the  Chattanooga  District: 
19th  Ann.  Rept.  U.  S.  Geol.  Surv.,  Pt.  II,  pp.  1-58. 

8.  DAVIS,  The  Seine,  the  Meuse,  and  the  Moselle:    Nat.  Geog.  Mag.,  Vol. 
VII,  pp.  181-202  and  228-238;    Rivers  and  Valleys  of  Pennsylvania:   Nat. 
Geog.  Mag.,  Vol.  I,  especially  pp.  203-219;    Stream  Contest  along  the  Blue 
Ridge:    Bull.  Geog.  Soc.  of  Philadelphia,  Vol.  HI,  pp.  213-244;   Geographic 
Cycle  in  an  Arid  Climate:  Jour.  Geol.,  Vol.  XIII,  p.  381;  and  Development 
of  River  Meanders:    Geol.  Mag.,  Vol.  X,  1903. 

9.  DALY,  Accordance  of  Summit  Levels  among  Alpine  Mountains:    Jour. 
Geol.,  Vol.  XII,  p.  105. 

10.  WALCOTT,  Natural  Bridge  of  Virginia:    Nat.  Geog.  Mag.,  Vol.  V, 
1893,  pp.  59-62. 

11.  CLELAND,  North  American  Natural  Biidges,  in  Pop.  Sci.  Mo.,  May, 
1911,  and  Bull.  Geol.  Soc.  Am.,  Vol.  XXI,  pp.  313-338. 

12.  MERRILL,  Principles  of  Rock  Weathering:  Jour,  of  Geol.,  Vol.  IV, 
1896,  pp.  704-724  and  850-872;  also  Rocks,  Rock  Weathering,  and  Soils, 
Parts  III,  IV,  and  V:  The  Macmillan  Co. 

13.  DUTTON,  Tertiary  History  of  the  Grand  Canyon  District,  especially 
Chapter  III:  Mono.  II,  U.  S.  Geol.  Surv.,  1882. 

14.  JOHNSON  (L.  C.),  The  Nita  Crevasse:  Bull.  Geol.  Soc.  of  Am.,  Vol. 
II,  pp.  20-25,  1891. 

15.  GANNET,  The  Flood  of  April,  1897,  in  the  Lower  Mississippi.     Scot. 
Geog.  Mag.,  Vol.  XIII,  1897,  p.  419. 


206  PHYSIOGRAPHY 

16.  JEFFERSON,  Limiting   Width  of  Meander  Belts:   Nat.  Geog.  Mag., 
Vol.  XIII,  pp.  373-384. 

17.  TOWER,  Development  of  Cut-off  Meanders:  Bull.  Am.  Geog.   Soc., 
Vol.  XXXVI,  pp.  589-599. 

18.  DAVIS,  River  Terraces  in  New  England:  Bull.  Mus.  Comp.    Zool., 
Geol.  Ser.,  Vol.  V,  pp.  282-346,  1902. 

19.  DODGE,  The  Geographical  Development  of  Alluvial  River  Terraces: 
Proc.  Boston  Soc.  Nat.  Hist.,  Vol.  XXVI,  pp.  257-273. 

20.  All  standard  text-books  on  geology. 


CHAPTER  V 
THE  WORK  OF  SNOW   AND   ICE 

WE  have  seen  that  the  atmosphere,  the  ground-water,  and  the 
waters  on  the  surface  of  the  land  bring  about  important  changes 
in  its  configuration.  We  are  now  to  study  the  work  of  water  in 
its  solid  form. 

Ice  beneath  the  surface.  The  wedge- work  of  ice  in  the 
crevices  of  rock  has  already  been  mentioned  (p.  73).  When 
the  great  areas  where  water  freezes  during  some  part  of  the  year 
are  considered,  it  appears  that  the  aggregate  effect  of  the  freezing 
of  water  in  the  pores  and  crevices  must  be  great  in  long  periods 
of  time.  The  water  which  freezes  in  the  soil  also  has  some  effect 
on  the  surface.  This  is  shown  by  the  disturbance  of  the  walls  of 
buildings  if  their  foundations  do  not  go  below  the  depth  of  freez- 
ing, and  by  the  working  up  of  stones  and  bowlders  through  the 
soil,  etc.  The  frozen  water  in  the  soil  makes  it  solid  temporarily, 
and  so  retards  or  prevents  surface  erosion,  thus  having  a  pro- 
tective effect.  The  moisture  rising  from  the  soil,  either  by  evap- 
oration or  by  capillary  action,  sometimes  freezes  as  it  reaches 
the  surface.  There  may  be  continued  additions,  from  below, 
to  the  frost  (ice)  thus  formed,  resulting  in  the  upward  growth  of 
ice,  as  shown  in  Fig.  219. 

Snow,  the  most  familiar  form  of  ice,  is  more  wide-spread  than 
any  other.  Besides  snow,  the  more  familiar  forms  of  ice  appear 
on  lakes,  rivers,  and  the  seas  of  high  latitudes,  and  on  the  lands 
of  high  mountains  or  of  high  latitudes. 

The  ice  of  lakes.  To  understand  the  formation  of  ice  on  ponds 
and  lakes,  we  may  follow  the  changes  which  take  place  as  the  cold 
of  winter  comes  on. 

Fresh  water  is  densest  at  a  temperature  of  about  39°  F. 
(about  4°  C.).  The  surface  water  of  ponds  and  lakes  in  middle 

207 


208 


PHYSIOGRAPHY 


latitudes  is  usually  much  warmer  than  39°  in  summer.  The 
waters  below  the  surface  are  generally  cooler,  but  for  some  dis- 
tance at  least,  and  often  to  the  bottom,  they  are  warmer  than 
39°  F.  As  the  surface  water  cools  in  autumn  and  winter,  it  becomes 
heavier  than  the  warmer  water  beneath,  and  slowly  sinks.  This 
process  goes  on,  or  tends  to,  until  all  the  water  from  top  to  bottom 
has  a  temperature  of  about  39°.  With  further  cooling  the  upper- 
most water  expands  slightly,  and  so  becomes  lighter  and  remains 


FIG.  219. — Ice-crystals  forming  in  the  upper  part  of  the  soil  grow  by  the 
addition  of  moisture  rising  from  below.  The  ice  added  below  pushes 
up  the  ice  already  formed.  Columns  of  ice  two  or  three  inches  in  height 
are  formed  in  this  way.  often  raising  small  stones.  (Photo,  by  Roberts.) 

at  the  surface.     When  cooled  to  32°  F.  (0°  C.),  it  freezes.     In 
freezing  it  expands  about  one-tenth  of  its  volume. 

Deep  lakes  in  middle  latitudes,  such  as  the  Great  Lakes  of  the 
United  States,  do  not  freeze  over  even  in  the  coldest  winters,  for 
the  body  of  the  water  of  such  lakes  is  not  cooled  to  39°  F.,  and  so 
long  as  their  deeper  parts  have  a  temperature  above  39°,  the  sur- 
face water  sinks  as  it  is  cooled,  and  so  does  not  reach  the  freezing 
temperature.  Such  lakes,  therefore,  freeze  over  only  about  their 
borders,  where  the  water  is  so  shallow  that  its  temperature  from 
top  to  bottom  is  reduced  to  the  temperature  of  greatest  density. 
Theoretically,  this  colder  water  near  shore  should  spread  to  the 


THE  WORK  OF  SNOW  AND  ICE 


209 


greater  depths  farther  from  shore;  and  it  actually  does  move  in 
this  direction  whenever  it  is  heavier  than  that  of  greater  depths, 
but  the  movement  is  often  too  slow  to  prevent  the  freezing. 


FIG.  220. — Ice  crowding  on  shore.     Lake  Mendota,  Wis. 
(Buckley,  Wis.  Geol.  Surv.) 


FIG.  221.— Shore  of  Wall  Lake,  Iowa.     (Photo,  by  Calvin.) 

Like  most  other  solids,  ice  contracts  as  its  temperature  is  re- 
duced. If  the  temperature  falls  notably  after  the  lake  or  pond  is 
frozen  over,  the  ice,  in  contracting,  pulls  away  from  the  shore 


210 


PHYSIOGRAPHY 


or  cracks  open,  sometimes  with  loud,  pistol-like  reports.     Water 
rises  between  the  ice  and  the  shore,  or  into  the  opened  cracks, 


FIG.  222. — A  low  terrace  of  gravel  and  sand  formed  by  ice.     Shore  of  Oco- 
nomowoc  Lake,  Wis.     (Fenneman,  Wis.  Geol.  Surv.) 

and  freezes,  and  the  ice-cover  again  fits  the  lake.     When  the  cold 
"spell"  is  over,  the  temperature  of  the  ice  rises,  and  the  ice  ex- 


FIG.  223. — The  shove  of  ice  on  the  shore  of  Lake  Mendota,  Wis. 
(Photo,  by  Buckley.) 

pands.     The  expanding  ice  may  be  crowded  up  on  the  shores, 
especially  if  their  slopes  be  very  gentle  (Fig.  220),  or  bowed  up 


THE  WORK  OF  SNOW  AND  ICE 


211 


away  from  the  shores.     In  the  former  case,  sand,  gravel,  and  bowl- 
ders frozen  into  the  bottom  of  the  ice  are  pushed  up  with  it.     Many 


FIG.  224. — Shove  of  shore  ice  where  the  shore  is  marshy.    The  ice  of  the 
marsh  is  pushed  up  into  ridges.     (Buckley,  Wis.  Geol.  Surv.) 


Marsh 


Lak 


FIG.  225. — Diagram  representing  the  effects  of  ice-shove  on  a  marsh  adjoin- 
ing a  lake,  and  on  a  high  steep  bank.  It  is  to  be  remembered  that 
the  ground  is  frozen  when  the  shove  takes  place,  and  therefore  more 
resistant  than  when  not  frozen.  The  thrust  must  therefore  be  strong 
to  produce  the  observed  result.  • 

"walled  lakes"  (Fig.  221),  that  is,  lakes  with  accumulations  of 
bowlders  resembling  walls  about  their  shores,  owe  their  peculiar 


212 


PHYSIOGRAPHY 


features  to  the  shove  of  shore  ice.  Low  terraces  along  shores,  and 
low  ridges  are  also  made  by  the  landward  shove  of  the  ice  (Fig.  222). 
When  the  shore  of  the  lake  is  steep  and  of  loose  earthy  matter, 
the  expanding  ice  sometimes  crowds  in  under  the  soil,  even  over- 
turning trees  near  the  shore  (Fig.  223). 

The  ice  of  a  lake  may  be  continuous  laterally  with  the  ice  of 
the  soil  (Fig.  224),  and  in  this  case  the  shove  of  the  ice,  on  ex- 
panding, may  thrust  up  the  frozen  soil,  making  conspicuous  ridges 
of  it.  Fig.  224  shows  a  ridge  formed  in  this  way  on  the  shores  of 
Lake  Mendota  near  Madison,  Wis.,  in  the  winter  of  1898-9.  The 
ridge  here  is  chiefly  of  the  ice  of  the  marsh  which  bordered  the 
lake  at  this  point. 

Ice  on  the  sea.  In  high  latitudes  ice  is  formed  along  the  sea- 
shore. Unlike  fresh  water,  sea-water  condenses  till  it  freezes,  at  a 
temperature  of  26°  to  28°  F.  The  variation  in  the  temperature  is 
due  to  the  varying  salinity  of  the  water. 

The  ice  crystals  formed  from  sea-water  are  individually  with- 
out salt;  but  a  mass  of  ice  formed  from  sea- water  contains  iri- 


FIG.  226. — Floe-ice  on  the  shore  of  Greenland. 

elusions  of  crystallized  salt  or  of  brine,  excluded  from  the  salt 
water  as  it  froze.  If  large  quantities  of  such  ice  be  melted,  the 
resulting  water  is  more  or  less  salty,  though  apart  from  these 
inclusions  the  ice  is  fresh.  In  polar  regions  the  sea  ice  attains  a 
depth  of  several  feet,  at  least  as  much  as  eight  or  ten.  Floating 
ice  of  much  greater  thickness  is  sometimes  seen,  but  it  is  doubtful 
if  these  great  thicknesses  represent  the  ice  formed  by  the  freezing 


THE  WORK  OF  SNOW  AND  ICE  213 

of  undisturbed  sea-water.  At  any  rate,  the  ice  formed  in  winter 
is  often  broken  up  in  the  summer  inj;o  floating  pieces,  floe-ice  (Fig. 
226),  and  the  floe-ice  is  sometimes  crowded  together  in  ice-packs, 
the  separate  pieces  being  so  jammed  together  that  some  of  them 
are  ended  up  and  stand  high  above  the  water.  If  the  ice-pack 
of  one  summer  is  still  far  enough  north  at  the  end  of  the  warm 
season,  it  is  frozen  together,  and  its  aggregate  thickness,  made  up 
as  it  is  of  blocks  of  ice  some  of  which  are  on  edge,  is  far  beyond  that 
of  normal  sea  ice. 

Ice-foot.  In  high  latitudes  the  snows  along  shore  begin  to 
accumulate  in  the  autumn,  before  the  sea-water  freezes,  and  the 
water  dashed  up  in  storms  freezes  on  and  in  the  snow,  converting 
it  into  ice.  The  first  sea  ice  may  be  forced  up  on  the  land  some- 
what above  normal  sea-level  by  waves  and  tides,  and  it  is  thickened 
by  the  snow  which  lodges  on  it.  In  these  ways  the  ice  on  the 
shore  sometimes  becomes  very  thick,  with  its  upper  edge  many 
feet  above  sea-level.  Such  shore  ice  is  known  as  an  ice-foot.  On 
the  ice-foot,  rock  fragments  broken  off  from  cliffs  above  often 
gather  in  quantity.  This  protects  the  ice  beneath  from  melting, 
and  remnants  of  it  may  endure  through  the  summer. 

Ice  in  rivers.  Rivers  also  freeze  over  in  cold  climates,  and 
when  the  ice  breaks  up  in  the  spring,  stones  and  bowlders  to  which 
it  was  frozen  in  the  banks  are  sometimes  floated  miles  down  the 
river.  Not  only  are  bowlders  frozen  into  the  ice  floated  away, 
but  huge  pieces  are  occasionally  torn  from  points  of  rock  which 
project  into  the  river.  At  Montreal  stone  buildings  30  to  50  feet 
square,  projecting  so  as  to  have  river  ice  form  about  them,  have 
been  moved  by  the  ice  of  the  St.  Lawrence. 

When  the  river  ice  breaks  up,  masses  of  it  may  be  carried 
down-stream,  and  may  accumulate  in  vast  fields  or  "jams  "  behind 
obstructions  in  the  river.  Where  they  are  formed  above  bridges, 
the  bridges  are  likely  to  be  swept  away.  The  jams  also  occasion 
disastrous  floods  above  their  sites,  and  when  they  break,  the  waters 
accumulated  above  may  sweep  down  the  valleys  with  destructive 
violence. 

Northward-flowing  rivers  in  the  northern  continents  are  espe- 
cially subject  to  such  floods.  The  snows  of  their  upper  basins 
often  melt  while  the  lower  parts  of  the  streams  are  still  frozen  over. 
The  free  discharge  of  the  upper  waters  is  thus  prevented,  and 
freshets  result. 


214  PHYSIOGRAPHY 

When  frozen  over,  many  rivers  of  northern  latitudes  serve  as 
roadways. 

Ground-ice.  Tee  sometimes  forms  on  the  bottoms  of  stony 
rivers  where  the  current  is  swift.  It  ultimately  freezes  around  the 
stones  and  bowlders  on  the  bed,  and  when  enough  of  it  freezes  to 
them,  they  may  be  raised  from  the  bottom  and  floated  away.  Ice 
sometimes  forms  in  quantities  on  the  bottom  (or  below  the  sur- 
face) of  shallow  seas,  such  as  the  Gulf  of  St.  Lawrence  and  the 
Baltic  Sea.  Ice  thus  formed  is  called  ground-ice  or  sometimes 
anchor-ice.  Small  vessels  are  said  to  be  occasionally  surrounded 
and  entrapped  by  the  sudden  appearance  of  large  quantities  of 
this  ice  at  the  surface. 

Ground-ice  in  rivers  seems  to  be  due  (1)  sometimes  to  the  fact 
that  the  bed  of  the  stream  is  frozen,  and  the  water  in  contact  with 
it  freezes;  and  (2)  sometimes  to  the  fact  that,  though  the  tempera- 
ture of  the  river  as  a  whole  is  slightly  below  32°  F.,  the  greater 
motion  of  the  upper  and  swifter  part  keeps  the  water  there  from 
freezing,  while  the  quieter  water  below  congeals. 

The  cause  of  the  development  of  ice  on  the  bottom  of  shallow 
seas  is  not  clear.  The  suggestions  made  above  as  to  the  cause 
of  river-bottom  ice  do  not  seem  applicable.  It  may  be  that  fresh- 
water springs  issuing  into  sea-water  which  is  below  32°  F.,  but 
above  the  freezing  temperature  of  salt  water,  sometimes  freeze 
before  being  thoroughly  mixed  with  the  salt  water.  The  ice  formed 
about  anchors  is  probably  sometimes  the  result  of  the  low  tempera- 
ture of  the  anchor  before  it  was  lowered.  Ice  due  to  this  cause 
would  not,  however,  endure  long. 

Snow.  When  the  moisture  in  the  air  condenses  at  a  tempera- 
ture of  less  than  32°  F.,  it  commonly  takes  the  form  of  snow- 
flakes  (Fig.  76).  Snowflakes  are  not  frozen  rain-drops;  they  are 
formed  instead  of  rain-drops  when  the  temperature  at  which  the 
water  vapor  condenses  in  the  air  is  below  the  freezing-point. 

Snow  falls  in  high  latitudes  during  much  of  the  year,  and  in 
middle  latitudes  during  the  winter  season.  Except  on  high  moun- 
tains, little  falls  in  low  latitudes,  and  the  little  that  does  fall  gen- 
erally melts  quickly.  The  period  of  snowfall  and  the  duration  of 
the  period  when  snow  lies  on  the  surface  increase  both  with  in- 
creasing altitude  and  latitude,  so  that  above  the  polar  circles 
most  of  the  precipitation  falls  as  snow,  and  snow  lies  on  most  of 
the  land  surface  all  the  time,  even  at  low  levels.  The  same  is 


THE  WORK  OF  SNOW  AND  ICE 


215 


true  in  very  high  altitudes,  even  in  tropical  latitudes.  In  such 
situations,  indeed,  the  snowfall  of  the  cold  summer  is  often  much 
greater  than  that  of  the  winter. 

While  snow  lies  on  the  surface,  it  serves  to  protect  it.  It  shields 
the  vegetation  beneath  from  excessive  changes  of  temperature, 
and  especially  from  the  repeated  thawings  (by  day)  and  freezings 
(by  night)  which  are  injurious  to  many  plants,  and  it  keeps  the 
dust  and  sand  beneath  from  being  blown  about  by  the  wind.  The 
conditions  which  preserve  the  snow  also  prevent  the  effective 
wear  of  the  surface  by  running  water,  so  long  as  the  snow  is  on  the 
ground. 


FIG  227. — Mt.  Hood,  a  snow-capped  mountain.     (By  permission  of 
Lipman,  Wolfe  &  Co.) 

Snow-fields.  Where  snow  endures  from  year  to  year  over  any 
considerable  area,  it  constitutes  a  snow-field.  Snow-fields  are 
widely  distributed.  Stated  in  general  terms,  they  occur  in  moun- 
tains in  nearly  all  latitudes,  but  the  altitude  which  is  neces- 
sary in  the  equatorial  region  is  great  (15,000  to  18,000  feet),  that  in 
the  temperate  region  less,  and  that  in  the  polar  regions  slight. 
In  the  polar  regions,  indeed,  snow-fields  occur  even  down  to  sea- 
level.  Stated  in  other  terms,  snow-fields  occur  in  sufficiently  high 
altitudes  in  any  latitude,  and  at  any  altitude  in  sufficiently  high 
latitudes. 


216  PHYSIOGRAPHY 

Snow-fields  are  by  no  means  rare  in  the  United  States.  They 
occur  in  the  high  mountains  of  California,  Colorado,  and  Utah 
(rare),  and  in  the  high  mountains  of  all  the  states  farther  north 
(Fig.  227).  The  snow-fields  of  the  more  northerly  states  are  more 
numerous  and  on  the  whole  larger  than  those  farther  south.  In 
the  mountains  north  of  the  United  States  they  are  still  more  ex- 
tensive, and  in  Alaska  some  of  them  attain  considerable  size  (Fig. 
228). 

Small  snow-fields  also  occur  in  the  high  mountains  of  Mexico 
and  South  America.  They  occur  in  the  Alps,  the  Pyrenees,  the 
Caucasus,  and  the  Scandinavian  mountains  of  Europe,  and  in  the 


FIG.  228. — Snow-fields  in  the  Skolai  Range  of  Alaska.     Chisana  glacier  in 
the  foreground.     (U.  S.  Geol.  Surv.) 

Himalayas  and  the  higher  mountains  of  the  regions  farther  north 
and  northeast  in  Asia.  They  occur  also  in  Africa,  even  very  near 
the  equator,  though  they  are  small  and  limited  to  very  high  moun- 
tains. Besides  these  and  other  small  fields  of  snow  and  ice,  there 
are  two  great  fields  in  Greenland  and  Antarctica.  The  snow- 
and-ice  field  of  Greenland  contains  much  more  snow  and  ice  than 
all  the  mountain  snow-fields  mentioned  above,  and  that  of  Antarc- 
tica contains  probably  several  times  as  much  as  that  of  all  other 
fields  together. 

It  is  not  improbable  that  there  are  as  much  as  a  million  cubic 
miles  of  snow  and  ice  now  on  the  land.  If  this  amount  of  ice 
were  all  melted  and  returned  to  the  sea.  it  would  raise  its  level 
about  30  feet. 

The  snow-line.  The  line  above  which  the  snows  of  winter 
are  not  all  melted  is  the  snow-line.  The  snow- line  may  fluctuate 


THE  WORK  OF  SNOW  AND  ICE  217 

somewhat  from  year  to  year,  but  during  any  given  period,  his- 
torically speaking,  it  is  relatively  constant.  Its  average  position 
for  a  period  of  years  should  perhaps  be  regarded  as  the  snow-line 
for  that  period. 

1.  The  position  of  the  snow-line  is  influenced  by  temperature. 
This  is  shown  by  the  general  fact  that  it  is  higher  in  lower  (warmer) 
latitudes  and  lower  in  higher  (colder);  but  in  various  mountains, 
the  Himalayas  for  example,  the  snow-line  is  much  higher  on  the 
north  side  than  on  the  south,  although  the  temperature  on  the 
south  side  is  higher  than  that  on  the  north.     It  is  therefore  evident 
that  something  besides  temperature  is  involved  in  the  position  o]  the 
snow-line. 

2.  An  addition al  factor  is  the  amount  of  snowfall.     The  southerly 
winds  blowing  over  the  Himalayas  carry  much  more  moisture  than 
the  northerly  ones.     The  result  is  that  the  fall  of  snow  on  the 
southern  slope  is  much  heavier  than  that  on  the  northern.    The 
same  is  true  in  the  mountains  of  Switzerland.     The  position  of  the 
snow-line  is  therefore  influenced  by  the  amount  of  snowfall  as  well 
as  by  the  temperature.     Six  inches  of  snow  on  the  colder  north 
slope  of  mountains  (northern  hemisphere)  may  disappear  in  the 
fewer  melting  days  of  summer,  while  as  many  feet  of  snow  on  the 
warmer  south  slope  may  not  disappear  during  the  longer  period  of 
melting  in  that  position. 

3.  Again,  snow  does  not  disappear  entirely  by  melting.     Some 
of  it   evaporates,   and    aridity  favors  evaporation.     A  snow-field 
in  a  dry  region    is  therefore  wasted  more  by  evaporation  than 
one  in  a  humid  region.     Wind  increases  evaporation  if  the  air  is 
dry. 

4.  Topographic  relations  also  affect  the  position  of  the  snow- 
line  in  a  given  place,  for  some  situations  favor  accumulation  and 
afford  protection  against  the  sun. 

(1)  Temperature  and  (2)  amount  of  snowfall  are  the  principal 
factors  which  determine  the  position  of  the  snow-line,  and  (3) 
humidity  (or  aridity)  and  (4)  topographic  relations  are  minor 
factors.  Since  these  factors  vary  from  place  to  place,  no  particu- 
lar altitude  in  any  particular  latitude  can  be  specified  as  the  one 
necessary  for  the  existence  of  perennial  snow. 

The  following  table  shows  the  position  of  the  snow-line  at  a 
few  points: 


218 


PHYSIOGRAPHY 


Bolivian  Andes,  west  side, 

Bolivian  Andes;  east  side, 

Chilean  Andes, 

Mexico 

Teneriffe, 

Himalayas,  north  side, 

Himalayas,  south  side, 

Caucasus  Mountains, 

Pyrenees  Mountains, 

Alps, 

Norway, 

Lapland, 

Alaska, 

Greenland. 


Near  equator. 
Near  equator, 
Lat.  33°  S., 

Lat.  33°  N., 
Lat.  about  28°  N., 
Lat.  about  28C  N., 
Lat.  40°  +  N., 
Lat.  40° +  N., 
Lat.  about  46  J°  N. 

Lat.  70°  N., 
Lat.  60°-70°  N., 


About  18,500  feet. 
About  16,000  feet. 
About  12,800  feet. 
About  14,800  feet. 
About  13,000  feet. 
About  16,700  feet. 
About  13,000  feet. 
About  8300  to  14,000  feet 
About  6500  feet. 
About  9000  feet. 
About  5000  feet. 
About  3000  feet. 
About  5500  feet. 
About  2200  feet. 


Ice-fields.  Every  considerable  snow-field  is  also  an  ice-field, 
for  where  snow  accumulates  to  great  depths  and  lies  long  upon  the 
surface,  the  greater  part  of  it  is  converted  into  ice.  The  beginning 
of  this  change  is  seen  in  the  snow  which  has  lain  for  a  few  days  at 
the  surface.  It  loses  its  flaky  character  and  becomes  coarse-grained, 
so  that  it  is  harsh  to  the  touch.  The  change  is  still  more  con- 
spicuous in  the  last  banks  of  snow  in  the  spring.  The  snow  of  such 
banks  is  made  up  of  coarse  granules,  often  of  considerable  size. 
The  change  of  the  flakes  into  granules  is  a  process  which  is  due, 
in  part,  to  the  melting  of  the  surface  snow  and  the  sinking  and 
re-freezing  of  the  water  below  the  surface;  but  since  the  change 
appears  to  go  on  even  where  there  is  no  melting,  melting  and  re- 
freezing  are  probably  only  a  part  of  the  process  of  change. 

While  this  transformation  is  going  on,  the  snow  becomes  more 
compact.  As  it  lies  on  the  surface,  its  own  weight  tends  to  com- 
press it.  The  sinking  water  which  re-freezes  below  the  surface 
tends  to  bind  the  granules  to  one  another,  and  as  a  result  of  the 
compression,  of  the  transformation  of  the  flakes  into  granules,  and 
of  the  binding  together  of  the  granules  themselves  by  the  freezing 
of  water  between  them,  the  whole  mass  tends  to  become  solid. 
Just  how  solid  and  how  dense  snow  must  become  before  it  is  to  be 
called  ice,  cannot  be  stated;  but  every  great  snow-field  is  really 
an  ice-field,  scarcely  more  than  frosted  over  with  snow.  The 
last  snow-banks  of  spring  are  often  essentially  ice. 


THE  WORK  OF  SNOW  AND  ICE  219 


GLACIERS. 

If  the  body  of  ice  developed  from  snow  becomes  great  enough, 
it  begins  to  spread  or  creep  out  from  its  place  of  accumulation. 
Ice  thus  moving  is  glacier  ice.  Not  all  snow-fields  give  origin  to 
glaciers,  but  nearly  all  glaciers  have  their  sources  in  snow-fields. 
The  distribution  of  glaciers  is  therefore  much  the  same  as  the  dis- 
tribution of  snow-fields. 

Types  of  glaciers.  Glaciers  assume  various  shapes,  depending 
chiefly  on  the  amount  of  ice  and  on  the  configuration  of  the  sur- 
face on  which  it  lies.  If  the  originating  snow-field  lies  on  the 
slope  of  a  mountain,  the  ice  moves  down  the  slope;  and  if  a  valley 


FlG.  229. — Summit  of  the  Nizina-Tanana  glacier,   Alaska. 
(Rohn,  U.  S.  Geol  Surv.) 

leads  out  from  the  area  of  the  snow-field,  the  movement  of  the  ice 
is  chiefly  concentrated  in  the  valley.  If  the  ice  lies  on  a  flat  sur- 
face, it  spreads  in  all  directions  from  its  centre. 

Glaciers  which  occupy  valleys  are  called  valley  glaciers.  In 
common  speech,  "a  glacier"  is  usually  understood  to  be  a  valley 
glacier.  All  valley  glaciers  are  sometimes  called  alpine  glaciers, 
because  they  belong  to  the  same  general  class  as  those  of  the  Alps ; 
but  the  valley  glaciers  of  high  latitudes  differ  in  some  ways,  espe- 
cially in  their  steeper  sides  and  ends,  from  those  of  the  Alps.  For 
this  reason,  valley  glaciers  may  be  classed  as  alpine  (Fig.  230) 
and  high-latitude  (Fig.  231)  glaciers. 

In  high  latitudes  glacier  ice  sometimes  lies  on  plains  or  plateaus. 
In  such  positions  glaciers  may  be  nearly  circular  in  outline,  and  may 


220 


PHYSIOGRAPHY 


FIG.  230 —The  Rhone  glacier.     (Photo,  by  Reid.) 


FIG.  231. — The  end  of  the  Bryant  glacier,  a  high-latitude  glacier  of  North 
Greenland.     (Photo,  by  Chamberlin.) 


THE  WORK  OF  SNOW  AND  ICE 


221 


spread  radially  from  their  centres.  Such  glaciers  are  ice-caps  or 
ice-sheets.  Ice-caps  may  be  large  or  small.  The  main  ice-caps 
of  Antarctica  and  Greenland  (Figs.  252  and  259)  are  large,  but 


30  Miles 


FIG.  232. — A  small   ice-cap   in   the   northwestern  part  of  Iceland. 
(After  Thoraddsen.) 


FIG.  233.— A  cliff  glacier,  coast  of  North  Greenland.      The  height  of  the 
cliff  is  perhaps  2000  feet.     The  water  in  the  foreground  is  the  sea. 

small  ones  of  the  same  type  are  found  on  various  promontories 
along  the  coast  of  Greenland,  on  Iceland  (Fig.  232),  and  on  some 
Arctic  islands. 


222  PHYSIOGRAPHY 

Glaciers  sometimes  occur  at  the  bases  of  mountains,  being 
formed  by  the  union  of  the  spreading  ends  of  valley  glaciers. 
Such  glaciers  are  piedmont  glaciers  (Fig.  260).  Again,  many  snow- 
fields  nestled  in  the  depressions  of  mountain  cliffs  give  origin  to 
small  glaciers  which  never  descend  to  a  valley.  Such  ill-formed 
and  poorly  developed  glaciers  are  cliff  glaciers  (Fig.  233).  Cliff 
glaciers  grade  into  valley  glaciers  (Figs.  234  and  235).  Ice  broken 


FIG.  234. — Glaciers  intermediate  in  type  between  a  cliff  glacier  and  a  valley 
glacier.     Cascade  Mts.,  Wash.     (Willis,  U.  S.  Geol.  Surv.) 

off  from  ice-sheets,  or  from  valley  glaciers  which  reach  a  cliff,  may 
accumulate  below,  freeze  together  again,  and  assume  movement. 
Such  a  glacier  is  sometimes  called  a  reconstructed  glacier. 

Of  these  types,  valley  glaciers  are  most  common  and  familiar, 
but  ice-caps  contain  far  more  ice.  The  leading  characteristics  of 
glaciers  may  be  studied  in  connection  with  the  most  familiar 
form. 


THE  WORK  OF  SNOW  AND  ICE 


223 


The  Valley  Glacier 

The  general  form  of  a  valley  glacier  (Fig.  236)  is  determined 
by  the  shape  of  the  valley  in  which  it  lies.  If  the  valley  is  crooked, 
the  glacier  curves  to  match  it,  and  if  the  bottom  of  the  valley  is 
uneven,  the  surface  of  the  ice  is  more  or  less  uneven,  in  keeping 
with  it.  The  valley  glacier  has  sometimes  been  called  a  "river  of 


FIG.  235. — Dana  glacier,  Mt.   Dana,  Cal.     A  glacier  of  the  same  type  as 
that  shown  in  Fig.  234. 

ice,"  but  the  differences  between  a  glacier  and  a  river  are  so  much 
greater  than  their  likenesses  that  this  definition  is  misleading. 

The  surface.  The  upper  end  of  a  valley  glacier  is  in  the  snow- 
field,  and  is  covered  with  snow  all  the  time,  while  the  lower  end 
may  be  covered  during  the  winter.  Some  glaciers  carry  so  much 
stony  and  earthy  debris  on  their  surfaces  as  to  conceal  the  ice  in 
some  places,  especially  near  the  lower  end. 

The  centre  of  a  valley  glacier  is  usually  higher  than  its  sides, 
so  that  its  upper  surface  is  generally  somewhat  convex  in  cross- 
section.  The  profile  of  the  surface  of  a  glacier  corresponds  some- 
what to  the  profile  of  the  bottom  of  the  valley  in  which  it  lies 
(Figs.  230  and  236),  as  already  noted,  but  its  slope  is  sometimes 


224 


PHYSIOGRAPHY 


notably  increased  near  its  lower  end,  because  of  the  steep  slope  of 

the  upper  surface  of  the  ice. 

The  surface  of  the  glacier 
is  often  uneven.  In  many 
cases  it  is  cracked,  and  the 
cracks  or  crevasses  frequently 
gape.  A  principal  cause  of 
the  crevasses  is  the  movement 
of  the  brittle  ice  over  an  un- 
even bed  (Fig.  238).  When 
the  slope  of  a  glacier  bed  in- 
creases suddenly,  an  ice  cas- 
cade is  developed  (Figs.  230 
and  237) ;  but  an  ice  cascade 
has  little  in  common  with  the 
rapids  or  falls  of  rivers. 
Crevasses  formed  by  the  pas- 
sage of  ice  over  a  steep  place 
in  its  bed  are  usually  trans- 
verse to  the  glacier.  Cre- 
vasses are  sometimes  parallel 
to  the  sides  of  the  glacier  and 
oblique  to  them,  and  such 
crevasses  are  due  to  other 

FIG.  236.— Aletsch  glacier,  Switzerland,    causes.      The  breaking  of   the 

ice  as  it  moves  is  one  of  the 

many  features  wherein  a  glacier  differs  from  a  river. 


PIG.  237. — Diagrammatic  and  longitudinal  sections  of  glaciers.    (After  Heim.) 

As  the  ice  moves  forward,  the   crevasses  sometimes  tend  to 
close,  though  they  rarely  heal  in  such  a  way  as  to  leave  the  surface 


THE  WORK  OF  SNOW  AND  ICE 


225 


of  the  ice  smooth.  So  long  as  a  crevasse  is  open,  the  sun's  rays 
and  the  sun-warmed  air  enter  it  and  melt  the  ice.  The  effect  of 
the  melting  is  to  widen  the  crevasse,  especially  its  upper  part. 


FIG.  238. — Crevassed  glacier,  the  cracking  due  to  change  in  grade  of  bed. 

North  Greenland. 

The  result  is  that  when  the  movement  tends  to  close  the  crevasse, 
the  opposing  faces  rarely  fit  together.    This  is  illustrated  by  Figs. 


FIG.  239. — Crevassing  in  the  upper  part  of  a  glacier  on  Mt.   Hood,   Ore. 

(Meyers.) 

240  and  241.     The  crevassing  and  the  subsequent  melting  are 
therefore  a  cause  of  unevenness  of  surface. 


226 


PHYSIOGRAPHY 


Another  cause  of  surface  irregularity  is  the  drainage  from  the 
surface.  The  valley  glacier  often  extends  far  below  the  snow 
line,  and  is  within  the  region  of  active  melting  during  the  summer 
season.  Some  of  the  surface  water  sinks  beneath  the  surface, 


FIG.  240. — Diagram  to  illustrate  one  reason  why  ice  crevasses  fail  to  heal 
as  explained  in  text. 

but  some  of  it  runs  in  little  streams  on  the  ice  until  it  reaches  a 
crevasse  or  the  edge  of  the  glacier.  These  surface  streams  wear 
notable  channels  (valleys)  in  the  ice  (Fig.  242),  which,  though 
rarely  deep,  help  to  destroy  the  smoothness  of  the  surface. 

The  stony  and  earthy  debris  which  many  valley  glaciers  carry 
on  their  surfaces  also  gives  rise  to  irregularities.     The  large  stones 


FIG.  241. — Seracs  of  glaciers.     (Photo,  by  Reid.) 

protect  the  ice  beneath  from  melting,  and  therefore  come  to  stand 
on  pedestals  of  ice,  as  the  unprotected  ice  about  them  is  melted 
away.  Considerable  aggregations  of  debris  of  any  sort  have  the 
same  effect,  by  protecting  the  ice  beneath  from  melting,  thus 
giving  rise  to  mounds  or  ridges  of  ice  covered  with  debris 


THE  WORK  OF  SNOW  AND  ICE 


227 


(Fig.  243).  Small  or  thin  stones  on  the  surface  of  the  ice  affect  its 

topography  in  the  opposite  way. 
Rock  absorbs  heat  better  than  the 
ice  does,  and  thin  pieces  of  rock 
are  warmed  through.  They  may 
then  melt  their  way  into  the  ice 
more  rapidly  than  the  sun  melts 
down  the  surface  about  them,  thus 
making  depressions  in  the  ice. 
Patches  of  dust  blown  on  the  ice 
have  the  same  effect.  The  de- 
pressions to  which  they  give  rise  are 
known  as  "dust-wells"  (Fig.  245). 
Dust-wells  are  sometimes  so  close 
together  that  one  must  watch  his 

FIG.  242.— Valley  of  a  superglaciai  steps  in  walking  over  the  ice.  Their 

depths  depend  upon  their  diameters 
and  the  angle  of  the  sun's  rays  (Fig. 

246).    Their  bottoms  do  not  descend  below  the  plane  where  the 

sun's  rays  strike  the  heat-absorbing  sediment.     Dust-wells  are 


stream  in  the   Bighorn  Mts. 
(Photo,  by  Blackwelder.) 


FIG.  243. — Bowlders  on  ice  pinnacles.     Forno  glacier,  Switzerland. 
(Photo,  by  Reid.) 

usually  full  of  water  at  the  end  of  a  warm  (melting  temperature) 


228 


PHYSIOGRAPHY 


FIG.  2436. 


FIG.  244. 


FIG.  243a. — Ice  columns  capped  by  slabs  of  rock,  on  Parker  Creek  glacier, 

California.     (U.  S.  Gepl.  Surv.) 
FIG.  243Z>.— An  ice  pyramid  on  Mt.  Lyell  glacier,  California.     The  protecting 

stone   has  fallen  from  the  column,  which  has  since  melted   into  the 

pyramidal  form.     (~U.  S.  Geol.  Surv.) 
FIG.  244. — Diagram  to  show  how  debris  on  ice  gives  rise  to  prominences. 

(Gilbert.) 


I  *  • 
» 
I 


*;  ,*  y-f* 


..          -* 

^   *  ^«*     *   *    .      - 

*  . .  .' ,    :-  • !. 

/**•       •  » 

%-r/  •:•    • 
^ 


-.. 


FIG.  245. — Dust-wells,  North  Greenland.     (Photo,  by  Chamberlin.) 


THE  WORK  OF  SNOW  AND  ICE 


229 


day,  but  the  water  usually  drains  out  at  night.  This  drainage 
shows  that  the  glacier  ice  is,  on  the  whole,  very  leaky.  Depressions 
resembling  dust-wells,  and  of  the  same  origin,  sometimes  develop 
on  the  compact  surface  of  snow  which  has  lain  for  some  time  on 
the  ground. 

Movement 

Waste  and  supply  of  ice.  The  ice  of  a  glacier  is  continually 
wasting.  The  waste  is  due  partly  to  surface  melting,  especially 
in  summer,  partly  to  melting  below  the  surface,  for  much  of  the 

*  P 


IS 


FIG.  246. 


FIG.  247. 


FIG.  246. — Diagram  to  illustrate  the  fact  that  wells  of  larger  diameter 
may  be  deeper  than  those  of  smaller  diameter.  The  slanting  lilies 
represent  the  direction  of  the  sun's  rays  when  the  sun  is  highest. 

FIG.  247. — Diagram  illustrating  certain  features  of  glacier  motion.  The 
figure  at  the  left  represents  a  vertical  section,  and  the  top  as  moving 
faster  than  the  bottom.  The  figure  at  the  right  represents  a  part  of 
the  surface,  and  the  central  part  as  moving  faster  than  the  sides. 

subsurface  ice  of  most  glaciers  is  at  the  melting  temperature  much 
of  the  time,  and  partly  to  evaporation. 

In  spite  of  the  rapid  waste  of  glaciers,  particularly  at  their 
lower  ends  and  in  summer,  they  often  remain  nearly  constant 
in  size  for  long  periods  of  time.  This  shows  that  there  must  be  a 
source  of  supply  to  replace  the  waste.  This  source  is  found  in 
the  snow-fields.  From  them  the  ice  creeps  down  the  valleys 
until  it  reaches  an  altitude  so  low  and  so  warm  that  the  waste 
(chiefly  melting)  at  its  end  balances  its  forward  motion. 

The  fact  of  movement  was  first  established  by  noting  (1)  that 
the  ends  of  glaciers  were  sometimes  farther  down  the  valleys  than 
they  had  been  at  earlier  times,  and  (2)  that  familiar  objects  at  the 
ends  of  glaciers  were  overturned  and  pushed  forward  by  the  ice. 

Rate  of  movement.  Once  the  fact  of  movement  was  estab- 
lished, various  means  were  devised  for  measuring  its  rate.  Rows 


230  PHYSIOGRAPHY 

of  stakes  were  set  across  a  glacier  in  a  straight  line,  and  their 
positions  with  reference  to  fixed  points  on  the  sides  of  the  valley 
marked.  After  a  time  they  were  found  to  have  moved  down  the 
valley.  In  most  cases  it  appears  that  those  in  the  central  part  of 
the  glacier  have  moved  faster  than  others,  as  shown  by  Fig.  247. 

In  this  and  in  other  similar  ways  the  rate  of  movement  of 
numerous  glaciers  has  been  determined.  It  ranges  from  an  amount 
so  small  as  to  be  measured  with  difficulty,  to  several  feet  per  day. 
One  very  large  glacier  in  north  Greenland  has  been  estimated  to 
move  100  feet  per  day,  but  this  is  certainly  far  beyond  the  rate  for 
any  of  the  more  accessible  and  better-known  glaciers.  Few  of  the 
better-known  mountain-valley  glaciers  move  more  than  a  foot  or 
two  a  day. 

Conditions  affecting  rate  of  movement.  The  rate  of  glacier 
movement  appears  to  depend  chiefly  on  (1)  the  depth  of  the  moving 
ice;  (2)  the  slope  of  the  surface  over  which  it  moves;  (3)  the  slope 
of  the  upper  surface  of  the  ice;  (4)  the  topography  of  the  bed  over 
which  it  passes;  (5)  the  temperature;  (6)  the  amount  of  water 
in  the  ice,  including  that  which  falls  upon  it  or  is  carried  to  it  by 
the  drainage  of  its  surroundings,  as  well  as  that  produced  by  the 
melting  of  the  glacier  itself;  and  (7)  the  amount  of  load  (debris) 
which  the  ice  carries,  especially  in  its  bottom.  Great  thickness, 
a  steep  slope,  smoothness  of  bed,  a  high  (for  ice)  temperature,  and 
much  water  favor  rapid  movement.  Since  some  of  these  conditions, 
notably  temperature  and  amount  of  water,  vary  with  the  season, 
the  rate  of  movement  for  any  given  glacier  is  not  constant  through- 
out the  year,  and  is  generally  greater  in  summer  than  in  winter. 
Other  conditions,  especially  the  first  of  those  mentioned  above,  vary 
through  longer  periods  of  time,  and  occasion  periodic  variations 
in  the  rate  of  movement. 

MAP  EXERCISE 

Topographic   Maps  Showing  Glaciers 

Study  the  following  maps  showing  existing  glaciers,  in  preparation 
for  conference: 

1.  Shasta,  Cal. 

2.  Mt.  Lyell,  Cal. 

3.  Mt.  Stuart,  Wash, 

4.  Glacier  Peak,  Wash. 

5.  Cloud  Peak,  Wyo. 


THE  WORK  OF  SNOW  AND  ICE  231 

In  each  note  (a)  the  altitude  of  the  glaciers,  (6)  their  size,  and 
(c)  their  exposure  (on  east,  west,  north,  or  south  slopes). 

Is  there  any  relation  between  the  heights  of  (a)  the  lower,  and  (6) 
the  upper  ends  of  the  glaciers,  and  the  exposures? 

Compare  the  heights  of  the  lower  ends  of  the  glaciers  in  California, 
Washington,  and  Wyoming.  Why  the  differences? 

Note  the  peculiar  shape  of  the  upper  ends  of  many  of  the  valleys 
shown  on  the  Cloud  Peak  Sheet,  for  example  that  of  the  South  Fork  of 
Clear  Creek.  Such  valley  heads  are  cirques,  and  cirques  generally 
indicate  the  former  existence  of  glaciers. 

Nature  of  glacier  movement.  Glacier  movement  has  been 
much  discussed,  but  no  general  agreement  concerning  its  nature 


FIG.  248. — The  spreading  end  of  a  glacier,  North  Greenland. 

has  been  reached.  From  the  fact  that  the  ice  moves  down  the 
valley,  conforming  to  it  somewhat  as  a  river  does  to  its  valley,  it 
has  been  thought  that  a  glacier  flows  like  a  stiff  liquid.  This 
idea  seemed  at  first  to  be  supported  by  the  fact  that  when  a  glacier 
moves  out  from  its  mountain  valley  to  the  plain  beyond,  it  gen- 
erally spreads  (Fig.  248),  somewhat  as  a  stiff  liquid  might.  It 
is  to  be  noted,  however,  that  the  spreading  or  deploying  end  cracks 
open.  In  further  support  of  this  explanation  of  glacier  motion, 
various  experiments  have  been  performed  upon  ice.  They  show 
that  a  bar  of  ice  may  be  bent  or  moulded  into  almost  any  desired 


232 


PHYSIOGRAPHY 


form,  if  it  be  subjected  to  sufficient  pressure,  applied  slowly  enough 
through  long  periods  of  time. 


+.  +.•*•  .•*••*•  +  .+ 


FIG.  249. — Diagram  to  show  relations  of  a  high-latitude  glacier  to  its 

valley  walls. 

But  in  spite  of  the  apparent  mobility  of  ice,  and  in  spite  of  the 
fact  that  in  so  many  ways  its  motion  seems  to  resemble  that  of  a 

stiff  liquid,  it  is  very  doubtful  if  its 
real  motion  is  one  of  flowage,  as 
that  term  is  ordinarily  understood. 
It  has  already  been  stated  that  the 
ice  often  cracks  open  when  it  passes 
over  irregularities  of  bed,  as  well  as 
under  some  other  circumstances. 
Cracking  open  is  not  a  characteristic 
of  liquids.  Many  glaciers  of  high 
latitudes  do  not  rest  against  the 
sides  of  the  valleys  in  which  they 
lie  (Figs.  249  and  250).  Such  gla- 
ciers are  often  crevassed  longitu- 
dinally, and  the  crevasses  sometimes 
have  great  length.  If  the  ice  flows, 
therefore,  it  must  be  supposed  to 
flow  until  it  cracks  open.  It  is 
not  evident  that  a  fluid,  however 
viscous,  would  do  this.  These  and 
many  other  considerations,  which 
will  not  be  detailed  here,  have  led 
to  the  view  that  the  resemblance 
between  glacier  motion  and  the 
motion  of  a  stiff  liquid  is  more 
seeming  than  real. 

The    fundamental    element    in 
glacier  motion  probably  consists  in  the  melting  and  re-freezing  of 
its  substance.      The  process  is  a  very  complex  one,  and  cannot 
be  fully  analyzed  here,  but  some  of  its  elements  may  be  stated. 
When  water  from  the  surface  sinks  into  the  glacier  and  freezes 


FIG.  250. — A  part  of  the  verti- 
cal side  of  a  North  Green- 
land glacier.  The  vertical  or 
even  overhanging  faces  are 
sometimes  more  than  100  feet 
high. 


THE  WORK  OF  SNOW  AND  ICE  233 

again  it  expands,  and  the  ice  where  the  freezing  takes  place  is 
subject  to  great  stress.  The  force  of  the  stress  which  freezing 
water  exerts  is  illustrated  by  the  familiar  fact  that  vessels  of  very 
considerable  strength  are  broken  when  water  freezes  in  them. 
The  freezing  of  the  water  which  has  descended  must  have  the  effect 
of  moving  the  ice,  and  the  movement  must  be  chiefly  down  the 
valley,  for  in  this  direction  gravity  helps,  while  in  the  opposite 
direction  it  hinders.  Furthermore,  the  water  before  re-freezing 
moves,  and  always  downward,  not  only  toward  the  bottom  of  the 
ice,  but  often,  at  least,  toward  the  lower  end  of  the  valley  as  well. 
The  flow  of  the  water  is  therefore  a  way  of  transferring  the  ice  of  the 
glacier  down-valley. 

There  are  causes  of  melting  and  re-freezing  other  than  those 
which  are  dependent  upon  the  direct  influence  of  the  sun  or  the 
heat  of  the  interior  of  the  earth.  These  are  bound  up  with  the 
movement  of  the  ice  itself,  and,  without  attempting  here  to  ex- 
plain the  principle  involved,  it  may  be  stated  that  it  is  now  be- 
lieved that  an  important  part  of  glacier  motion  is  to  be  explained 
by  the  melting  which  results  from  the  pressure  involved  in  motion, 
and  in  the  re-freezing  of  the  water  thus  produced.  It  is  believed 
therefore  that  though  the  aggregate  motion  of  a  glacier  is  in  a 
way  comparable  to  the  motion  of  a  viscous  body,  the  actual  motion 
is  probably  that  of  a  solid,  small  parts  of  which  frequently  pass 
from  a  solid  to  a  liquid  condition  for  brief  spaces  of  time. 

Another  element  of  glacier  motion  is  sliding,  for  parts  of  a 
glacier  sometimes  slide  or  shear  over  other  parts  (Fig.  251).  The 
motion  of  the  lower  portion  of  a  glacier  which  carries  much  debris 
is  greatly  retarded  by  its  load.  The  relatively  clean  ice  above 
the  bottom  moves  less  slowly,  and  appears  very  often  to  be  thrust 
forward  or  sheared  over  the  debris-laden  portion  below.  This 
phase  of  motion  is  probably  much  more  common  and  of  much 
more  consequence  than  was  formerly  supposed.  It  is  best  seen 
in  the  glaciers  of  high  latitudes,  where  the  vertical  edges  and 
ends  allow  the  structure  of  the  ice  to  be  well  seen.  Under  some 
conditions  a  glacier  may  probably  slide  over  its  bed.  Sliding  is 
not,  however,  believed  to  be  a  principal  element  in  glacier  motion. 

Size.  There  are  in  the  Alps  nearly  2000  glaciers.  The  longest 
of  them  is  about  10  miles  long.  Less  than  40  are  as  much  as  five 
miles  long,  and  the  great  majority  are  less  than  one  mile  in  length. 
Some  of  them  are  but  a  few  hundred  feet  wide,  and  few  of  them 


234 


PHYSIOGRAPHY 


are  so  much  as  a  mile  wide.  The  thickness  of  ice  is  rarely  known, 
except  at  the  lower  ends,  but  it  is  generally  to  be  measured  by 
hundreds  of  feet,  rather  than  by  denominations  of  a  higher  order. 
Larger  alpine  glaciers  occur  in  the  Caucasus  Mountains  of 
Europe  and  in  Alaska.  Seward  Glacier  in  Alaska  is  more  than 
50  miles  long,  and  3  miles  wide  at  the  narrowest  part.  The  glaciers 
of  the  western  mountains  of  the  United  States  (south  of  Alaska) 
are  mostly  shorter  than  the  longer  glaciers  of  the  Alps.  Many 


FIG.  251. — Shearing  planes  in  ice,  well  defined. 

of  them  are  indeed  cliff  glaciers,  or  intermediate  in  type  between 
valley  glaciers  and  cliff  glaciers  (Figs.  234  and  235). 

Ice-caps 

Ice-caps  lie  on  plains  or  plateaus  instead  of  occupying  moun- 
tain valleys.  As  already  stated,  they  may  be  large  or  small. 
Large  ones  may  cover  valleys  and  hills  alike.  Very  large  ones 
are  sometimes  called  continental  glaciers.  At  the  present  time 
the  ice-caps  of  Greenland  and  Antarctica  are  the  only  ones  which 
attain  great  size. 


THE  WORK  OF  SNOW  AND  ICE 


235 


The  area  of  Greenland  has  been  variously  estimated  at  from 
400,000  to  600,000  square  miles,  and  all  of  it,  except  its  borders, 
is  covered  with  a  vast  field  of  snow  and  ice  (Fig.  252).  Near 
its  margin,  occasional  mountain  tops  project  above  the  snow, 
and  here  its  surface  carries  some  debris;  but,  except  about  its 


FIG.  252. — Map  showing  the  ice-cap  of  Greenland, 
the  island  are  free  of  ice. 


Only  the  borders  of 


edge,  nothing  is  visible,  so  far  as  known,  through  the  entire 
island,  save  one  vast  plateau  of  snow-covered  ice.  The  sur- 
face snow  is  frequently  driven  by  the  wind  into  rolling  billows. 
The  snow-  and  ice-covered  plateau  rises  gradually  toward  the 
centre  of  the  island,  where  it  attains  an  elevation  of  8000  or  9000 
feet.  The  thickness  of  the  ice  is  not  known,  but  where  thickest 
it  is  probably  some  thousands  of  feet. 


236 


PHYSIOGRAPHY 


The  ice  of  this  great  field  is  creeping  slowly  outward.     The 
rate  of  movement  has  not  been  determined  and  is  probably  not 


FIG.  253. — Edge  of  the  Greenland  ice-sheet. 

the  same  at  all  points;  but  it  has  been  estimated  not  to  exceed  a 
foot  a  week.      Near  its  margin  the  ice-cap  is  much  crevassed; 


Fia.  254. — A  mountain  projecting  up  through  the  ice,  North  Greenland. 

but  the  interior  is  comparatively  smooth  and  unbroken,  so  far 
as  now  known. 

The  ice-cap  of  Greenland  is,  in  one  sense,  more  of  a  desert 


THE  WORK  OF  SNOW  AND  ICE 


237 


FIG.  255. — A  nunatak  projecting  up  through  a  Greenland  glacier. 


FIG.  256. Three  small  glaciers  descending  to  the  sea,  North  Greenland, 


238 


PHYSIOGRAPHY 


FIG.  257. — Front  of  Miles  glacier,  Alaska,  where  it  reaches  Copper  River. 
(U.  S.  Geol.  Surv.) 


FIG.  257a. — Glacier  and  Icebergs. 


FIG.  258. — Iceberg,  coast  of  Greenland. 


THE  WORK  OF  SNOW  AND  ICE 


239 


than  the  Sahara,  since  it  is  inhabited,  even  less  than  that  desert, 
by  plants  and  animals.  There  are,  it  is  true,  tiny  red  plants  upon 
it  at  various  points  about  its  border.  Taken  singly,  they  are  too 
small  to  be  readily  noticed,  but  they  sometimes  occur  in  such 
multitudes  as  to  give  the  snow  a  distinctly  red  color,  known  as 
"red  snow."  Occasional  small  animals,  especially  the  larvas  of 


FIG.  259. — Map  of  Antarctica.     The  dotted  line  represents  the  approximate 
limit  of  abundant  floating  ice.     (After  Bartholomew.) 

certain  insects,  are  also  found  on  the  snow  some  little  distance  at 
least  back  from  its  margin. 

Where  the  edge  of  the  Greenland  ice-cap  lies  back  a  few  miles 
from  the  coast,  the  rock  plateau  outside  it  is  affected  by  numerous 
valleys  which  lead  down  to  the  sea.  Where  the  edge  of  the  ice- 
cap reaches  the  heads  of  these  valleys,  ice  moves  down  them  in 
advance  of  the  edge  of  the  ice-cap,  making  valley  glaciers.  Many 
of  the  valley  glaciers  move  down  to  the  sea,  where  their  ends  are 


240 


PHYSIOGRAPHY 


broken  off  (Fig.  256)  and  floated  away  as  icebergs.  Many  of  these 
glaciers  are  far  larger  than  any  of  the  Swiss  glaciers,  and  some 
are  even  larger  than  the  great  Seward  Glacier  of  Alaska.  While 
their  number  is  very  large,  the  total  amount  of  ice  in  them  is 
trivial  compared  with  the  amount  in  the  one  great  ice-cap  glacier 
from  which  they  originate. 

The  Antarctic  snow-  and  ice-cap  is  far  more  extensive  than 
that  of  Greenland,  but  its  area  is  not  even  approximately  known 
(Fig.  259).  It  appears  to  be  some  millions  of  square  miles  in 
extent,  but  mountains  project  up  through  it,  as  determined  by 
the  Schackleton  expedition.  At  many  points  the  ice  descends  to 
sea,  where  huge  blocks  of  it  are  broken  off  and  floated  away  as 
icebergs.  Whether  this  Antarctic  ice-cap  rests  on  a  continuous 
land-mass  which  if  free  from  ice  would  be  called  a  continent ,  or 
whether  it  rests  on  numerous  islands  which  but  for  the  ice  would 
be  separated  by  shallow  water,  is  not  known. 


FIG.  260. — Malaspina  glacier,  a  piedmont  glacier  in  Alaska.     (After  Russell.) 

Piedmont  Glaciers. 

In  Alaska  a  number  of  large  alpine  glaciers  emerge  from  adja- 
cent valleys  of  the  St.  Elias  range  upon  a  low  plain,  where  their 
ends  spread  and  unite  to  form  a  vast  plateau  of  ice,  70  miles  long 


THE  WORK  OF  SNOW  AND  ICE 


241 


and  20  to  25  miles  wide.  This  peculiar  body  of  ice  is  the  Malas- 
pina  Glacier  (Fig.  260).  Its  area  is  considerably  more  than  that 
of  the  state  of  Delaware.  Its  central  portion  is  free  from  rock 
debris,  but  is  interrupted  by  thousands  of  crevasses.  On  warm 
summer  days  hundreds  of  rivulets  flow  in  channels  of  clear  ice 
until  they  lose  themselves  in  yawning  crevasses.  The  deep  roar 
of  some  stream  in  its  tunnel  far  below  the  surface  is  frequently 
heard.  Nearer  the  margin,  where  the  ice  is  not  so  broken,  there 
are  many  small  ponds  with  high  walls  of  ice.  A  belt  along  the 
margin  five  miles  or  less  in  width  is  covered  by  earthy  matter  and  is 


FIG.  261. — Forest  on  the  southern  border  of  Malaspina  glacier. 
(Photo,  by  Russell.) 

densely  forested  (Fig.  261).  The  undergrowth  is  here  so  thick 
that  travelers  have  to  cut  their  paths,  and  on  the  edge  of  the  ice 
there  are  trees  three  feet  in  diameter.  The  forest  extends  four  or 
five  miles  from  the  edge  of  the  ice.  The  ice  beneath  the  surface 
debris  is  probably  1000  feet  thick.  Another  large  but  unexplored 
glacier  of  the  same  type  lies  a  few  miles  west  of  the  Malaspina. 
Others  occur  about  north  Greenland. 

Piedmont  glaciers  are  of  slight  importance,  from  a  quantitative 
point  of  view,  but  they  constitute  an  interesting  type. 


242 


PHYSIOGRAPHY 


THE    WORK    OF    GLACIERS. 

Glaciers  do  a  twofold  work  They  wear  or  erode  the  surface 
over  which  they  pass,  and  they  carry  away  and  ultimately  deposit 
the  material  which  they  acquire  by  erosion,  as  well  as  all  that  falls 
or  blows  upon  them. 

Erosion.  As  the  snow-field  develops,  it  often  lies  upon  a  sur- 
face which  is  uneven  and  more  or  less  covered  with  loose  pieces 
of  rock  (Figs.  72  and  274).  As  the  snow  accumulates,  all  projecting 


FIG.  262. — Surface  of  rock  rounded  and  smoothed  by  ice.     Bronx  Park 
New  York  City.     (U.  S.  Geol.  Surv.) 

stones  are  covered  and  enclosed  by  it,  and,  when  the  snow  becomes 
ice  and  begins  to  move,  these  masses  of  rock  are  carried  along  in 
its  bottom.  The  ice  therefore  has  some  load  when  it  starts. 

Where  the  snow  and  ice  accumulate  about  projecting  points  of 
bed-rock,  the  ice  tends  to  break  them  off  when  it  moves.  If  they 
are  too  strong  to  be  broken  off  bodily,  their  surfaces  are  worn  by 
the  passage  of  the  ice  carrying  rock  debris  in  its  bottom.  Again, 
as  a  glacier  creeps  out  over  surfaces  covered  with  soil  or  other 
mantle  rock,  the  ice  freezes  to  the  soil,  etc.;  that  is,  the  ice  above 
the  ground  becomes  continuous  with  the  ice  in  the  soil.  This 
union  is  brought  'about,  in  part  at  least,  by  the  freezing  of  descend- 
ing water.  When  this  has  been  done,  further  movement  causes 
more  or  less  of  the  soil  to  be  moved  along. 

The  first  effects  of  the  glacier  therefore  are  (1)  to  clean  off 
the  loose  debris  from  the  surface,  and  (2)  to  break  or  wear  off 


THE  WORK  OF  SNOW  AND  ICE 


243 


projecting  points   of  the  bed-rock   over   which   it   passes.     The 
general  effect  of  the  movement  of  the  ice  may  be  comoared  to 


FIG.  263. — Ice-worn  rock,  Bell's  Island,  Lake  Huron.     (Bell.) 

the  effect  of  a  flexible  rasp  which  fits  itself,  though  sometimes  with 
difficulty,  to  the  irregularities  of  the  surface  over  which  it  moves. 


FIG.  264. — Diagram  representing  a  hill  unworn  by  ice,  and  the  irregular 
contact  of  soil  and  rock. 


FIG.  265. — Diagram  showing  the  effect  of  glacial  wear  on  a  hill  such  as  is 
shown  in  Fig.  264. 

Clean  ice,  moving  over  smooth,  solid  rock,  would  erode  little;  but 
rock-shod  ice  wears  the  surface  over  which  it  moves,  even  where 
that  surface  is  smooth,  solid  rock. 


244 


PHYSIOGRAPHY 


An  ice-sheet  glacier  is  generally  much  thicker   than    a  valley 
glacier,  and  it  generally  moves  over  a  surface  which  has  less  slope. 


FIG.  266. — A  mountain  valley  which  has  been  strongly  glaciated, 
Wasatch  Mountains.     (Photo,  by  Church.) 

An  ice-sheet  of  great  thickness  may  move  over  considerable  hills 
and  valleys,  without  being  notably  turned  from  its  course  by 


FIG.  267. — A  mountain  valley  in  the  same  range  as  the  last,  but  not 
glaciated.     (Photo,  by  Church.) 


them.  The  hills  and  projecting  points  of  rock  overridden  are  worn 
down  and  smoothed  off  (compare  Figs.  264  and  265),  the  wear  being 
greatest  on  the  side  of  the  hill  against  which  the  ice  moves.  The 


THE  WORK  OF  SNOW  AND  ICE 


245 


result  is  that  glacial  erosion  sometimes  so  shapes  the  rock  hills 
that  their  forms  indicate  the  direction  of  movement. 


FIG.  268. — A  normally  eroded   mountain   mass   not  affected   by  glaciatioa 

(Davis.) 


FIG.  269. — The  same  mountain  mass  shown  in  Fig.  268  affected  by  glaciers 
which  still  occupy  its  valleys.     (Davis.) 

Valleys  tnrough  which  glaciers  pass  are  widened  and  deepened 
and  their  walls  made  smoother.     Valley  glaciers  tend  to  trans- 


246 


PHYSIOGRAPHY 


form  Y-shaped  valleys  into  U-shaped  ones,  a  result  often  con- 
spicuous in   mountain   regions    (Figs.   266   and   271).     Where   a 


FIG.  270. — The  same  mountain  mass  shown  in  the  two  preceding  figures 
after  the  ice  has  melted.     (Davis.) 

glacier  deepens  a  mountain  valley  notably,  it  brings  about  a  lack 
of  adjustment  between  the  valley  which  is  deepened,  and  its  tribu- 


FIG.  271. — A  hanging  valley  near  Lake  Kootenay.     (Photo,  by  Atwood.) 

taries  which  are  not  so  deepened.  The  effect  is  illustrated  by 
Figs.  270  and  271.  The  lower  ends  of  the  tributary  valleys  are 
well  above  the  bottoms  of  their  mains.  Such  valleys  are  called 


THE  WORK  OF  SNOW  AND  ICE 


247 


hanging  valleys.     Hanging  valleys  abound  in  the  mountains   of 
the  West,  where  glaciers  were  formerly  much  more  extensive  than 


FIG.  272. —  Figure  showing  contrast  between  glaciated  rock  surface  below 
and  non-glaciatqd  crests  above.  Kearsarge  Pinnacles,  Bubbs  Creek 
Canyon,  Cal. 


FIG.  273. — A  glacial  cirque  with  a  small  glacier  in  its  head.     Bighorn  Mts. 
Wyo.     (Photo,  by  Blackwelder.) 

now.    Valley  glaciers  descending  to  the  sea  sometimes  deepen  the 
the  lower  ends  of  their  valleys  so  that  they  become  narrow  bays  or 


248 


PHYSIOGRAPHY 


fjords,  when  the  ice  melts.     Glacier  erosion  is  not,  however,  the 
only  factor  in  fjord-making  (p.  173). 


FIG.  274. — A  glacial  cirque  north  of  Grizzly  Peak,  Colo.     (Photo,  by  Hole.) 

Fig.  237  shows  that  there  is  often  a  steep  descent  of  a  moun- 
tain glacier  near  its  head.  This  steep  slope,  and  especially  its  lower 
part,  is  the  site  of  great  erosion,  which  carries  the  head  of  the 


FIG.  275. — Striated  rock  surface.    Kingston,  Des  Moines  Co.,  la. 
(U.  S.  Geol.  Surv.) 

valley  back  farther  and  farther  into  the  mountain,  and  at  the 
same  time  gives  it  steep  slopes  at  sides  and  head  (Fig.  273  and  PI. 
XVIII).  The  big,  blunt,  steep-sided  heads  of  valleys  developed 
by  the  erosion  of  valley  glaciers  are  known  as  cirques.  Cirques 
have  remarkable  development  in  the  Uintas,  the  Bighorns,  the 


PLATE  XVfl 


•C^/'f'  ^y 

,oV^°    - 
aX&  — ;  y 

•WASHINGTON.     - f^C^ 


Glaciers  on  Glacier  Peak,  Washington.    Scale  2  —  miles  per  inch.    (Glacier  Peak 
Sheet,  U.  S.  Geol.  Surv.) 


PLATE  XVIII 


A  portion  of  the  Bighorn  Mountains,  showing  glaciated  valleys,  the  heads  of  which 
are  in  many  cases  cirques.  Scale  2—  miles  per  inch.  (Cloud  Peak,  Wyo.,  Sheet, 
U.  S.  Geol.  Surv.) 


THE  WORK  OF  SNOW  AND  ICE 


249 


Sierras,  and  many  other  mountains  of  the  West.  There  are  often 
basins  excavated  in  the  solid  rock  in  the  cirques,  and  such  basins 
are  the  sites  of  some  of  the  numerous  little  lakes  which  add  so 
much  to  the  beauty  of  scenery  in  mountains  which  have  been 
recently  affected  by  ice. 

The  effect  of  ice-sheets  on  valleys  is  less  obvious  than  that  of 


FIG.  276. — Rock  grooved  by  glaciation.  The  gorge  was  probably  formed  by 
a  stream  under  the  ice,  and  then  worn  by  the  ice.  Kelley's  Island, 
Lake  Erie.  (U.  S.  Geol.  Surv.) 

valley  glaciers,  because  in  this  case  the  ice  affected  divides  as  well 
as  valleys.  It  is  probable  that  the  valleys  through  which  the 
ice  of  a  large  ice-cap  moves  are  deepened  more  than  the  neigh- 
boring hilltops  are  cut  down.  If  so,  glacier  erosion  increases  the  relief 
of  the  rock  surface.  At  the  same  time,  it  probably  reduces  the 
roughness  of  the  surface  by  reducing  the  steepness  of  slopes,  and 
by  obliterating  many  minor  irregularities  of  hill  and  valley  slopes. 


250 


PHYSIOGRAPHY 


If  the  ice  of  an  ice-sheet  crosses  valleys,  as  the  ice  of  great 
ice-caps  often  does,  the  valleys  are  not  deepened  notably,  though 
their  upper  slopes  may  be  much  worn. 


FIG.  277. — Small  protuberances  of  rock  showing  the  effect  of  ice  wear. 
The  movement  was  from  left  to  right.  Near  Darlington,  Ind.  (U.  S. 
Geol.  Surv.) 

As  the  ice  wears  the  surface,  it  makes  distinct  scratches,  called 
strice  (Fig.  275),  on  the  bed-rock  over  which  it  passes.  Grooves 
(Fig.  276)  may  be  developed  instead  of  striae  under  favorable  con- 
ditions. The  striae  are  made  by  the  stones  carried  in  the  bottom 
of  the  ice.  The  grooves  are  developed  where  the  bed-rock  is  softer, 


FIG.  278. — Diagram  showing,  by  the  wear  in  the  depressions,  the  direction 
of  ice  movement,  left  to  right. 

or  where  great  bowlders  are  held  firmly  in  the  bottom  of  the  ice, 
and  urged  along  under  great  pressure.  Fine  clayey  material 
in  the  bottom  of  the  ice  polishes  the  rock  below.  The  polish, 
the  striae,  and  the  grooves  left  on  the  surface  of  the  rock  after  the 
ice  has  melted  are  among  the  most  distinctive  marks  of  the  former 
existence  of  glaciers.  In  any  limited  area,  these  striae  are  gen- 
erally nearly  parallel  to  one  another  and  show  the  direction,  or 
one  of  two  directions,  in  which  the  ice  moved.  Between  these 
two  directions  it  is  usually  possible  to  decide  by  the  help  of  the 
little  irregularities  of  surface,  as  shown  in  Figs.  277  and  278. 

The  stones  in  the  bottom  part  of  the  ice  are  rubbed  against 
one  another,  as  well  as  against  the  bed  of  the  glacier,  and  are 


THE  WORK  OF  SNOW  AND  ICE 


251 


scratched  much  as  the  bed-rock  is  (Figs.  279  and  280).  Since 
the  stones  in  the  ice  shift  their  positions  from  time  to  time  as 
the  ice  moves,  they  are  frequently  striated  on  two  or  more  sides. 
As  the  materials  carried  by  the  ice  rub  against  one  another 
and  against  the  bed  over  which  they  are  carried,  they  become 
finer  and  finer.  The  finest  products  of  the  grinding  constitute 
"rock  flour,"  while  coarser  parts  have  the  size  of  sand  grains, 
pebbles,  or  even  large  stones.  Thus  it  happens  that  the  materials 
gathered  and  shaped  by  the  ice  are  of  all  grades  of  coarseness, 
from  huge  masses  many  feet  in  diameter  down  to  the  finest  earth 


FIG.  279. — Stones  striated  by  glacial  wear. 

(Fig.  281).  The  larger  masses  of  rock  are  bowlders;  the  smaller 
pieces  are  cobble-stones,  pebbles,  etc.,  while  the  finer  materials  are 
sand  and  ground-up  rock  (rock  flour),  popularly  called  clay. 

Materials  gathered.  From  its  mode  of  erosion  it  will  readily 
be  seen  that  the  bottom  of  a  glacier  may  be  charged  with  various 
sorts  of  material.  There  may  be  (1)  bowlders  which  the  ice  has 
picked  up  from  the  surface,  or  which  it  has  broken  off  from  pro- 
jecting points  of  rock  over  which  it  has  passed;  (2)  smaller  pieces 
of  rock  picked  up  in  the  same  way;  (3)  the  fine  products  (rock 
flour)  produced  by  the  grinding  of  the  debris  in  the  ice  on  the 
rock-bed  over  which  it  passes,  and  similar  products  resulting 
from  the  rubbing  of  stones  in  the  ice  against  one  another;  and 
(4)  sand,  clay,  soil,  vegetation,  etc.,  derived  from  the  surface 
overridden.  Thus  the  materials  which  the  ice  carries  are  of  all 
grades  of  coarseness  and  fineness,  from  large  bowlders  to  fine 


252 


PHYSIOGRAPHY 


FIG.  280. — Stones  in  the  drift  striated  and  beveled  by  glacial  wear. 


FIG.  281. — Section  of  drift  showing  its  heterogeneity. 


THE  WORK  OF  SNOW  AND  ICE  253 

clay.  The  coarser  material  may  be  angular  or  round  at  the 
outset,  and  its  form  may  be  changed  and  its  surface  striated  as 
it  is  moved  forward.  Whether  one  sort  of  material  or  another 
predominates  depends  primarily  on  the  nature  of  the  surface 
overridden. 

Disposition  of  debris  in  transit.  The  larger  part  of  the 
material  carried  by  a  glacier  is  carried  in  its  basal  portion;  but 
some  is  carried  in  the  body  of  the  ice,  well  above  its  bottom,  and 
in  the  case  of  most  glaciers,  some  is  carried  on  the  surface  of  the 
ice. 

The  position  of  the  material  in  the  base  of  the  ice  is  readily 
understood  from  the  manner  in  which  the  debris  is  gathered. 
The  material  above  the  base  reaches  its  position  in  various  ways. 


FIG.  282. — Diagram  illustrating  one  way  in  which  a  glacier  gets 
englacial  material. 

Sometimes  the  ice  passes  over  a  considerable  elevation.  In  this 
case  material  may  be  torn  from  its  top  and  carried  along  at  a 
level  corresponding  somewhat  to  that  from  which  it  was  derived. 
This  is  illustrated  by  Fig.  282.  Under  some  circumstances,  too, 
ice  moves  from  the  bottom  of  the  glacier  upward  (Fig.  283),  and 
carries  debris  with  it,  and  in  this  way  debris  which  was  once  at 
the  bottom  may  later  find  itself  in  a  higher  position. 

The  material  on  the  surface  of  a  glacier  reaches  its  position  in 
various  ways.  Where  the  slopes  above  the  glacier  are  steep,  rock 
material  may  fall  or  slide  down  to  the  surface  of  the  ice.  Great 
masses  of  snow  (avalanches)  sometimes  slide  down  upon  a  glacier 
from  the  steep  slopes  above,  bringing  quantities  of  debris,  and 
dust  is  blown  upon  the  ice. 

The  material  falling  or  sliding  down  to  a  glacier  from  the 
cliffs  above  tends  to  accumulate  near  the  margin  of  the  ice,  and 
as  it  lies  on  the  surface  of  the  glacier,  constitutes  lateral  moraines 
(Fig.  284).  If  two  glaciers  unite,  as  is  sometimes  the  case,  the 


254 


PHYSIOGRAPHY 


FIG.  283. — End  of  a  North  Greenland  glacier,  showing  the  upturning  of 
the  layers  of  ice  at  the  end.  At  one  point  a  few  stones  are  seen  on  the 
surface  of  the  ice,  where  an  upturned  layer  comes  to  the  surface.  This 
structure  is  common  in  North  Greenland. 


FIG.  284.— Figure  showing  the  union  of  glaciers  and  the  development  of 
medial  moraines  by  the  union;  also  the  position  of  lateral  moraines. 
(After  Tyndall.) 


THE  WORK  OF  SNOW  AND  ICE  255 

two  lateral  moraines  of  the  adjacent  sides  may  unite,  forming  a 
single  medial  moraine  (Fig.  284). 

Both  medial  and  lateral  moraines  arise  in  other  ways.  If  a 
glacier  passes  over  an  elevation  which  reaches  well  up  into  the 
ice,  material  torn  from  the  elevation  is  at  first  carried  along  in 
the  ice;  but  as  the  lower  end  of  the  glacier  is  approached,  surface 
melting  may  bring  the  surface  of  the  ice  down  to  the  level  of  the 
debris,  when  it  appears  at  the  surface  as  a  medial  moraine  (Fig. 
282).  In  other  cases  lateral  and  medial  moraines  arise  by  the 
upturning  of  the  layers  of  ice,  as  shown  in  Fig.  285.  Moraines 
arising  in  this  way  are  common  on  the  glaciers  of  North  Greenland. 


FIG.  285. — Diagram  illustrating  a  way  in  which  lateral  and  medial  moraines 
are  formed  in  many  of  the  North  Greenland  glaciers.  The  horizontal 
line  at  the  base  represents  sea-level. 

Deposition  by  Glaciers. 

While  the  ice  is  in  motion  it  is  depositing  more  or  less  debris 
beneath  itself,  against  the  hills  and  projecting  bosses  of  rock. 
Again,  the  bottom  of  the  ice  may  become  heavily  loaded  in  passing 
over  a  region  which  yields  load  readily,  and  a  part  of  it  may  be 
deposited  farther  on,  where  changed  conditions  of  movement  have 
rendered  the  load  excessive.  Debris  deposited  at  one  time  may 
be  taken  up  at  another.  Thus  beneath  the  moving  ice  there  is 
more  or  less  deposition  in  progress  all  the  time,  though  much  of 
it  is  not  permanent.  In  this  respect,  deposition  by  glaciers  is 
somewhat  like  the  deposition  of  streams. 

The  position  of  the  end  of  the  glacier  is  determined  by  the 
relation  between  ice  waste  and  forward  movement.  When  the 
ice  advances  as  much  as  it  is  melted  back,  its  end  or  edge  is  con- 
stant in  position.  But  it  is  to  be  remembered  that  even  when 
the  end  of  a  glacier  is  constant  in  position,  the  ice  itself  is  in  con- 
tinual movement.  Under  these  circumstances,  material  is  con- 
tinually brought  to  the  end  or  edge  of  the  glacier  and  left  there. 
The  result  is  that  if  the  end  of  a  valley  glacier  or  the  edge  of  an 
ice-cap  be  constant  or  nearly  constant  in  position  for  a  long  period 
of  time,  a  thick  body  of  drift  is  accumulated  beneath  its  margin 
(Figs.  286  and  287). 


256 


PHYSIOGRAPHY 


FIG.  286. — Thick  accumulation  of  drift  under  the  end  of  a  glacier.  The 
end  has  probably  been  in  about  the  same  place  for  a  long  time.  McCor- 
mick  Bay,  North  Greenland. 


FIG.  287. — An  accumulation  similar  to  that  shown  in  Fig.  286,  after  the  ice 
has  melted  away;  near  the  last. 


THE  WORK  OF  SNOW  AND  ICE  257 

The  terminal  moraine.  The  belt  of  thick  drift  accumulated 
beneath  the  end  of  a  valley  glacier  or  beneath  the  edge  of  an 
ice-cap  is  a  terminal  moraine.  The  terminal  moraine  becomes 
massive  only  when  the  end  of  the  glacier  remains  nearly  constant 
in  position  for  a  long  time.  The  same  term  is  sometimes  applied 
to  the  debris  on  the  end  of  a  valley  glacier,  or  on  the  edge  of  an 
ice-sheet.  The  material  of  this  super-glacial  terminal  moraine  is 
added  to  the  sub-marginal  terminal  moraine  when  the  ice  melts. 
In  general,  the  sub-marginal  accumulation  is  much  greater  than 
that  which  is  let  down  from  the  top  when  the  ice  melts. 

The  ground  moraine.  When  a  glacier  melts,  all  the  debris 
which  it  carried  is  deposited.  When  the  ice  is  gone,  therefore,  the 
whole  surface  which  it  covered  is  likely  to  be  strewn  with  its  debris. 
All  the  rock  debris  left  by  the  ice  is  drift.  The  drift  deposited 
by  the  ice  but  not  aggregated  into  thick  belts  at  its  edge  is  ground 
moraine.  The  area  of  the  ground  moraine  is  much  more  exten- 
sive than  the  area  of  the  terminal  moraine. 

Many  spots  once  covered  by  the  ice  are  left  without  drift 
when  the  ice  melts,  for  the  ice  does  not  always  carry  debris  at 
every  point  in  its  bottom.  Areas  of  bare  rock  are  therefore  found, 
and  sometimes  commonly,  in  the  area  from  which  glacier  ice  has 
melted. 

Lateral  moraines.  The  term  lateral  moraine  is  applied  not 
only  to  certain  aggregations  of  drift  on  valley  glaciers  (Fig.  284) , 
but  also  to  certain  aggregations  of  drift  which  are  left  by  a 
valley  glacier  after  melting.  When  a  valley  glacier  is  melted, 
the  lateral  moraines  of  its  surface  are  left  in  the  valleys  which  it 
occupied;  but  these  lateral  moraines  are  not  commonly  massive 
enough  to  be  conspicuous  after  the  ice  is  gone.  On  the  other 
hand,  the  lateral  moraines  which  remain  when  a  valley  glacier  has 
disappeared  are  often  large  (Fig.  288).  In  many  cases,  indeed, 
they  are  the  most  conspicuous  deposits  left.  They  are  often  hun- 
dreds of  feet  high,  and  in  some  cases  even  more  than  a  thousand. 

The  making  of  these  huge  lateral  moraines  is  a  somewhat  com- 
plex process.  They  are  made  up  in  small  part  of  the  lateral  moraines 
which  were  on  the  ice,  and  in  much  larger  part  of  material  accumu- 
lated beneath  the  lateral  margin  of  the  glacier.  Their  explanation 
seems  to  be  found  in  the  fact  that  a  valley  glacier  moves  not  only 
down  the  valley,  but  also  sidewise  from  the  center  toward  either 
side.  Spreading  sidewise  from  the  center,  the  ice  is  constantly 


258 


PHYSIOGRAPHY 


shifting  debris  from  the  axis  of  the  valley  to  the  edge  of  the  ice  on 
either  side.  The  lateral  moraine  left  after  the  ice  is  gone  is  there- 
fore of  the  nature  of  a  terminal  moraine  beneath  the  lateral  margin 
of  the  ice. 

Lateral  moraines  of  valley  glaciers  are  much  more  likely  to 
remain  after  the  ice  is  gone  than  the  corresponding  terminal 
moraines,  for  the  latter  are  more  likely  to  be  washed  away  by  the 
waters  issuing  from  the  ice  or  flowing  down  subsequently  through 
the  valley. 

Ice-caps  do  not  develop  lateral  moraines,  because  they  have  no 
lateral  margins. 


FIG.  288. — A  lateral  moraine  left  by  a  former  glacier  in  the  Bighorn  Moun- 
tains of  Wyoming.     (Photo,  by  Blackwelder.) 

Distribution  and  Disposition  of  Drift  Glaciers  distribute  their 
debris  unevenly  over  the  surface.  The  drift  of  terminal  moraines 
is  generally  much  thicker  than  that  of  the  ground  moraine  near 
at  hand,  and  the  drift  in  the  lateral  moraines  of  valley  glaciers 
is  often  very  thick.  The  drift  therefore  modifies  the  topography 
in  some  notable  measure. 

In  valleys,  the  terminal  moraines  often  constitute  dams,  and 
so  pond  the  waters  of  the  streams  above,  and  make  lakes  (Fig. 
289),  though  not  all  the  lakes  of  glaciated  valleys  are  due  to 
moraine-dams  (p.  311). 

The  drift  of  ice-sheets  is  more  likely  to  be  abundant  in  valleys 


260 


PHYSIOGRAPHY 


and  other  low  places  than  on  hills  and  ridges.  The  drift  of  ice- 
sheets,  therefore,  usually  diminishes  rather  than  increases  the  relief 
of  the  surface  (Fig.  290). 


FIG.  290. —  Diagram  to  illustrate  how  drift  may  decrease  the 
relief  of  the  surface. 

Glacier  drift  is  often  irregularly  disposed,  so  that  its  own  sur- 
face is  somewhat  rough,  even  where  it  diminishes  surface  relief. 
Its  surface  is  marked  in  many  places  by  hillocks,  mounds,  etc.,  of 
drift,  and  by  basin-like  depressions  (Figs.  291  and  292).  In  some 


FIG.  291. — Sketch  of  drift   (terminal  moraine)  topography  near 
Hackettstown,  N.  J.     (N.  J.  Geol.  Surv.) 

of  the  basins  water  stands,  making  lakes,  ponds,  or  marshes. 
The  surface  of  drift  is  therefore  very  unlike  the  surface  developed 
by  the  erosion  of  running  water,  for  in  the  latter  the  depressions 
have  outlets,  and  the  hills  and  ridges  stand  in  a  very  definite  re- 
lation to  the  valleys  (compare  Figs.  106,  139,  and  292). 

R6sum6.  The  more  distinct  marks  which  a  valley  glacier  leaves 
behind  it  are  the  following:  (1)  A  U-shaped  valley,  often  with  its 
tributary  valleys  hanging  (Fig.  271),  and  with  its  head  in  the 
form  of  a  cirque  (Fig.  273) ;  (2)  the  upper  end  of  the  valley  which 


262 


PHYSIOGRAPHY 


it  occupied  well  cleaned  out  (Fig.  293),  the  loose  rock-debris  hav- 
ing been  carried  down  the  valley;  (3)  the  rock  of  the  valley 
smoothed  and  striated  (Fig.  294);  (4)  rock  basins  in  the  bottom 
of  the  valley,  especially  near  its  head;  (5)  a  body  of  drift  com- 
posed of  coarse  and  fine  material,  often  without  trace  of  stratifi- 
cation or  orderly  arrangement;  (6)  the  stones  of  the  drift  are 
often  worn,  but  not  rounded  as  streams  or  waves  round  them,  and 


FIG.  293.— Portion  of  the  upper  part  of  a  valley  cleaned  out  by  ice.  The 
figure  shows  also  the  contrast  between  glaciated  topography  below 
and  the  non-glaciated  above.  Needle  Mountains,  Colo.  (U.  S.  Geol. 
Surv.) 

they  often  have  planed  and  striated  faces;  (7)  the  drift  is  disposed 
as  no  other  transporting  agent  disposes  the  material  which  it  leaves 
(Fig.  295).  The  singular  lateral-moraine  ridges  of  valley  glaciers, 
and  the  terminal  moraines  which  often  partially  or  wholly  ob- 
struct valleys,  giving  rise  to  lakes,  ponds,  and  marshes,  are  among 
the  distinctive  deposits  of  valley  glaciers.  (8)  Another  distinc- 
tive though  less  common  mark  of  glacier  deposits  is  the  huge 
bowlders  in  delicately  balanced  positions  (perched  bowlders)  (Fig. 

296). 

Ice-sheets  likewise  leave  (1)  bodies  of  drift  very  much  like  that 


THE  WORK  OF  SNOW  AND  ICE 


2G3 


FIG.  294. — Striae,  grooves,  etc.,  in  a  canyon  tributary  to  the  Big  Cotton- 
wood  Canyon.     Wasatch  Mts.,  Utah.     (Photo,  by  Church.) 


FIG.  295. — The  moraines  about  the  lower  end   of  a  glaciated  mountain 
valley.    Bloody  Canyon,  Cal.     (U.  S.  Geol.  Surv.) 


264 


PHYSIOGRAPHY 


FIG   296. — A  perched  bowlder,  size  12X8X8  feet.  East  of  Englewood,  N.  J. 

(N.  J.  Geol.  Surv.) 


FIG.  297. — Topography  of  drift  shown  in  contours;  an  area  near  Minneapolis, 
Minn.     Scale  about  one  inch  to  the  mile.      (U.  S.  Geol.  Surv.) 


THE  WORK  OF  SNOW  AND  ICE 


266 


of  mountain  glaciers,  though  often  less  coarse.  This  drift  is  in  the 
form  of  terminal  moraines.  (Figs.  291  and  297)  and  ground  moraines 
(Fig.  298),  the  surfaces  of  which  are  marked  by  (2)  numerous  lakes, 
ponds,  and  marshes,  which  fill  the  kettle-like  or  saucer-like  de- 
pressions in  the  surface  of  the  drift.  (3)  Ice-sheets  also  smooth, 
striate,  and  groove  the  surface  of  the  rock  over  which  they  move. 


FIG.  298. — One  phase  of  ground  moraine  topography.  Elongated  hills  of 
drift  of  the  type  shown  here  are  called  drumlins.  Southeastern  Wis- 
consin. (U.  S.  Geol.  Surv.) 


Fluvio-glacial  Deposits. 

Even  while  glaciers  are  growing,  their  ice  is  melting  to  some 
extent,  and  when  they  disappear,  it  is  primarily  by  melting.  Drain- 
age is  vigorous  at  the  edge  of  an  ice-sheet,  and  below  the  ends  of 
valley  glaciers  much  of  the  time,  and  in  the  summer,  when  the 
ice  is  melting  rapidly,  the  streams  which  carry  off  the  water  are 
greatly  swollen.  Water-work,  therefore,  accompanies  ice-work 
in  all  cases,  and  since  there  is  on  the  whole  about  as  much  water 
as  ice,  and  since  the  water  has  the  last  chance  at  the  material  left 
by  the  ice,  it  follows  that  some  of  the  drift,  as  deposited  by  the 
glacier,  is  more  or  less  modified  by  water  subsequently.  The 
streams  which  flow  from  glaciers  carry  away  much  debris  de- 
rived from  the  ice.  At  the  outset,  this  consists  of  both  coarse 


266 


PHYSIOGRAPHY 


and  fine  material,  but  the  gravel  and  small  bowlders  are  soon 
dropped  and  only  the  finer  material  is  carried  far.  Many  such 
streams  carry  so  much  silt  in  suspension  that  the  water  is  turbid. 
If  the  silt  is  whitish,  as  it  often  is,  the  streams  are  said  to  be 
"milky." 

Profile  of  Valley  Train 

FIG.  299. — Diagram  to  illustrate  the  profile  of  a  valley  train,  and  its  rela- 
tions to  the  terminal  moraine  in  which  it  heads. 

By  the  deposition  of  the  gravel,  sand,  and  silt,  the  valleys  below 
glaciers  are  often  aggraded  to  some  extent  by  the  fluvio-glacial 
debris.  Deposits  of  this  sort  are  stratified,  and  so  are  in  contrast 
with  the  deposits  made  by  the  ice.  Furthermore,  the  surface  of 
the  stream  deposits  is  generally  plane,  and  therefore  in  contrast 
with  the  topography  of  the  drift  deposited  by  the  ice. 


FIG.  300. — The  outwash  plain  and  the  terminal  moraine  near  Baraboo,  Wis. 
(Photo,  by  Atwood.) 

The  material  deposited  by  the  stream  in  the  valley  below  a 
glacier  is  a  valley  train.  It  is  simply  an  alluvial  plain  developed 
under  special  circumstances.  Valley  trains  are  best  developed 
just  outside  terminal  moraines  (Fig.  299). 

In  the  case  of  an  ice-cap,  the  water  which  issues  from  the  ice 
often  fails  to  find  a  valley.  Each  issuing  stream  thus  tends  to 
develop  an  alluvial  fan.  By  growth,  these  fans  may  merge, 
making  an  alluvial  plain.  Such  a  plain,  composed  of  material 
washed  out  from  the  ice  is  an  outwash  plain  (Fig.  300),  which  is 


THE  WORK  OF  SNOW  AND  ICE 


267 


often  wider  than  long.  It  is  of  coarse  material  next  the  ice  and 
of  finer  material  farther  away.  Outwash  plains,  like  valley  trains, 
are  best  developed  just  outside  the  terminal  moraines  of  ice-sheets, 
and  their  materials  are  stratified. 

A  glacier  may  obstruct  surface  drainage.  If  in  its  forward 
movement  the  ice  obstructs  the  lower  end  of  a  valley,  the  water 
above  accumulates  and  constitutes  a  lake.  Drainage  from  the 
melting  ice  often  builds  deltas  in  lakes,  just  as  other  streams 
build  deltas  in  the  bodies  of  standing  water  into  which  they  flow. 


FIG.  301.— An  esker  in  Finland. 

In  addition  to  the  drainage  outside  the  ice,  there  is  running 
water  in  the  ice  and  under  it.  The  streams  beneath  the  ice  (sub- 
glacial  streams)  sometimes  deposit  gravel  in  their  channels.  These 
channels  may  be  so  built  up  that  when  the  ice  melts,  the  old  bed 
of  the  stream  appears  as  a  low  but  narrow  ridge,  called  an  esker, 
composed  chiefly  of  gravel  and  sand  (Fig.  301).  The  sub-ice 
channels  sometimes  have  the  effect  of  tubes  through  which  the 
water  is  forced  with  considerable  velocity.  As  it  issues  from 
beneath  the  ice,  its  velocity  is  checked,  and  it  sometimes  makes 
extensive  deposits  of  gravel  and  sand  at  the  margin  of  the  ice. 
These  deposits  are  stratified,  but  the  stratification  is  often  irregular. 


268 


PHYSIOGRAPHY 


They  are  often  left  against  the  edge  of  the  ice,  and  when  the 
edge  melts,  the  deposits  appear  as  mounds  and  ridges,  called 
kames  (Fig.  302). 

In  warm  weather,  there  are  many  small  streams  on  the  surfaces 
of  glaciers  (Fig.  242),  especially  if  the  surface  of  the  ice  is  not 
much  crevassed;  but  these  surface  streams  rarely  make  deposits 
of  consequence.  On  ice-caps  there  is  little  debris  on  the  ice 


FIG.  302. — A  group  of  kames  near  Connecticut  Farms,  N.  J. 
(N.  J.  Geol.  Surv.) 

except  at  its  immediate  edge,  so  that  the  surface  streams  have 
access  to  little  debris,  and  they  are  commonly  clear. 

As  the  ice  melts  away,  the  waters  produced  by  the  melting 
flow  over  the  surface  of  the  drift  which  the  ice  had  already  depos- 
ited, and  modify  its  surface  to  some  extent  by  eroding  in  some 
places  and  depositing  in  others. 


THE  WORK  OF  SNOW  AND  ICE  269 

As  a  result  of  all  these  phases  of  water-work,  much  of  the  drift 
is  stratified.  The  stratified  drift  is  sometimes  above  the  unstrati- 
fied,  sometimes  below  it,  and  sometimes  interbedded  with  it.  In 
some  cases,  too,  it  lies  beyond  the  limit  which  the  ice  reached. 

Icebergs. 

Where  glaciers  move  down  into  the  sea,  their  ends  may  be 
broken  off  and  floated  away  as  icebergs.  The  breaking-off  is 
brought  about  in  various  ways.  As  the  ice  pushes  out  into 
deepening  water,  it  may  reach  a  point  where  the  depth  of  water 
is  so  great  that  its  buoyant  effect  breaks  off  the  ice,  which  is 
lighter  than  water.  The  ice  is  often  already  partly  broken  by 
crevasses  before  it  reaches  the  sea. 

Bergs  derived  from  Greenland  are  found  as  far  south  as  New- 
foundland in  considerable  numbers.  Occasionally  they  reach  still 
lower  latitudes,  but  by  the  time  they  have  moved  so  far  from 
their  source,  they  have  usually  become  small  by  melting.  The 
bergs  from  Greenland  are  rarely  200  feet  out  of  water,  and  most 
of  them  are  not  more  than  100  feet,  even  near  their  sources.  They 
are  sometimes  a  mile  or  more  across.  In  the  South  Polar  regions 
bergs  are  still  larger  in  area,  though  higher  ones  are  perhaps  not 
common.  A  berg  200  feet  out  of  water,  disregarding  projecting 
points,  may  be  1000  to  1500  feet  thick.  Though  river  or  lake  ice 
is  about  nine-tenths  as  heavy  as  water,  glacier  ice  is  less  heavy, 
unless  loaded  down  with  rock  debris.  This  is  because  the  snow- 
ice  does  not  become  so  compact  as  the  ice  formed  on  rivers  and 
lakes. 

As  the  icebergs  sail  away,  they  carry  more  or  less  of  the  debris 
which  was  in  the  bottom  of  the  glacier.  In  the  water,  melting 
takes  place,  and  the  debris  which  was  held  by  the  melted  parts 
falls  to  the  bottom.  If,  as  is  often  the  case,  the  iceberg  capsizes  as 
it  sets  sail,  the  bottom  debris  of  the  glacier  may  appear  on  the 
sides  or  top  of  the  iceberg,  if  it  does  not  at  once  slide  off  and  sink. 
This  debris  absorbs  more  of  the  sun's  heat  than  the  ice  does,  and 
is  soon  melted  out  of  the  ice;  or  more  strictly,  the  ice  is  melted 
away  from  around  it.  If  it  was  on  the  side,  it  drops  out  into  the 
sea. 

Icebergs  frequently  turn,  or  cant,  because  of  (1)  the  cutting 
of  the  waves,  (2)  the  splitting  off  of  pieces  of  ice,  (3)  unequal 


270  PHYSIOGRAPHY 

melting,  etc.,  all  of  which  tend  to  shift  their  centers  of  gravity 
and  so  disturb  their  equilibrium. 

Observations  on  northern  icebergs  indicate  that  they  do  not 
carry  much  debris  far.  The  average  berg  is  probably  free  of 
debris  before  it  has  floated  100  miles.  The  common  notion  that 
the  banks  of  Newfoundland  were  made  largely  by  berg  deposits 
probably  has  no  foundation  in  fact. 

Where  Icebergs  ground  in  large  numbers,  as  on  the  shores  of 
Labrador  and  Newfoundland,  they  may  erode  the  bottom  to  come 
slight  extent. 

Icebergs  in  the  North  Atlantic  occasionally  reach  the  tract  of 
transatlantic  commerce.  Since  they  are  sometimes  surrounded 
by  fog,  they  may  be  a  menace  to  shipping  and  travel. 

ANCIENT  GLACIERS  AND  ICE-SHEETS. 

There  have  been  times  in  the  earth's  history  when  glaciers  were 
much  more  extensive  than  now.  The  latest  of  these  periods  is 
known  as  the  glacial  period.  During  this  period  the  glaciers  of  the 
western  mountains  were  very  much  larger  than  now,  and  glaciers 
were  numerous  in  many  mountains  where  there  are  none  now. 
Small  ones  existed  even  in  the  mountains  of  New  Mexico,  Arizona, 
and  Nevada.  The  amount  of  ice  in  the  glaciers  of  Utah  or  Colo- 
rado at  that  time  was  far  in  excess  of  all  that  now  exists  in  the 
United  States  south  of  Alaska.  The  glaciers  in  the  western  moun- 
tains north  of  the  United  States  also  were  correspondingly  larger 
than  now,  while  east  of  the  mountains  an  area  some  4,000,000 
square  miles  in  extent  (Fig.  303), lying  partly  in  Canada  and  partly 
in  the  United  States,  was  covered  with  an  ice-sheet  or  continental 
glacier. 

The  ice-sheet  of  North  America  seems  to  have  originated  in 
two  principal  centers,  one  on  either  side  of  Hudson  Bay.  The 
beginning  of  each  was  doubtless  a  great  snow-field.  The  snow- 
and  ice-fields  grew  by  the  fall  of  snow,  and  later  by  the  spread 
of  the  ice  to  which  the  snow  gave  rise.  The  two  ice-sheets  finally 
became  one  by  growth  (Fig.  303).  It  is  to  be  noted  that  the 
great  continental  glacier  did  not  originate  in  mountains,  but  on 
high  plains.  In  addition  to  the  large  valley  glaciers  of  the  western 
mountains,  bodies  of  ice  of  the  ice-sheet  type  were  developed  in 
favorable  situations  in  these  mountains,  though  their  continuity 


THE  WORK  OF  SNOW  AND  ICE 


271 


was  much  interrupted  by  the  crests  and  peaks.  The  valley  glaciers 
often  merged  on  the  plains  below,  where  piedmont  glaciers  of  great 
size  were  developed. 

At  the  time  of  its  greatest  extent,  the  ice-sheet  of  North  America 


FIG.  303. — Sketch-map  showing  the  area  in  North  America  covered  by  ice 
at  the  maximum  stage  of  glaciation.     (Chamberlin.) 

extended  south  so  as  to  cover  all  of  New  England,  the  northern 
parts  of  New  Jersey  and  Pennsylvania,  and  much  of  Ohio  and 
Indiana.  Its  edge  crossed  the  Ohio  River  where  Cincinnati  now 
stands,  and  advanced  a  few  miles  into  Kentucky.  Farther  west 
it  reached  almost  to  the  southern  end  of  Illinois.  Its  edge  crossed 
the  Mississippi  near  St.  Louis,  and  followed,  in  a  general  way, 


272  PHYSIOGRAPHY 

the  course  of  the  Missouri  River  to  Montana.  Most  of  the  con- 
tinent north  of  this  line  was  covered  with  snow  and  ice,  but  there 
was  an  area  of  eight  or  ten  thousand  square  miles,  mainly  in 
southwestern  Wisconsin,  which  the  ice  did  not  cover.  Because 
of  the  absence  of  drift  in  this  region,  it  is  known  as  the  Driftless 
Area. 

The  conditions  for  extensive  glaciation  existed  in  Europe  at 
about  the  same  time.  The  glaciers  of  the  Alps,  for  example,  were 
many  times  as  large  as  those  of  the  present  time.  To  the  south 
they  extended  quite  through  the  mountain  valleys  and  spread 
themselves  out  on  the  plains  of  northern  Italy.  On  other  sides 
also  the  glaciers  were  correspondingly  larger  than  now.  This 
great  extension  of  the  glaciers  is  known  from  the  moraines,  and 
from  the  striated  rock,  etc.,  which  the  ice  left  where  it  melted. 
Similar  conditions  existed  in  the  other  mountains  of  Europe  where 
glaciers  now  exist,  and  in  some  where  glaciers  are  not  now  present. 

In  northern  Europe,  as  in  the  northern  part  of  North  America, 
there  was  an  extensive  ice-sheet,  but  its  area  was  only  about  half 
that  of  the  ice-sheet  of  North  America.  The  center  from  which 
the  ice-sheet  radiated  was  the  high  mountains  of  Scandinavia, 
with  perhaps  subordinate  centers  in  the  highlands  of  Scotland, 
and  in  the  Urals.  At  the  time  of  its  greatest  extension,  this  ice- 
sheet  covered  all  but  the  southernmost  part  of  Great  Britain,  all 
of  northern  Germany,  and  much  of  Russia  (Fig.  304). 

Great  ice-sheets  are  not  known  to  have  developed  in  other 
continents,  but  their  mountain  glaciers  were  greatly  enlarged. 

In  both  Europe  and  North  America  the  history  of  the  con- 
tinental glaciers  was  most  complex.  In  each  continent  there 
were  several  successive  ice-sheets,  separated  from  one  another  by 
considerable  intervals  of  time.  The  sequence  of  events  in  North 
America  was  somewhat  as  follows:  After  the  development  of  the 
first  great  ice-sheet,  it  shrank  to  small  proportions,  or  disappeared 
altogether,  probably  because  of  a  change  of  climate.  The  dwin- 
dling of  the  first  ice-sheet  was  followed  by  a  relatively  warm  period, 
during  which  plants  and  animals  took  possession  of  the  region 
abandoned  by  the  ice.  Another  continental  ice-sheet  then  de- 
veloped, overspreading  the  region  from  which  the  first  had  with- 
drawn, and  extending  still  farther  south.  As  it  advanced,  the 
second  ice-sheet  occasionally  buried  the  soil  which  had  formed  on 
the  top  of  the  drift  deposited  by  the  ice  of  the  first  epoch.  Such 


THE  WORK  OF  SNOW  AND  ICE 


273 


soils,  sometimes  with  the  remains  of  plants  which  can  be  identified. 
lying  between  a  sheet  of  drift  below  and  another  above,  are  one  of 
the  means  by  which  it  is  known  that  there  was  more  than  one 
continental  glacier.  By  this  and  other  means,  a  third,  fourth, 
and  fifth  ice-sheet,  each  somewhat  smaller  than  its  predecessor, 
developed  and  disappeared.  In  other  words,  there  were  at  least 
five  epochs  when  ice-sheets  were  extensive,  separated  by  epochs 


FIG.  304.— Sketch-map  showing  the  area  of  Europe  covered  by  the  con- 
tinental glacier  at  the  time  of  its  maximum  development.  (After 
Jas.  Geikie.) 

when  the  ice  was  greatly  diminished,  or  when  it  disappeared  alto- 
gether.    The  ice-sheets  of  Europe  had  a  similar  history. 

Cause  of  the  Glacial  Epochs. 

The  cause  of  the  development  of  the  great  ice-sheets  was  doubt- 
less climatic,  the  chief  factor  being  a  reduction  of  temperature. 
The  cause  of  this  cold  climate  is  not  certainly  known.  Various 
hypotheses  have  been  proposed  to  explain  it,  but  to  most  of  them 
there  seem  to  be  fatal  objections.  This  subject  will  not  be  dis- 
cussed here,  but  it  may  be  stated  that  the  only  hypothesis  which 


274  PHYSIOGRAPHY 

seems  not  to  be  discredited  is  that  which  refers  the  change  of 
climate  to  a  change  in  the  constitution  of  the  atmosphere.  It 
appears  that  an  increase  in  the  amount  of  carbonic-acid  gas  and 
water  vapor  would  result  in  an  amelioration  of  climate,  while  a 
decrease  in  these  elements  would  result  in  a  reduction  of  tempera- 
ture. This  hypothesis  cannot  be  elaborated  here,  but  it  may  be 
stated  that  plausible  reasons  have  been  suggested  for  fluctuations 
in  the  amounts  of  these  substances  in  the  atmosphere,  and  also 
for  the  relatively  heavy  precipitation  (which  is  as  necessary  as 
low  temperature  for  glaciation)  in  the  regions  where  the  ice-sheets 
developed. 

CHANGES  PRODUCED  BY  THE  CONTINENTAL  GLACIERS. 

The  ice-sheets  of  North  America  modified  the  surface  which 
they  covered  to  some  notable  extent.  A  brief  resume"  of  the 
changes  they  produced  will  serve  to  review  and  emphasize  the 
work  of  ice-sheets.  The  changes  wrought  by  the  ice-sheet  fall 
into  two  classes:  (1)  those  brought  about  by  the  erosion  of  the  ice, 
and  (2)  those  brought  about  by  the  deposition  of  the  drift. 

It  is  important  to  remember  that  the  continental  glacier  of 
North  America  developed  on  the  surface  of  a  rather  high  plain, 
the  topography  of  which  had  been  shaped  in  large  measure  by 
rain  and  river  erosion.  This  is  inferred  from  the  topography  of 
the  area  not  covered  by  the  ice. 

Changes  Produced  by  Erosion, 

1.  On  elevations.     The  ice  was  thick  enough  to  pass  over  the 
hills  and  low  mountains,  such  as  those  of  New  England  and  north- 
ern New  York,  within  the  area  shown  in  Fig.  303.     As  it  over- 
spread these  and  lesser  elevations,  it  wore  off  their  tops.     It  re- 
duced all  points  which  stood  up  above  the  general  surface,  and  so 
tended  to  make  the  surface  less  rough.     The    general  effect  on 
elevations  is  shown  by  Figs.  264  and  265. 

2.  In  valleys.     The  ice  also  deepened  the  valleys  through  which 
it  moved.     In  many  cases  it  deepened  them  as  much  as  it  lowered 
the  hills,  or  even  more.     In  the  latter  case,  the  relief  of  the  surface 
was  increased;   but  even  where  this  was  true,  the  roughness  of  the 
surface  was  often  diminished,  for  roughness  depends  on  the  fre- 


THE  WORK  OF  SNOW  AND  ICE  275 

quency  with  which  elevations  and  depressions,  such  as  hills  and 
valleys,  succeed  one  another,  and  on  the  steepness  of  their  slopes, 
quite  as  much  as  on  the  amount  of  relief  (Fig.  305).  Where  the 
edge  of  an  ice-sheet  was  differentiated  into  valley  glaciers  which 
moved  down  to  the  sea,  the  ice  sometimes  gouged  out  the  valleys 
far  below  sea-level,  giving  rise  to  narrow  bays,  or  fiords,  after  the 
ice  melted. 

3.  Rock  basins.  Another  effect  of  ice  erosion  wras  to  gouge 
out  hollows  where  the  underlying  rock  was  relatively  weak.  The 
result  was  the  formation  of  basins  in  the  surface  of  the  rock.  Such 
rock  basins  are  probably  less  common  in  the  area  of  the  con- 
tinental ice-sheet  than  in  mountain  valleys  affected  by  glaciers. 


FIG.  305. — Diagram  to  show  that  roughness  of  surface  and  amount  of  relief 
are  not  necessarily  the  same.  A  represents  greater  relief,  but  B  might 
be  regarded  as  a  rougher  surface. 

The  ice  also  polished,  striated,  and  grooved  the  surface  of  the  rock 
over  which  it  moved,  though  these  effects  are  not  important 
topographically. 

Changes  Produced  by  Deposition. 

Sooner  or  later  the  ice  deposited  all  of  the  material  which  it 
eroded  from  the  surface  over  which  it  passed.  Had  the  drift 
been  equally  thick  everywhere,  its  effect  would  have  been  to 
raise  the  surface  without  altering  its  topography;  but  it  is  dis- 
tributed with  great  inequality,  and  this  inequality  modified  the 
topography. 

1.  General  distribution  of  the  drift.  The  tendency  of  the  mov- 
ing ice  was  always  to  transfer  its  drift  from  the  point  where  it 
was  picked  up  toward  the  margin  of  the  ice.  In  general,  there 
fore,  the  drift  left  by  the  continental  glaciers  is  thicker  toward 
their  former  margins,  and  thinner  toward  their  centers.  It  is  very 
thick,  for  example,  in  a  belt  extending  from  western  New  York 
through  Ohio,  Indiana,  Illinois,  Wisconsin,  Minnesota,  and  Iowa, 


276 


PHYSIOGRAPHY 


to  Dakota  and  Montana.  In  considerable  tracts  north  of  the 
boundary  of  the  United  States,  on  the  other  hand,  toward  the 
center  of  the  ice-fields,  little  drift  was  left. 


FIG.  306. — Terminal   moraine   topography  near  Oconomowoc.  Wis. 
(Wis.  Geol.  Surv.) 


FIG.  307. — Map   showing  the  position   of   some   of  the   principal  terminal 
moraines  of  the  United  States. 

Terminal  moraines.      The  last    ice-sheet,  especially,  developed 
stout  terminal  moraines.     The  position  of  some  of  them  is  shown 


THE  WORK  OF  SNOW  AND  ICE  277 

in  Fig.  307.  It  will  be  seen  that  they  lie  well  north  of  the  southern- 
most margin  of  the  drift,  because  the  ice-sheet  which  made  them 
did  not  advance  so  far  to  the  south  as  some  of  its  predecessors  had 
done. 

As  the  edge  of  the  ice  was  melted  back,  it  sometimes  halted  for 
a  time  far  back  from  the  position  of  its  maximum  advance.  Beneath 


FIG.  308. — Bowlders   on  the   terminal  moraine   of  the   Okanagan  glacier, 
Wash.     (U.  S.  Geol.  Surv.) 

the  edge  in  such  positions,  terminal  moraines  were  made.  Such 
terminal  moraines  are  sometimes  called  recessional  moraines. 
They  are  terminal  to  the  ice  at  the  time  they  were  made,  but  not 
terminal  to  the  drift  sheet  as  a  whole.  This  explains  why  one 
ice-sheet  came  to  develop  several  terminal  moraines. 

The  terminal  moraine  of  an  ice-sheet  is  not  always,  and  per- 


FIG.  309. — A  single  bowlder  in  the  area  shown  in  Fig.  308. 
(Willis,  U.  S.  Geol.  Surv.) 

haps  not  usually,  a  conspicuous  ridge,  though  it  is  often  conspicu- 
ous in  a  region  of  slight  relief.  Its  topography  is  much  more  dis- 
tinctive than  its  size.  Its  surface  is  often  marked  by  hillocks, 
mounds,  ridges,  etc.,  associated  with  depressions  of  similar  shapes 
(Figs.  291,  292  and  306).  While  this  sort  of  topography  is  so  wide- 
spread as  to  be  characteristic,  it  is  not  pronounced  in  all  terminal 
moraines.  The  depressions  often  contain  ponds,  lakes,  or  marshes. 


278 


PHYSIOGRAPHY 


Bowlders  frequently  abound  on  a  surface  of  terminal  moraines 
(Fig.  308). 

The  ground  moraine.  The  area  of  the  ground  moraine  is 
much  more  extensive  than  that  of  the  terminal  moraines,  and  its 
topography  is,  in  general,  less  rough.  The  hills  and  hollows  are 


FIG.  310.— "Pilot  Rock.' 


A  glacial  bowlder  near  Coiile"  City,  Wash. 
(Carrey.) 


less  steep-sided,  and  the  curves  of  the  surface  broader  (Fig.  311, 
and  PI.  XIX).  Portions  of  the  ground  moraine  are  sometimes  in 
the  form  of  elongate  or  oval  hills,  called  drumlins.  Drumlins  occur 
in  many  places,  some  of  the  best  known  being  in  Wisconsin  and 
New  York  (Figs.  298  and  312-314).  At  the  battle  of  Bunker  Hill, 
the  Americans  occupied  and  fortified  a  drumlin. 


Fio.  311. — Ground  moraine  topography.     (At wood.) 

Effect  of  drift  on  topography.  The  drift  is  sometimes  so 
disposed  as  to  increase  the  relief  of  the  surface  (Fig.  315),  but 
oftener  so  as  to  decrease  it  (Fig.  290),  because  more  drift,  on 
the  whole,  was  left  in  the  low  places  than  on  the  high  ones.  On 
the  other  hand,  the  drift  was  sometimes  left  in  such  a  way  as  to 


PLATE  XIX 


/    T..  Cr.    :^ 


Characteristic  drift  topography.     Scale  1—  mile  per  inch.     (Eagle,  Wis. 
Sheet,  U.  S.  Geol.  Surv.) 


THE  WORK  OF  SNOW  AND  ICE 


279 


FIG.  312. — Drumlins  in  contour,  near  Clyde,  N.  Y.     (U.  S.  Geol.  Surv.) 


FlQ.  313. — A  Wisconsin  drumlin  seen,  from  the  side.    Two  miles  north  of 
Sullivan.    (Alden,  tT.  S.  Geol.  Surv.) 


280 


PHYSIOGRAPHY 


make  the  surface  rougher  than  the  surface  of  the  rock  below, 
even  where  the  relief  was  decreased.  > 

Effect  of  drift  deposits  on  drainage.  The  drift  left  by  the 
ice  sometimes  filled  valleys  at  some  points,  but  not  at  others. 
Drift  fillings  in  valleys  make  dams,  above  which  water  is  likely 


FIG.  314. — The  same  drumlin  shown  in  Fig.  313  seen  from  the  end. 
(U.  S.  Geol.  Surv.) 

to  accumulate,  making  lakes.  If  a  valley  was  filled  in  two  places, 
as  sometimes  happened,  the  unfilled  place  between  became  a 
basin  fit  for  a  lake.  The  number  of  lakes  developed  in  this  way 
is  very  large.  The  finger  lakes,  of  New  York  and  Devil's  Lake, 
Wisconsin  (Fig.  316),  are  good  examples. 


FIG.  315. — Diagram  to  show  how  drift  may  be  so  disposed  as  to  increase 
the  relief  of  the  surface. 

Rock  basins  have  already  been  referred  to;  but  it  often  hap- 
pened tk&t'T&wms  the  bottoms  of  which  are  in  rock  were  made 
deeper  by  the  deposition  of  drift  about  their  rims.  The  Great  Lakes 
probably  occupy  rock  basins,  but  their  margins  were  built  up  by 
drift,  making  them  deeper. 

The  ice-sheets  gave  rise  to  lakes  and  ponds  in  other  ways'a'lso. 


THE  WORK  OF  SNOW  AND  ICE 


281 


Many  of  them  occupy  depressions  in  the  surface  of  the  drift.  Such 
lakes  are  especially  numerous  in  terminal  moraines,  but  they  are 
not  rare  in  the  ground  moraine,  and  are  by  no  means  unknown 
in  the  stratified  drift.  Glaciation  affords  the  explanation  of 
the  numerous  lakes  of  North  America,  nearly  all  of  which  are  in 
the  area  which  was  covered  by  the  ice-sheet  or  by  mountain 


FIG.  316. — Sketch  showing  a  lake  in  a  former  river  valley,  held  in  by  drift 
dams.     The  dotted  areas  are  terminal  moraines. 


glaciers.  They  are  most  numerous  in  the  area  covered  by  the 
ice  of  the  last  glacial  epoch  (Fig.  303),  as  in  North  Dakota,  Minne- 
sota, Wisconsin,  Michigan,  New  York,  and  New  England.  Except 
in  special  situations,  where  they  are  of  wholly  different  types, 
lakes  do  not  occur  south  of  the  drift. 

Some  lakes  developed  by  the  ice  had  but  a  temporary  exist- 
ence.    Some  of  them  came  into  existence  along  the  margin  of 


282 


PHYSIOGRAPHY 


the  ice-sheets,  the  ice  itself  often  forming  one  border  of  the  lake. 
Such  lakes  disappeared  when  the  ice  melted. 

One  of  the  largest  of  the  marginal  lakes  (Lake  Agassiz)  lay  in 
the  valley  of  the  Red  River  of  the  North  (Fig.  317).  When  this 
lake  was  largest,  its  length  was  about  700  miles,  and  its  maximum 
width  about  250  miles.  Its  area  was  about  110,000  square  miles, 


'  .=     ;^S  9ca  / 


<»jff™R»'l 


J 


FIG.  317. — Map  of  the  extinct  Lake  Agassiz,  and  a  few  other  glacial  lakes. 
Lake  Winnipeg  occupies  a  part  of  the  old  basin  of  Lake  Agassiz.  (Upham , 
U.  S.  Geol.  Surv.) 

or  nearly  one-fifth  more  than  the  combined  area  of  all  the  Great 
Lakes,  but  the  water  was  not  very  deep.  It  came  into  existence 
when  the  ice  at  the  north  obstructed  drainage  in  that  direction. 
The  water  rose  in  the  basin  until  it  overflowed  to  the  south.  When 
the  ice  at  the  north  melted,  a  new  and  lower  outlet  was  opened 
in  that  direction,  and  the  lake  was  drained.  Lake  Winnipeg  and 
several  smaller  lakes  may  be  looked  upon  as  remnants  of  this  great 
lake,  for  they  occupy  the  deepest  depressions  in  the  old  basin. 

The  borders  of  the  former  lake  are  marked  by  old  beaches, 
and  locally  by  deltas.  The  silt-covered  bottom  of  the  Jake  is  one 
of  the  most  important  wheat-producing  areas  in  the  United  States. 


THE  WORK  OF  SNOW  AND  ICE 


283 


FIG.  318. — The  beginning  of  the  Great  Lakes.  The  ice  still  occupied  the 
larger  parts  of  the  present  lake  basins.  (After  Taylor  and  Leverett, 
U.  S.  Geol.  Surv.) 


FIG.  319. — A  later  stage  in  the  development  of  Lakes  Chicago  and  Maumee. 
The  ice  has  retreated,  and  the  outlet  of  Lake  Maumee  has  been  shifted. 
(After  Leverett  and  Taylor,  U.  S.  Geol.  Surv.) 


284 


PHYSIOGRAPHY 


The  Great  Lakes  of  the  present  day  were  greatly  expanded 
while  the  ice  blocked  their  present  outlets.     A  part  of  their  history, 


FIG.  320. — The  Great  Lakes  at  the  Algonquin-Iroquois  stage. 
(After  Taylor.) 


FIG.  321. — A  still  later  stage  of  the  Great  Lakes.     The  sea  is  thought  to 
have  covered  the  area  shaded  by  lines  at  the  east.     (After  Taylor.) 

dating  from  the  time  of  the  last  ice  occupancy,  is  suggested  by 
Figs.  318  to  321.     The  basins  of  these  lakes  did  not  exist,  so  far 


THE    WORK    OF  SNOW    AND    ICE 


285 


as  known,  before  the  glacial  period,  but  considerable  rivers  may 
have  flowed  along  the  lines  of  their  axes.  From  these  river  val- 
leys, lake  basins  appear  to  have  been  developed  as  a  result  of  (1) 
the  deepening  of  portions  of  the  valleys  by  ice  erosion,  (2)  the 
building  up  of  the  rims  of  the  basins  by  the  deposition  of  drift,  and 
(3)  perhaps  the  down-warping  of  the  sites  of  the  basins. 


FIG.  322. — Sketch-map  showing  the  drainage  of  the  upper  Ohio  basin  as  it 
is  believed  to  have  been  before  the  glaciation  of  the  region.  (Tight, 
U.  S.  Geol.  Surv.) 

The  irregular  disposition  of  drift  also  deranged  the  rivers. 
After  the  ice  melted,  the  surface  drainage  followed  the  lowest 
lines  open  to  it,  but  these  lines  did  not  always  correspond  with 
the  former  valleys,  for  some  of  them  had  been  filled,  and  most  of 
them  were  blocked  up  in  some  places.  After  the  ice  melted,  there- 
fore, the  surface  waters  followed  former  valleys  in  some  cases, 
and  in  others  flowed  across  areas  where  there  had  been  no  valleys. 
In  choosing  their  new  courses,  the  streams  sometimes  fell  over 


PHYSIOGRAPHY 


cliffs  or  ran  down  steep  slopes.  Thus  arose  falls  and  rapids  which, 
on  the  whole,  are  rather  common  in  the  streams  of  the  glaciated 
area.  Figs.  322  and  323  indicate  something  of  the  changes 
effected  by  the  deposition  of  the  drift  in  the  basin  of  the  upper 
Ohio. 

We  have  already  seen  that  rapids  and  falls  are  marks  of  young 
streams.  Most  lakes  also  are  marks  of  youth.  Rivers  are,  on 
the  whole,  hostile  to  lakes,  for  outflowing  waters  cut  down  their  out- 
lets, and  inflowing  waters  bring  in  sediment  which,  when  deposited 
in  the  basins,  tends  to  fill  them  up.  Many  small  lakes  have  already 


FIG.  323. — Present  drainage  of  the  area  shown  in  Fig.  322. 

become  extinct  in  these  ways,  and  many  others  have  been  made 
sensibly  smaller.  The  fact  that  so  many  falls,  rapids,  lakes,  etc., 
still  exist  within  the  glaciated  area  shows  that  the  time  since  the 
melting  of  the  last  ice-sheet  has  not  been  long  enough  for  these 
features  to  be  destroyed. 

Marshes  also  abound  within  the  glaciated  area.  In  some  cases 
they  represent  the  beds  of  former  lakes  and  ponds,  while  in  others 
they  are  simply  basins  too  shallow  to  hold  bodies  cf  water  suffi- 
ciently deep  to  prevent  the  growth  of  plants. 


THE  WORK  OF  SNOW  AND  ICE  287 

Lakes,  ponds,  marshes,  falls,  rapids,  etc.,  are  much  more 
abundant  in  the  area  covered  by  the  last  ice-sheet  than  in  the  area 
of  drift  outside  of  the  last  ice-sheet.  This  is  largely  because  the 
southernmost  part  of  the  drift,  as  now  exposed,  is  older,  and 
has  been  subject  to  rain  and  river  erosion  long  enough  for  sur- 
face drainage  to  destroy  most  of  the  lakes.  The  oldest  drift 
of  the  glacial  period  is  believed  to  be  many  times  (probably  as 
many  as  twenty-five)  as  old  as  the  youngest. 

Stratified  drift.  Valley  trains,  outwash  plains  (p.  266),  deltas 
(p.  198),  etc.,  were  developed  by  the  continental  glaciers,  but 
only  those  of  the  last  ice-sheet  are  well  preserved.  Some  of 
the  valley  trains  are  long,  and  in  some  the  deposits  are  deep.  Thus 
the  Rock  River  in  southern  Wisconsin  filled  its  valley  with  gravel 
and  sand  to  a  depth  of  300  to  400  feet  just  below  the  terminal 
moraine  of  the  last  glacial  epoch,  while  the  ice-water  flowed  through 
it.  The  Columbia  River,  swollen  by  the  waters  from  the  melting 
ice,  filled  its  valley,  locally,  to  the  depth  of  700  feet  with  material 
washed  out  from  the  ice. 

Since  the  ice  melted,  most  of  the  valley  trains  have  been  par- 
tially carried  away,  and  the  remnants  of  the  old  plains  of  aggrada- 
tion are  now  terraces  (Fig.  217). 

Outwash  plains  (p.  266)  also  are  extensive.  Thus  much  of  the 
southern  part  of  Long  Island  is  an  extensive  outwash  plain,  spread- 
ing southward  from  the  terminal  moraine  which  makes  the  back- 
bone of  the  island.  Just  southeast  of  Brooklyn  the  outwash  plain, 
in  preference  to  the  hills  of  the  adjacent  moraine,  has  been  occupied 
by  several  suburbs. 

Kames  and  eskers  (p.  267)  also  diversify  the  surface  of  the 
drift  at  numerous  points,  the  former  being  far  more  numerous  than 
the  latter  (Figs.  301  and  302). 

Effects  of  glaciation  on  human  affairs.  The  effects  of  glacia- 
tion  have  had  much  influence  on  the  industrial  history  of  the  region 
which  the  ice  covered. 

The  increase  of  mantle  rock  in  the  United  States,  as  a  result  of 
glaciation,  is  of  significance.  This  increase  is  of  value  in  regions 
where  slopes  were  considerable,  for  where  such  slopes  are  found 
in  driftless  regions,  the  soil  is  often  very  thin  or  absent,  and  the 
area  of  arable  land  is  thereby  restricted.  Abundant  soil  is  much 
more  likely  to  be  found  on  similar  slopes  in  the  glaciated  area. 
Furthermore,  since  the  general  effect  of  glaciation  was  to  reduce 


288  PHYSIOGRAPHY 

slopes,  it  tended  to  reduce  the  areas  where  the  slopes  were  too 
steep  to  be  cultivated. 

Again,  the  quality  of  the  soil  was  improved  in  many  places  by 
glaciation,  but  this  is  not  true  everywhere.  It  is  worth  noting 
that  most  of  the  wheat  and  hay  grown  in  the  United  States  east 
of  the  Rocky  Mountains,  are  within  the  area  which  was  glaciated. 
This  is  probably  not  altogether  because  of  the  drift,  but  partly 
because  of  the  climate. 

The  reduction  of  roughness  and  the  smoothing  of  slopes  effected 
by  glaciation  made  the  construction  of  roads  easier,  and  so,  on  the 
whole,  has  facilitated  transportation.  Locally,  however,  the  sur- 
face was  made  rougher,  with  disadvantageous  results. 

The  falls,  rapids,  and  lakes  which  resulted  from  glaciation 
have  increased  the  water-power,  and  the  lakes,  ponds,  and  marshes 
which  serve  as  reservoirs  have  tended  to  equalize  the  flow  of  the 
streams  throughout  the  year.  The  flow  of  streams  from  lakes  is 
much  steadier  than  the  flow  of  streams  which  have  no  permanent 
reservoirs  to  draw  upon.  The  drift  is  much  thicker,  on  the  whole, 
than  the  mantle  rock  of  other  regions.  This  greater  thickness  of 
loose  material  on  the  surface  tends  to  hold  back  the  rain-water 
after  it  falls,  the  porous  drift  itself  serving  as  a  sort  of  reservoir 
which  yields  up  the  water  slowly. 

The  economic  significance  of  lakes  is  noted  elsewhere  (p.  316). 

The  drift  materials  are  somewhat  extensively  utilized.  Thus 
much  of  the  drift  clay  (rock-flour)  is  used  for  the  manufacture  of 
brick,  tile,  etc.,  and  the  gravel  is  used  for  road-making,  and  in  the 
manufacture  of  various  sorts  of  cements. 

Such  are  some  of  the  beneficent  results  of  glaciation.  There 
are  also  some  considerations  on  the  other  side. 

In  some  places  the  quality  of  the  soil  has  been  injured,  for 
in  many  areas  the  drift  is  stony,  and  great  labor  is  necessary  to  put 
it  in  workable  condition.  In  some  places,  too,  it  is  too  sandy 
or  gravelly  to  make  good  soil,  and  in  other  places  its  surface  is 
too  rough  to  allow  of  successful  tilling.  In  still  other  situations, 
as  in  much  of  New  England,  the  ice  left  a  thin  stony  mantle  of 
drift  covering  a  rough  hilly  surface.  This,  combined  with  a  some- 
what unfavorable  climate,  made  agriculture  unprofitable  in  much  of 
this  region,  and  so  favored  the  early  development  of  the  fisheries, 
and,  together  with  abundant  water-power,  has  made  New  England 
a  manufacturing  rather  than  an  agricultural  region. 


THE  WORK  OF  SNOW   AND  ICE  289 

In  spite  of  these  adverse  considerations,  it  seems  probable,  on 
the  whole,  that  the  glaciated  area  of  the  United  States  was  con- 
siderably benefited  by  the  work  of  the  ice. 

MAP   EXERCISE. 
Maps  /or  the  Study  of  the  Topographic  Effects  of  Glaciation. 

I.  The  maps  to  be  studied  in  preparation  for  conference: 

1.  Lancaster,  Wis. — la.  8.  Passaic,  N.  J. 

2.  Eagle,  Wis.  9.  Palmyra,  N.  Y. 

3.  Whitewater,  Wis.  10.  Canada  Lake,  N.  Y. 

4.  Muskego,  Wis.  11.  Paradox  Lake,  N.  Y. 

5.  Geneva,  Wis.  12.  Leadville,  Colo. 

6.  St.  Croix  Dalles,  Wis.— Minn.  13.  Hayden  Peak,  Utah. 

7.  Brooklyn,  N.  Y. 

II.  Questions  to  be  answered  in  writing: 

1.  Contrast  the  Lancaster  Sheet   (which  represents  a  part  of  the 
"Driftless  Area")  with  the  Eagle  Sheet  (a  glaciated  area),   and  indi- 
cate  the  essential  ways  in  which  the  two  areas  differ  topographically. 

2.  Locate  (a)  terminal  moraine  belts  and  (6)  outwash  plains  on  at 
least  two  maps. 

3.  Interpret  the  peculiar  topography  shown  on  the  Palmyra,  N.  Y., 
Sheet.     How  were  the  numerous  hills  formed,  what  are  they  called,  and 
why  do  they  all  trend  in  the  same  direction? 

4.  Select   three   maps   showing   regions   whose   present   topography 
is  controlled  largely  by  the  drift;   three  where  the  topography  is  con- 
trolled chiefly  by  the  underlying  rock. 

5.  Under  what  conditions  does  drift  control  present  topography? 
Under  what  conditions  does  the  underlying  rock  control  topography 
within  the  glaciated  area? 

6.  The  probable  origin  of  the  lakes  on 

(a)  The  Paradox  Lake  Sheet. 

(6)  The  Eagle  and  Geneva  sheets. 

(c)  The  Leadville,  Colo.,  Sheet,  especially  Twin  Lakes. 

7.  What  altitude  appears  to  have  been  necessary  to  develop  glaciers 
in  the  area  of  the  Leadville  and  Hayden  Peak  sheets? 

REFERENCES. 
A.  EXISTING  GLACIERS. 

1.  CHAMBERLIN  AND  SALISBURY,  Geologic  Processes,  Vol.  I,  Chapter  V: 
Henry  Holt  &  Co.,  1903;    and  other  standard  text-books  on  Geology. 

2.  RUSSELL,  Glaciers  of  North  America:   Ginn  &  Co.,  1897. 

3.  RUSSELL,  Glaciers  of  Mount   Rainier:    18th  Ann.  Rept.  U.  S.  Geol. 
Surv.,  Pt.  II,  1896-1897,  pp.  349-415. 


290  PHYSIOGRAPHY 

4.  Alaskan  Glaciers.      REID,  16th  Ann.  Kept.  U.  S.  Geol.  Surv.,  Pt.  I, 
1894-1895,  pp.  421-459,  and  Nat.  Geog.  Mag.,  Vol.  IV,  1891,  pp.  19-55. 
RUSSELL,  Nat.  Geog.  Mag.,  Vol.  Ill,  1891,  pp.  176-188;  Jour,  of  Geol.,  Vol.  I, 
pp.  219-245;  and  13th  Ann.  Rept.  U.  S.  Geol.  Surv.,  Pt.  II,  1891-1892,  pp. 
7-91.     GILBERT,  Glaciers,  Harriman  Alaskan  Expedition:  Doubleday,  Page 
&  Co. 

5.  Glaciers  of  Greenland.     CHAMBERLIN,  Jour,  of  Geol.,  Vol.  II,   1894, 
pp.  649-666  and  768-788;    Vol.   Ill,   1895,  pp.  61-69,   198-218,  469-480, 
565-582,  668-681,  and  833-843;  and  Vol.  IV,  1896,  pp.  582-592.    SALISBURY, 
Jour,  of  Geol.,  Vol.  IV,  1896,  pp.  769-810. 

6.  SHALER  AND  DAVIS,  Glaciers:   James  R.  Osgood  &  Co.,  1881. 

7.  Variations  of  Glaciers.     REID,  Jour,  of  Geol.,  Vol.  Ill,  1895,  pp.  278- 
288;  Vol.  V,  1897,  pp.  378-383;  Vol.  VI,  1898,  pp.  473-476;  Vol.  VII,  1899, 
pp.  217-225;    Vol.  IX,  1901,  pp.  250-254;    Vol.  X,  1902,  pp.  313-328;    Vol. 
XI,  1903,  pp.  285-288;    Vol.  XII,  1904-,  pp.  252-263;   and  Vol.  XIII,  1905, 
pp.  313-318.     GILBERT,  Bull.  Sierra  Club,  Vol.  V,  pp.  20-25. 

8.  TYNDALL,  The  Glaciers  of  the  Alps:   Murray,  1860. 

B.  GLACIER  MOTION. 

9.  CHAMBERLIN  AND  SALISBURY,  Geologic  Processes,  Vol.  I,  pp.  308-323: 
Henry  Holt  &  Co.,  1903. 

10.  REID,  Mechanics  of  Glacier  Motion:   Jour,  of  Geol.,  Vol.  IV,  p.  912. 

11.  AITKIN,  Am.  Jour.  Sci.,  Vol.  V,  1873,  p.  305;    Vol.  XXXIV,  1887, 
p.  149;   and  Nature,  Vol.  XXXIX,  1888,  p.  203. 

12.  RUSSELL,  The  Influence  of  Debris  on  the  Flow  of  Glaciers:    Jour,  of 
Geol.,  Vol.  Ill,  p.  823. 

C.  RESULTS  OF  GLACIATION. 

13.  CHAMBERLIN  AND  SALISBURY,  Earth  History,  Vol.  Ill,  pp.  327-446: 
Henry  Holt  &  Co.,  1906. 

14.  SALISBURY,  The  Drift:    Jour,  of    Geol.,  Vol.  II,  pp.  708-724   and 
837-851,  and  Vol.  Ill,  pp.  70-97. 

15.  CHAMBERLIN,  Genetic  Classification  of  the  Drift:  Jour,  of  Geol.,  Vol.  II, 
pp.  517-538. 

16.  CHAMBERLIN,   A    Preliminary   Paper  on  the   Terminal  Moraines  of 
the  Second  Glacial  Epoch:    3d  Ann.  Rept.  U.  S.  Geol.  Surv.,   1881-1882, 
pp.  295-401. 

17.  J.  GEIKIE,  The  Great  Ice  Age,  3d  Ed.:   D.  Appleton  &  Co.,  1895,  and 
Earth  Sculpture,  Chapters  X  and  XI:  Putnams. 

18.  A.  GEIKIE,  Scenery  of  Scotland,  chapters  on  Glacial  Action:  Macmillan 
Co.,  1887. 

19.  Folios  of  the  U.  S.  Geol.  Surv.,  especially  those  of  the  high  mountains 
of  the  West. 

20.  LEVERETT,    Illinois   Glacial   Lobe:     Mono.    XXXVIII,    U.    S.    Geol. 
Surv.;  Glacial  Formations,  etc.,  of  the  Erie  and  Ohio  Basins:   Mono.  XLI,  U.. 
S.  Geol.  Surv.     STONE,  Glacial  Gravels  of  Maine:  Mono.  XXXIV,  U.  S.  Geol. 
Surv. 


THE   WORK   OF   SNOW   AND    ICE  291 

21.  SALISBURY,  Glacial  Geology  of  New  Jersey:   N.  J.  Geol.  Surv.,  Vol.  V, 
1902. 

22.  CHAMBERLIN,  Rock  Scorings  of  the  Great  Ice  Age:  7(h  Ann.  Kept.  U.  S. 
Geol.  Surv.,  pp.  155-248,  1885-1886. 

D.  EXPLORATION  IN  POLAR  REGIONS. 

23.  PEARY,  Northward  over  the  Great  Ice:   Fred.  A.  Stokes  Co.,  1898. 

24.  NANSEN,  First  Crossing  of  Greenland:   Longmans,  Green  &  Co.,  1890; 
and  Farthest  North:   Harper  &  Bros.,  1897. 

25.  Most  other  books  on  the  Arctic  and  Antarctic  regions  give  some 
account  of  the  ice. 

E.  GLACIAL  LAKES. 

26.  TAYLOR,  Short  History  of  the  Great  Lakes,  in   Studies  in  Indiana 
Geography:  Inland  Publishing  Co.,  Terre  Haute,  Ind.,  1897.    See  also  20  above. 

27.  CHAMBERLIN  AND  SALISBURY,  Earth  History,  Vol.  Ill,  pp.  394-403: 
Henry  Holt  &  Co.,  1906. 

28.  UPHAM,  Glacial  Lake  Agassiz:    Mono.  XXV,  U.  S.  Geol.  Surv. 

29.  SALISBURY  AND  KUMMEL,  Lake  Passaic,  an  Extinct  Glacial  Lake: 
Geol.  Surv.  of  N.  J.;   Ann.  Rept.  of  the  State  Geologist  for  1893. 

30.  FAIRCHILD,  Glacial  Genesee  Lakes:    Bull.  Geol.  Soc.  Am.,  Vol.  VII, 
pp.  423-452. 


CHAPTER  VI 
LAKES   AND   SHORES 

GENERAL  FACTS 

Definition.  In  general,  a  lake  is  an  inland  body  of  standing 
water  larger  than  a  pool  or  a  pond;  but  the  term  is  sometimes 
applied  to  the  widened  parts  of  rivers  (Fig.  324),  and  sometimes 


i 


FIG.  324. — Lake  Pepin,  a  widened  part  of  the  Mississippi  River  between 
Wisconsin  and  Minnesota.  Maximum  width  about  2A  miles.  The 
widening  of  the  river  is  apparently  due  to  the  detritus  brought  down 
by  the  Chippevva  River  and  deposited  in  the  Mississippi.  (Miss.  Riv.  Com. ) 

to  bodies  of  water  which  lie  along  coasts,  even  when  they  are  at 
sea-level,  and  sometimes  when  they  are  in  direct  connection  with 
the  sea  (PI.  XX). 

The  distinction  between   lakes  and   similar  bodies   of  water 

292 


LAKES  AND  SHORES  293 

which  are  not  lakes  is  rather  arbitrary.  The  amount  of  widening 
which  a  river  must  undergo  before  it  is  called  a  lake  is  as  arbitrary 
as  the  size  which  the  body  of  standing  water  must  attain  before 
this  name  may  be  applied  to  it.  In  the  interior  of  the  United 
States,  a  pond  is  usually  understood  to  mean  a  body  of  water 
smaller  or  shallower  than  a  lake;  but  this  usage  is  not  universal, 
for  some  beautiful  lakes  (for  example,  Green  Pond  in  New  Jersey) 
are  called  ponds.  Ponds  and  lakes  differ  from  inland  seas,  bays, 
and  lagoons  (1)  in  being  more  completely  (in  most  cases  altogether) 
shut  off  from  the  ocean,  and  (2)  in  being  for  the  most  part  at  a 
level  above — very  rarely  below — that  of  the  sea;  but  between  bays 
and  lagoons  which  are  nearly  enclosed,  and  coastal  lakes,  there 
are  all  possible  gradations. 

Most  lakes  are  fresh,  but  a  few,  like  Great  Salt  Lake  and  the 
Dead  Sea,  are  much  more  salt  than  the  sea  itself. 

Distribution  of  Lakes 

1.  In  latitude.     Lakes  occur  in  most  latitudes,  but  they  are 
more  abundant  in  high  latitudes  than  in  low.     They  do  not  abound, 
however,  in  all  high  latitudes.     Northern  Asia,  for  example,  has 
relatively    few.     This    distribution    of    lakes    is    connected    with 
former  glaciation,  a  connection  which  has  already  been  pointed 
out. 

2.  In  mountains.     Lakes  are  abundant  in  some  mountain  regions 
but  not  in  all.     They  are  numerous  in  the  western  mountains  of 
the  United  States,  especially  at  the  North,  but  they  are  essentially 
absent  from  the  Appalachian  Mountains  south  of  northern  Penn- 
sylvania.    They  are,  in  general,  more  abundant  in  high  moun- 
tains than  in  low  ones,  and  if  they  occur  in  low  mountains  at 
all,  it  is  likely  to  be  in  high   latitudes  only.      In  other  words 
lakes    are    common    in    mountains    which    have    been    recently 
glaciated. 

3.  Along  rivers.     Another  situation  where  lakes  occur,  though 
less  commonly,  is  along  rivers;    but  they  do  not  occur  along  all 
rivers.     Outside  of  high  latitudes  and  high  mountains  (glaciated 
regions),  lakes  are  common  only  along  streams  which  have  low 
gradients  and  wide  flats.     There  are  numerous  lakes,  for  example, 
on  the  alluvial   plain   of   the  Mississippi  (Fig.  197),  and  on  the 
flats  of  some  of  its  tributaries,  such  as  the  Missouri  and  the  Red 


294 


PHYSIOGRAPHY 


River  of  Louisiana  (Fig.  325).  In  many  of  these  cases  the  origin 
of  the  lakes  is  clearly  connected  with  the  changing  of  the  river 
channel  (Fig.  197). 

4.  Along  coasts.  Another  situation  where  lakes  are  of  rather 
common  occurrence  is  along  coasts  (PI.  XX  and  Fig.  326) ,  though 
many  coasts  are  without  them.  Coastal  lakes  stand  in  no 
apparent  relation  to  latitude  and  are  always  at  low  altitudes. 


.: L:Cruss  ..•  •'•'• 

•. '•'.'.'•£'. .  ShrevQport 


FIG.  325. — Lakes  along  the  Red  River  of  Louisiana.     The  lakes  are  at  the 
lower  ends  of  the  tributary  streams. 

The  level  of  the  water  in  them  is  often  nearly  or  quite  the  same 
as  that  of  the  sea. 

5.  On  coastal  plains.     Low   lands  in  proximity   to    the  sea, 
though  back  from  the  coast,   are  sometimes  affected  by  lakes, 
especially  if  the  climate  is  moist.     This  is  illustrated  by  Florida 
(Fig.  2,  PI.  XX),  where  the  number  and  abundance  of  lakes  is 
comparable  to  that  of  equal  areas  in  the  northern  part  of  the 
United  States. 

6.  Glaciated  plains  and  plateaus.      Plains  and  plateaus  which 
have  been  recently  glaciated  are  likely  to  abound  in  lakes,   as 
indicated  in  the  preceding  chapter. 

7.  On    plateaus.     Lakes    sometimes    occur    on    plateaus    even 
where    there    has    been    no    glaciation.     The    most    considerable 


PLATE  XX 


FIG.  1. — Coastal  lakes  formed  by  the  blocking  of  the  ends  of  drowned  valleys. 
Scale  1—  mile  per  inch.  (Marthas  Vineyard,  Mass.,  Sheet,  U.  S.  Geol. 
SunO 


Fio.  2. — A  group  of  lakes  on  the  coastal  plain  of  Florida.     Scale  1  —  mile 
oer  inch.     (Williston  Sheet,  U.  S.  Geol.  Surv.) 


PLATE  XXI 


The  upper  end  of  Seneca  Lake,  New  York.  The  flat  between  Montour  Falls 
and  Watkins  is  a  delta  which  has  been  built  out  into  the  lake  by  the 
inflowing  creek.  Scale  1—  mile  per  inch.  (Watkin's  Sheet,  U.  S 
Geol.  Surv.) 


LAKES  AND  SHORES 


295 


examples  are  the  great  lakes  of  central  and  southern  Africa;  but 
there  are  not  a  few  lakes,  mostly  shallow,  on  the  Great  Plains  (pla- 
teaus) of  the  United  States,  even  where  the  rainfall  is  not  great. 

8.  Other  situations.  Lakes  occur  in  a  few  other  situations,  as 
in  the  tops  of  some  volcanic  mountains,  and  on  plains  which  have 
not  been  glaciated  and  which  are  far  from  the  coast. 


FIG.  326. — Lakes  on  the  coaet  of  New  Jersey. 

Area,  Topographic  Position,  Depth,  etc. 

Area  and  topographic  position.  Lake  Superior,  Lake  Huron, 
Lake  Michigan,  Lake  Erie,  and  Lake  Ontario  are  examples  of 
great  lakes.  These  lakes,  indeed,  constitute  the  greatest  chain 
of  lakes  in  the  world,  and  have  an  aggregate  area  of  nearly  95,000 


296  PHYSIOGRAPHY 

square  miles.  Five  of  the  great  lakes  of  the  Dominion  of  Canada 
have  a  combined  area  of  more  than  32,000  square  miles.  All  these 
large  lakes  lie  at  relatively  low  altitudes. 

Lakes  have  a  great  range  in  altitude  as  well  as  in  size.  The 
Yellowstone  Lake  is  the  highest  lake  of  much  size  in  the  United 
States.  It  is  7738  feet  above  sea-level,  and  its  area  140  square 
miles.  Lake  Titicaca,  the  largest  lake  (except  Lake  Maracaibo) 
in  South  America,  is  both  higher  (12,500  feet)  and  larger  (3200 
square  miles).  The  surfaces  of  a  few  great  lakes  are  below  sea- 
level.  This  is  true  of  the  Caspian  Sea  (  —  85  feet),  the  Dead  Sea 
(-1268  feet),  and  the  Sea  of  Tiberias  (-682  feet). 

Depth.  Most  lakes  are  rather  shallow.  The  number  in  which 
the  water  is  less  than  50  feet  deep  probably  far  exceeds  the  num- 
ber in  which  the  depth  is  greater;  but  some  lakes  are  exceedingly 
deep.  It  need  hardly  be  said  that  the  popular  notion  that  many 
lakes  are  bottomless, -is  without  foundation.  Many  of  the  lakes 
which  are  locally  believed  to  be  without  bottom  are,  indeed, 
shallow. 

Lake  Superior  has  a  maximum  depth  of  about  1000  feet,  and 
Lakes  Michigan,  Huron,  and  Ontario  all  have  depths  exceeding 
700  feet.  Lake  Erie,  on  the  other  hand,  is  much  shallower,  its 
maximum  depth  being  only  about  200  feet. 

A  few  lakes  have  much  greater  depths.  The  deepest,  so  far  as 
known,  is  Lake  Baikal,  in  Siberia,  which  is  stated  to  have  a  maxi- 
mum depth  of  about  4700  feet,  or  about  one-seventh  the  depth 
of  the  deepest  part  of  the  ocean.  The  Caspian  Sea,  really  a  lake, 
is  next  deepest,  and  has  a  maximum  depth  of  about  3200  feet. 
Other  lakes  of  great  depth  are  Crater  Lake,  Oregon,  about  2000 
feet;  Lake  Tahoe,  California,  1645  feet;  Lake  Chelan,  Washington, 
about  1500  feet;  and  lakes  Maggiore,  Como,  and  de  Garda,  in 
northern  Italy,  and  the  Dead  Sea,  all  of  which  have  depths  of 
more  than  1000  feet. 

The  bottoms  of  most  lakes  are  well  above  sea-level;  but  in 
exceptional  cases  their  bottoms  are  far  below.  The  lowest  point 
in  the  bottom  of  the  Caspian  Sea  is  a  little  more  than  3000  feet 
below  sea-level,  and  the  lowest  point  in  the  basin  of  Lake  Baikal 
is  nearly  as  far  down.  The  lowest  point  in  the  bottom  of  Lake 
Ontario  is  about  500  feet  below  sea-level,  in  Superior  about  400  feet, 
and  in  Lake  Chelan  more  than  400  feet.  Except  in  lakes  along 
the  coast,  the  bottoms  of  the  small  lakes  are  rarely  so  low  as  sea- 


LAKES   AND   SHORES 


297 


level;    but  the  bottoms  of  the  three  north-Italian  lakes  mentioned 
are  all  several  hundred  feet  below  the  ocean. 

Various  facts  concerning  the  lakes  which  exceed  10,000  square 
miles  in  area,  and  concerning  a  few  others,  are  given  in  the 
following  table,  though  few  lakes  have  such  remarkable  dimen- 
sions: 


Name. 

Approximate 
area  in  square 
miles. 

Approximate 
altitude  of 
surface.1 

Approximate 
maximum  depth 
in  feet. 

Caspian  

170,000 

-85 

3,200 

Superior.  .                  .... 

31,200 

602 

1,008 

Victoria  Nyanza  

26,000 

3,800 

240 

Aral                                 . 

25,050 

160 

1,200 

Michigan  

22,500 

581 

870 

Huron  

22,320 

581 

700 

Nyassa.  .  .             .  .  .  .  -.  .  . 

14,2002 

1,500 

2,300 

Baikal  

13,000 

1,700 

4,7003 

Tanganyika  

12,000 

2,700 

2,100 

dreat  Bear  

11,200 

390 

270 

Erie   

9,960 

573 

200 

Winnipeg.  .          

9,400 

710 

70 

Balkash                  

8600 

900 

80 

Ontario  

7,240 

247 

738 

Chad  

6  OOO4  to  40,000 

900 

8  to  20 

Titicaca  

3,200 

12,500 

700 

Dead  Sea  

360 

-1,268 

1,300s 

'Garda  

189 

215 

1,135 

Chelan  

85 

1,079 

1,500 

Como  

60 

650 

1,340 

Crater.   .              

25 

6,239 

2,000 

Great  as  is  the  depth  of  the  water  in  some  of  the  lakes,  the 
^shape  of  their  basins  is  often  very  different  from  that  which  might 
be  imagined  from  the  mere  statement  of  the  depths.  Fig.  327 
represents  the  cross-sections  of  the  basins  of  some  of  the  Great 
Lakes,  drawn  to  scale.  The  basins  of  many  smaller  lakes  (Fig. 
328)  are  much  more  striking  in  cross-section. 

Volume.  No  accurate  estimate  of  the  volume  of  water  in 
lakes  has  ever  been  made,  but  their  combined  volume  is  insignifi- 
cant when  compared  with  the  sea.  If  the  water  of  all  lakes  were 
added  to  the  ocean,  it  would  probably  not  raise  its  surface  two 
feet. 

1  The  negative  sign  means  below  sea-level. 

2  Sometimes  given  as  low  as  10,200. 

*  Depth  of  5,618  feet  recently  reported. 
4  Range  between  wet  and  dry  seasons. 
6  Sometimes  given  as  low  as  1,171  feet. 


298 


PHYSIOGRAPHY 


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LAKES  AND  SHORES 


299 


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PHYSIOGRAPHY 


Movements  of  lake  water.  The  waters  of  all  lakes  are  af- 
fected by  waves,  and  the  waters  of  many  lakes  by  movements  of 
other  sorts.  In  some  lakes  there  is  a  more  or  less  well-defined 
system  of  currents  (Fig.  329)  or  drifts.  A  sudden  change  of 
atmospheric  pressure  on  one  part  of  a  large  lake  causes  changes 
of  level  everywhere.  If  the  pressure  is  increased  in  one  place, 
the  surface  of  the  water  there  is  lowered  and  the  surface  else- 
where correspondingly  raised.  If  the  change  is  one  which  les- 
sens the  pressure  locally,  the  water  surface  beneath  the  lessened 


FIG.  329. — Diagram  showing  the  currents  in  the  Great  Lakes. 
(U.  S.  Weather  Bureau.) 


pressure  rises,  while  it  falls  elsewhere.  Once  these  changes  are 
set  up,  there  is  some  pulsation  of  the  water-level  before  equilibrium 
is  again  established.  These  movements  are  called  seiches.  Seiches 
have  been  much  studied  in  the  Swiss  lakes.  In  very  large  lakes 
tides  may  be  observed,  though  they  are  not  usually  detected  except 
by  instruments  devised  to  record  them.  Slumping  about  the 
shores  of  a -lake,  earthquakes,  etc.,  also  cause  movements  of  its 
waters. 

Changes  of  level.     The  levels  of  lakes  without  surface  outlets 
change  notably  from  time  to  time,  according  to  the  amount  of 


LAKES  AND  SHORES  301 

precipitation  on  their  surroundings.     Many  small  lakes  rise  several 
feet  in  wet  weather,  and  fall  correspondingly  in  drought. 

Conditions  Necessary  for  the  Existence  of  Lakes 

The  conditions  necessary  for  the  existence  of  lakes  are  (1) 
depressions  without  outlets,  and  (2)  a  sufficient  supply  of  water. 

Depressions  without  outlets  must  not  be  understood  to  mean 
that  lakes  have  no  outlets.  It  means  that,  below  the  level  of  the 
lake-outlet,  there  is,  in  each  case,  a  depression  which  has  no  outlet. 
It  is  this  depression  without  an  outlet  which  holds  the  water  which 
makes  the  lake.  When  the  water  reaches  the  top  of  this  depres- 
sion it  overflows. 

A  sufficient  supply  of  water  means  enough  so  that  water  con- 
stantly 1  remains  in  the  depression.  If  the  bottom  of  a  basin  be  of 
porous  material,  such  as  gravel,  more  water  may  be  necessary,  in 
order  that  water  may  stand  in  the  depression,  than  if  the  bottom 
be  of  compact  material  like  clay.  If,  however,  the  ground-water 
surface  in  the  surroundings  of  the  basin  is  above  the  level  of  the 
bottom  of  the  basin  (Fig.  330),  the  water  will  not  escape  from  the 


FIG.  330. — If  the  water  table  about  a  lake  is  above  the  lake  level,  there  will 
be  no  leakage  from  the  lake,  even  if  its  basin  be  of  porous  material. 

basin,  even  if  the  latter  be  of  porous  material.  The  humidity  of  the 
atmosphere  also  affects  the  amount  of  water  necessary  for  a  lake. 
In  moist  regions,  most  considerable  depressions  without  outlets 
contain  lakes,  while  those  of  arid  regions  are  often  lakeless. 

The  sources  of  lake  water.  The  sources  of  lake  water  are 
rain,  melting  snow  and  ice,  springs  and  rivers,  and  immediate 
run-off.  Since  springs  and  rivers  are  dependent  upon  rain  and 
snow,  the  source  of  lake  water  may  be  said  to  be  atmospheric 
precipitation. 

1  There  are  temporary  lakes,  in  the  basins  of  which  water  is  not  always 
present. 


302  PHYSIOGRAPHY 


Changes  now  taking  Place  in  Lakes 

Various  changes  are  now  in  progress  in  all  lakes,  and  a  study 
of  these  changes  throws  light  on  the  past  and  the  future  of  lakes. 

The  filling  of  their  basins.  Lake  basins  are  being  filled  con- 
stantly. In  many  cases,  the  filling  will  in  time  obliterate  the 
basin,  and  the  lake  will  then  disappear.  The  filling  takes  place 
in  various  ways. 

1.  In  the  first  place,  all  streams  and  other  surface  waters  which 
flow  into  lakes  bring  sediment,  and  essentially  all  this  sediment 
is  left  in  their  basins.     This  is  shown  by  the  fact  that  the  streams 
which  flow  from  lakes  are  usually  clear,  even  when  those  which 
enter  bear  much  sediment. 

In  some  lakes  deltas  are  being  built,  and  a  delta  reduces  the 
area  of  the  lake  in  which  it  is  built.  In  rare  cases,  deltas  have  been 
built  across  the  middle  parts  of  narrow  lakes,  separating  the  one 
lake  into  two,  as  at  Interlaken,  Switzerland.  Deltas  have  been 
built  at  the  ends  of  some  of  the  Finger  Lakes  of  New  York,  as 
shown  in  PI.  XXI,  shortening  them  sensibly.  Some  of  the  impor- 
tant cities  about  these  lakes  are  on  deltas.  Ithaca  is  an  example. 
Sheet-floods  and  all  rain-wash  which  enters  a  lake,  even  though 
not  organized  into  streams,  bring  detritus  to  the  basin  and  help 
to  fill  it. 

2.  Lake  basins  are  also  being  filled  by  the  work  of  the  waves. 
The  waves  of  lakes  are  cutting  into  their  shores  at  some  points 
most  of  the  time,  and  most  of  the  material  thus  worn  from  the 
land  is  deposited  in  the  lake  basin.     The  lake  may  extend  its  area 
by  wave-cutting,  but  most  of  the  material  worn  away  from  the 
shore  is  deposited  in  the  lake  basin. 

Rivers  and  waves  are  the  principal  agents  which  are  filling 
lake  basins  and  diminishing  the  volume  of  their  waters,  but  they 
are  not  the  only  ones. 

3.  Numerous   shell-bearing   animals   live   in   lakes.     The   ma- 
terial for  their  shells  is  extracted  from  the  water,  and  the  shells 
are  left  on  the  bottom  when  the  animals  die,  thus  helping  to  fill 
the  basins. 

4.  Plants  grow  in  lakes,  especially  in  their  shallow  borders, 
and.  the  organic  matter  helps  to  fill  the  basins  when  the  plants 
die.      Some  lake  plants  secrete  lime  carbonate  and  others  silica, 


LAKES  AND  SHORES  303 

and  these  materials  help  to  fill  the  basin  the  same  as  the  shells  of 
animals. 

5.  Winds  blow  dust  and  sand  from  the  land  into  lakes,  and 
thus  help  to  fill  their  basins. 

In  all  these  ways  the  lake  basins  are  being    gradually  filled. 

The  lowering  of  their  outlets.  Most  lake  basins  are  being 
affected  in  another  way.  The  water  flowing  out  of  a  lake  cuts  down 
the  level  of  the  outlet,  and,  as  this  is  lowered,  the  depth  of  the 
basin  below  the  outlet  is  diminished.  The  limit  to  which  the  out- 
flowing water  may  cut  the  outlet  of  the  lake  is  base-level. 

Fate  of  lakes.  A  lake  may  be  destroyed  by  the  lowering  of 
its  outlet,  if  its  bottom  is  sufficiently  high;  but  where  the  bottom 
is  below  sea-level,  river  erosion  could  never  cut  the  outlet  down 
to  it.  In  such  cases,  filling  and  cutting  may  accomplish  what 
cutting  alone  could  not.  As  a  result  of  these  processes,  all  existing 
lakes  must  ultimately  become  extinct.  In  their  destruction,  rivers 
are  probably  the  most  important  agents.  Their  relation  to  lakes  is 
such  as  to  have  led  to  the  epigram:  "Rivers  are  the  mortal  enemies 
of  lakes."  This  has  especial  reference  to  lakes  which  are  not  on 
valley  flats. 

Lakes  are  occasionally  destroyed  by  drying  up.  This  some- 
times results  from  a  change  of  climate,  but  it  may  also  result  from 
a  diversion  of  inflowing  waters. 

The  Origin  of  Lake  Basins 

Lake  basins  originate  in  many  different  ways.  Most  of  them 
are  the  result  of  gradational  processes,  but  some  of  them  are  due 
to  vulcanism  and  some  to  diastrophism,  and,  while  these  latter 
topics  have  not  yet  been  studied,  we  may  anticipate  their  con- 
sideration sufficiently  to  note  the  ways  in  which  they  produce 
depressions  without  outlets  in  the  surface  of  the  land. 

Diastrophism.  This  term  includes  all  crustal  movements, 
whether  up  or  down.  Movements  of  the  earth's  crust  give  rise  to 
basins  in  various  ways.  There  are  often  basin-like  depressions 
beneath  the  shallow  water  over  the  continental  shelves.  If  such 
areas  were  converted  into  land,  either  by  their  own  rise  or  by  the 
withdrawal  of  the  sea  from  them,  the  basins  would  appear  on  the 
surface  of  the  new  land.  Newly  emerged  portions  of  the  sea 
bottom  are  therefore  regions  where  lake  basins  sometimes  occur. 


304  PHYSIOGRAPHY 

Some  of  the  lake  basins  of '  Florida- '"and  of  the  plains  of  Siberia 
perhaps  arose  in  this  way.  At  the  outset,  lakes  in  such  basins 
would  be  salt,  but  they  might  become  fresh  (p.  314). 

A  lake  basin  may  arise  by  crustal  warpinj  within  land  areas. 
Thus,  if  a  portion  of  a  flat  area  were  warped  downward,  while  its 
surroundings  were  not,  there  would  be  a  basin,  and.  under  proper 
climatic  conditions,  it  might  become  the  site  of  a  lake. 

Lake  basins  may  originate  by  the  warping  of  a  river  valley, 
where  the  warping  leaves  one  part  of  the  valley  higher  than  a  part 
farther  up-stream.  The  up-warp  constitutes  a  dam,  and  the  waters 
above  are  ponded  (p.  174)  and  a  lake  produced.  This  origin  has 
been  assigned  to  the  basin  of  Lake  Geneva  in  Switzerland.  Lakes 
produced  in  this  way  are  likely  to  be  but  short-lived,  for  in  most 
cases  the  outflowing  water  would  soon  cut  down  the  obstruction. 

A  portion  of  a  valley  may  sink  through  faulting,  thus  giving 
rise  to  a  basin.  Such  an  origin  has  been  ascribed  to  the  basin  of 
the  Dead  Sea  and  to  certain  lakes  in  Oregon  (Figs.  39  and  331). 


FIG.  331. — Section   showing  the  structure  of  the   rock  about  Abert   and 
Warner  Lake,    Oregon.     (U.  S.  Geol.  Surv.) 

According  to  one  interpretation,  Lakes  Stefanie,  Rudolf,  Albert. 
Tanganyika,  Leopold,  and  Nyassa,  in  Africa,  all  lie  in  a  great  rift 
(or  sunken)  villey. 

Lakes  have  probably  originated  at  various  times  in  the  past 
when  mountains  have  been  made  by  the  folding  of  rock  strata. 
Wherever  two  parallel  folds  are  developed  in  the  surface  of  the 
lithosphere,  a  trough  is  formed  between  them.  Such  troughs 
have  probably  sometimes  been  deeper  in  the  middle  than  at  either 
end,  and  lakes  have  probably  resulted.  Lakes  made  in  this  way 
would  be  short-lived,  as  a  rule,  since  they  are  situated  favorably 
for  receiving  abundant  drainage,  and  the  outflowing  water  would 
soon  reduce  the  outlets  so  as  to  drain  them.  It  is  sometimes  dif- 
ficult to  distinguish  between  basins  produced  by  faulting  and 
those  produced  by  warping,  especially  if  the  solid  rock  is  covered 
by  a  heavy  mantle  of  drift  or  soil.  Thus  lake  basins  are  known 
to  have  originated  during  earthquakes,  sometimes  it  may  be  by 
warping,  but  perhaps  more  commonly  by  faulting.  In  1811  and 


LAKES  AND  SHORES 


305 


1812  a  considerable  area  in  the  Lower  Mississippi  Valley  sank  dur- 
ing a  time  of  earthquakes.  One  of  the  areas  most  depressed  be- 
came the  site  of  Reelf  oot  Lake,  which  lies  in  the 
flat  of  the  Mississippi,  partly  in  Tennessee  and 
partly  in  Kentucky. 

Vulcanism.     In  the  tops  of  some   extinct 
volcanic  mountains  there  are  basins  known  as  g 
craters,  some  of  which  are  occupied  by  lakes,   g 
Such   lakes   occur   near  Rome   (Lake   Nemi),  | 
Naples  (Lake  Averno),  and  in  France.     Ponds  a 
or  small  lakes  are  known  in  craters  in  many  | 
other  places,  even  in  such  dry  regions  as  Nevada  Ji 
and  northern  Arizona.     Streams  of  lava  some- 
times obstruct  river  valleys,  giving  rise  to  lakes. 
Snag  Lake,  in  California,  and  Tiberias  Lake,  in 
the    valley   of   the   Jordan,    are   illustrations. 
Others  occur  in  France  and  in  other  regions  of 
recent  volcanoes.     Crater  Lake  in  Oregon  (Figs. 
333   and   334),  a  lake  five   or   six    miles    in 
diameter  and  2000  feet  deep  (p.  297),  occupies 
a  basin  or  caldera  made  by  the  sinking  of  the 
top  of  a  volcanic  mountain.     While  it  lies  in  a 
depression  in  the  top  of  an  extinct  volcano,  its 
basin  is  really  due  to  diastrophism  rather  than 
vulcanism.     This  lake  is  of  such  extraordinary 
interest  that  the  area  about  it  has  been  set  off 
as  a  National  Park.     The  general  conception  of 
its  history  is  represented  by  the  hypothetical 
Figs.  335  and  336,  the  former  representing  the 
volcanic   mountain  as  it  is  supposed  to  have 
been  before  the  top  sank  in,  and  the  latter  the 
present  basin,  free  from  water.     The  island  in 
Figs.    333    and  334  is   a  small  volcanic  cone 
developed  since  the  sinking  of  the  top.    Basins 
.which  become  the   sites  of   lakes  occasionally 
Idevelop  on  the  surfaces  of  lava-flows,  perhaps 
as  the  result  of  the  changes  of  surface  incident 
'  to  cooling. 

Gradation.     Various  agents  of  gradation  produce  lake  basins, 
and  some  of  them  produce  them  in  several  different  wavs. 


306 


PHYSIOGRAPHY 


FIG.  333. — Map  of  Crater  Lake,  Ore.     Contour  interval  200  feet.     Sound- 
ings in  feet.     Lake  surface  6239  feet  above  the  sea-level. 
(U.  S.  Geol.  Surv.) 


FIG.  334. — Western  border  of  Crater  Lake  from  Victor  Rock  to  Llao  Rock. 

(U.  S.  Geol.  Surv.) 


LAKES  AND  SHORES 


307 


FIG.  335. — Mount  Mazama  (the  name  given  to  the  former  mountain  where 
Crater  Lake  now  is),  as  it  is  conceived  to  have  been  before  the  collapse 
which  gave  rise  to  the  lake  basin.  (U.  S.  Geol.  Surv.) 


FIG.  336.— The  r?m  of  Crater  Lake.     (U.  S.  Geol.  Surv.) 


308  PHYSIOGRAPHY 

i.  River  lakes.  Reference  has  already  been  made  (p.  293)  to 
lakes  on  the  flood  plains  of  streams,  formed  by  the  meandering  and 
later  the  cutting  off  of  the  streams;  but  rivers  give  rise  to  lakes 
in  other  ways.  If  a  tributary  brings  to  its  main  more  sediment 
than  the  latter  can  carry  away,  the  excess  is  deposited  as  an  obstruc- 
tion, and  ponds  the  water  above  (Fig.  324).  If  a  main  stream 
aggrades  its  channel,  it  tends  to  obstruct  the  inflow  of  its  tribu- 
taries, giving  rise  to  lakes  along  them.  This  has  been  the  com- 
monly accepted  explanation  of  the  lakes  along  the  Red  River  of 
Louisiana  (Fig.  325);  but  it  now  appears  that  the  obstructions 
to  the  tributaries  are  due  to  organic  accumulations,  rather  than 
to  sediment  in  the  ordinary  sense  of  the  term.1 

It  is  well  known  that  "rafts"  sometimes  form  in  streams.  The 
"rafts"  are  jams  formed  of  timber  which  falls  into  the  river  as  the 
result  of  the  caving  in  of  the  forested  banks,  due  to  lateral  plana- 
tion  of  the  meandering  stream.  The  trees  thus  floated  down  the 
stream  lodge  against  the  banks  at  favoring  points,  and  once  this 
lodgment  is  started,  the  jam  continues  to  grow.  The  branches 
of  the  trees  greatly  aid  in  the  growth  of  the  raft  by  helping  to 
catch  and  hold  the  floating  trees. 

The  Red  River  is  known  to  have  been  the  site  of  a  great  raft. 
It  commenced  to  form  somewhere  below  Alexandria  (Fig.  336a), 
and  its  head  had  reached  the  vicinity  of  Alexandria  by  the  latter 
part  of  the  sixteenth  century.  The  raft  was  really  a  series  of 
more  or  less  disconnected  jams,  each  completely  filling  the  river. 
The  effect  of  the  early  jams  was  to  pond  the  water  of  the  river 
above,  and  to  force  it  out  of  its  old  channel  through  low  places 
in  its  banks.  The  whole  river  was  thus  diverted  to  a  new  course 
below  Alexandria  (Fig.  336&).  Driftwood  accumulated  about  the 
new  outlet,  forming  another  jam,  and  in  this  way  the  raft  gradu- 
ally grew  until  it  extended  itself  up  the  river  about  160  miles. 
Between  1820  and  1872,  its  average  rate  of  growth  was  about 
four-fifths  of  a  mile  per  year,  but  in  two  instances  accumulations 
of  over  five  miles  are  recorded  during  freshets.  As  the  raft  grew 
up-stream,' -it  obstructed  tributaries,  and  developed  lakes  along 
them. 

There  is  no  good  record  of  the  lakes  along  the  lower  part  of 
the  raft-ridden  section.  At  the  time  of  the  early  settlement,  its 

1  Veatch,  A.  C.,  Professional  Pap3r  46,  U.  S.  Gcol.  Surv.,  1906. 


LAKES  AND  SHORES 


509 


lower  end  was  near  Natchitoches,  and  the  location  of  this  city 
was  largely  determined  by  the  fact  that  the  foot  of  the  raft  was 
the  head  of  ordinary  navigation  Record  of  the  lakes  along  the 
upper  portion  of  the  raft  is  much  fuller.  The  group  of  lakes  near 
Shreveport  was  formed  near  the  end  of  the  eighteenth  century. 
Before  1873,  when  the  raft  was  finally  removed,  it  had  advanced 
almost  to  the  Arkansas  line,  forming  Poston  Lake,  the  most  north- 
erly of  the  series  (Fig.  336a). 


FIG.  336a. — The  lakes  of  Red  River  Valley,  La.,  at  their  fullest  recorded 
development.     (Veatch,  U.  S.  Geol.  Surv.) 

Since  the  removal  of  the  raft,  the  river  has  lowered  its  channel 
15  feet  at  a  point  15  miles  above  Shreveport,  and  3  feet  at  Shreve- 
port. As  a  result  of  the  deepening  of  the  channel,  tributaries  have 
lowered  their  valleys  in  the  attempt  to  adjust  themselves  to  their 
•main  stream,  and  the  lakes  are  being  drained.  In  many  places 
land  which  was  formerly  covered  by  lake  water  is  now  under  cul- 
tivation. When  the  topographic  adjustment  of  the  tributaries  has 


310 


PHYSIOGRAPHY 


been  completed,  other  areas  still  submerged  will  also  be  available 
for  cultivation. 

Rivers  are  partly  or  wholly  responsible  for  a  class  of  lakes 
which  may  be  called  delta  lakes.  Lake  Pontchartrain  in  Louisiana 
is  an  example  (Fig.  209).  Here  detritus  brought  down  by  the  river 


FIG.  3366. — Map  showing  the  diversion  of  the  Red  River  below  Alexandria. 
The  shaded  areas  are  subject  to  overflow.     (Veatch,  U.  S.  Geol.    Surv.) 

was  deposited  around  an  area  of  shallow  water,  converting  the  latter 
into  a  basin.  Marshes  and  ponds  and  lakes  are  sometimes  made  by 
the  building  of  alluvial  cones  or  fans  across  a  valley.  Lake  Tulare 
in  California  owes  its  basin  to  an  alluvial  fan  made  by  a  stream 
(King  River)  descending  from  the  Sierras. 


LAKES.  AND  SHORES  311 

2.  Waves  and  shore  currents.    Waves  and  shore  currents  give 
rise  to  lakes  by  shutting  in  the  drowned  ends  of  valleys  or  other 
bays.     Illustrations  are  numerous  along  many  coasts  (PI.  XX  and 
Fig.  337). 

3.  Glacial  lakes.    The  relation  between  the  distribution  of  lakes 
and  the  distribution  of  ice  in  former  times  is  so  close  that  it  cannot 
be  looked  upon  as  accidental,  and  the  study  of  lakes  has  shown 
that  many  of  their  basins  are  due  to  glaciation.    Glaciers  give  rise 


FIG.  337. — Maps  showing  lakes  (ponds)  along  the  shore  of  Lake  Ontario, 
shut  off  from  the  main  lake  by  sand-bars.     (U.  S.  Geol.  Surv.) 

to  lake  basins  in  many  ways,  some  of  which  have  already  been 
mentioned. 

a.  The  mountain  glacier  descending  over  a  steep  slope  often 
digs  out  a  basin  at  the  base  of  that  slope  (Fig.  338).     Hundreds 
of  lake  basins  in  the  valleys  of  the  western  mountains  of  the  United 
States,  and  many  hi  similar  positions  in  other  parts  of  the  world, 
were  formed  in  this  way.     Such  lakes  are  usually  small.     They 
occupy  rock  basins,  and  often  lie  in  cirques  (p.  247,  and  PI.  XVIII). 

b.  Where  glaciers  pass  over  rocks  of  unequal  hardness,  they 
sometimes  erode  the  weaker  more  than  the  stronger,  thus  gouging 
out  hollows.     Lake  basins  formed  in  this  way  are  common  in 


312 


PHYSIOGRAPHY 


mountain  valleys,  and  are  not  unknown  within  'the  Krea  covered 
by  the  ice-sheets. 

Lake  basins  of  the  above  types  are  due  to  glacial  erosion. 

c.  A  glacier  descending  a  mountain  valley  may  obstruct  the 
lower  end  of  a  tributary  "valley,  giving  rise  to  a  lake  above.     Such 
a  basin  has  been  called  an  ice-barrier  basin.     The  Marjelen  See 
in  Switzerland  is  an  example  of  such  a  lake.     Many  former  lakes 
due  to  ice  barriers,  for  example  Lake  Agassiz,  have  become  extinct. 

d.  By  far  the  larger  number  of  lake  basins  due  to  glaciation 
arose  through  the  disposition  of  the  debris  which  the  ice  carried 


FIG.  338. — Shadow  Lake,  in  a  rock  basin  of  glacial  origin,  near  the  head  of 
San  Joaquin  Valley,  Sierra  Nevada  Mountains,  Cal.     (Fairbanks.) 

and  left  on  the  surface  when  it  melted.  Of  such  basins  there  are 
many  varieties:  (1)  The  terminal  moraines  of  mountain  glaciers 
often  cross  and  obstruct  valleys,  giving  rise  to  basins  and  so  to 
lakes  (Fig.  289).  (2)  In  many  other  cases,  drift  is  so  deposited 
with  relation  to  a  rock  slope  as  to  leave  a  depression  between  the 
main  body  of  drift  and  the  rock.  Basins  of  this  sort  are  enclosed 
partly  by  solid  rock  and  partly  by  drift.  Illustrations  are  furnished 
by  many  of  the  lakes  of  the  United  States  and  Europe.  (3)  Drift 
may  fill  a  valley  at  twro  points,  leaving  the  intermediate  portion 
unfilled.  The  intermediate  part  becomes  a  basin  and,  under 
proper  conditions  of  water-supply,  a  lake.  (4)  Other  lake  basins 
owe  their  origin  to  the  unequal  disposition  of  the  drift  itself.  Prob- 
ably the  larger  number  of  lake  basins  in  the  northern  part  of  North 
America  and  northern  Europe  are  merely  depressions  in  the  surface 


LAKES  AND  SHORES  313 

of  the  drift.  Lakes  whose  basins  are  of  this  type  are  not  among 
the  larger  or  the  deeper  lakes. 

In  some  of  the  states  within  the  glaciated  area  of  North 
America,  ponds  and  lakes  are  numbered  by  the  thousand.  The 
area  of  the  lakes  in  Minnesota  alone  has  been  estimated  at  more 
than  5000  square  miles. 

Many  glacial  lakes  owe  their  origin  to  a  combination  of  the 
above  conditions  and  relations.  Here  belong  the  Great  Lakes. 
As  already  indicated,  these  basins  are  probably  due  (1)  partly  to 
glacial  excavation,  which  gouged  them  out  to  considerable  depths; 
(2)  partly  to  the  piling  up  of  the  debris  thus  eroded  about  the 
rims  of  the  basins;  and  perhaps  (3)  partly  to  the  downward  warping 
of  the  surface  beneath  the  water. 

Lakes  due  to  slumping.  Valleys  are  sometimes  obstructed 
by  landslides,  thus  giving  rise  to  basins  which  become  the  sites 
of  lakes.  Such  a  lake,  five  miles  long  and  more  than  seven  hun- 
dred feet  deep,  was  formed  on  the  Upper  Ganges  in  1892.  Two 
years  later,  the  dam  which  held  back  the  water  broke,  and  the 
resulting  flood  wrought  great  destruction  in  the  valley  below. 

Solution,  weathering,  wind,  etc.  Basins  suitable  for  ponds 
and  lakes  sometimes  originate  by  solution  of  the  underlying  rock. 
Limestone  sinks  (p.  97)  may  become  the  sites  of  ponds  and  per- 
haps of  lakes,  but  considerable  basins  of  this  origin  are  not  known. 
Basins  are  sometimes  formed  by  solution  of  the  surface  rock. 
Some  of  the  lake  basins  of  Florida  probably  arose  in  this  wTay. 

The  surface  of  rock  weathers  (p.  110)  unequally.  If  the  weather- 
ing of  one  area  is  greater  than  that  of  its  surroundings,  the  weathered 
material  may  be  blown  away,  leaving  a  depression  suitable  for 
holding  water.  Wind-driven  sand  sometimes  scours  out  small  de- 
pressions which  hold  pools  of  water,  though  basins  of  such  size 
as  to  become  the  sites  of  lakes  are  not  known  to  have  originated  in 
this  way.  Eolian  sand  is  sometimes  piled  up  about  low  places, 
enclosing  them,  thus  giving  rise  to  marshes,  ponds,  and  even 
lakes. 

Glacial  lakes  an  index  of  topographic  age.  Since  rivers  are 
antagonistic  to  lakes,  and  since  rivers  are  always  active,  it  follows 
that  a  region  of  abundant  lakes  is  a  region  in  its  topographic  youth, 
unless  the  lakes  are  in  valley  flats.  Lakes  at  high  altitudes  are  of 
relatively  recent  origin. 


314  PHYSIOGRAPHY 


SALT  LAKES' 

While  most  lakes  are  fresh,  a  few,  such  as  Great  Salt  Lake,  the 
Caspian,  'Aral,  and  Dead  seas,  are  salt.  Salt  lakes  are  found  chiefly 
in  arid  climates. 

Fresh  lakes  may  becrme  salt,  and  salt  lakes  may  become  fresh. 
These  changes  are  usually  the  result  of  changes  of  climate.  If 
aridity  increases  so  that  evaporation  from  a  fresh  lake  exceeds 
the  intake  (precipitation  and  inflow),  a  fresh  lake  may  become 
salt.  If  evaporation  from  a  salt  lake  becomes  less  than  the  intake 
of  fresh  water,  the  lake  will  be  freshened,  and,  under  proper  con- 
ditions, may  become  altogether  fresh.  The  best-known  illustra- 
tion of  these  changes  is  furnished  by  Great  Salt  Lake  and  the 
earlier  lake  which  preceded  it  in  the  same  region. 

1.  The  first  lake  which  occupied  this  basin  appears  to  have 
been  fresh.     A  climatic  change  seems  then  to  have  taken  place, 
a  relatively  humid  climate  giving  place  to  an  arid  one.     When 
this  change  had  been   accomplished,  evaporation  from  the  sur- 
face of  the  lake  exceeded  the  intake  of  fresh  water,  and  the  level 
of  the  lake  was  lowered.     As  the  water  evaporated,  the  mineral 
matter  which  it  held  in  solution  was  left  behind.     Salt  was  one 
of  these  substances,  and  as  more  and  more  water  evaporated,  the 
salinity  of  that  which  remained  increased,  and  the  lake  became 
salt. 

2.  Another  change  of  climate,  this  time  in  the  direction  of  in- 
creased humidity,  ensued.     The  intake  of  fresh  water  then  ex- 
ceeded evaporation  from  the  lake,  and  the  saltness  of  the  water 
was  diminished  by  dilution.     At  the  same  time,  the  level  of  the 
lake  rose  until  it  overflowed,  finding  an  outlet  by  way  of  Snake 
River  to  the  Columbia.     The  continued  inflow  of  fresh  water  and 
the  continued   outflow  of  the  diluted  salt  water  resulted  in  the 
progressive  freshening  of  the  lake,  and  it  finally  became  fresh. 
The  expanded  lake  of  former  times  in  this  basin  is  known  as  Lake 
Bonnevilk   (Fig.  339),  which,  at  its  maximum,  covered  an  area 
of  about  17,000  square  miles.     Its  surface  was  about  1000  feet 
higher  than  that  of  Great  Salt  Lake. 

Another  change  of  climate,  this  time  in  the  direction  of  aridity, 
reduced  Lake  Bonneville.  Its  surface  sank  below  its  outlet,  and, 
as  this  happened,  its  waters  gradually  became  saline.  Evapora- 


LAKES  AND  SHORES 


315 


,tion  in  excess  of  intake  continuing,  the  former  great  lake  was  in 
time  reduced  to  the  relatively  small  Great  Salt  Lake  of  the  present, 
with  an  area  of  about  2000  square  miles  and  an  average  depth  of 
only  about  15  feet.  Its  waters  are  saturated  with  salt,  and  much 
salt  has  been  deposited. 


FIG.  339. — Former  lakes  of  the  Great  Basin.     (U.  S.  Geol.  Surv.) 

Farther  west  a  similar  series  of  changes  is  recorded  by  the  former 
Lake  Lahontan  (Fig.  339),  and  by  Mono  Lake. 

Salt  Lake,  and  the  sites  of  some  extinct  salt  lakes,  yield  salt 
in  commercial  quantities.  Great  Salt  Lake  was  estimated,  a  few 
years  ago,  to  contain  400,000,000  tons  of  common  salt,  besides 
large  quantities  of  other  mineral  matter.  Much  of  the  mineral 
matter  formerly  held  in  solution  by  this  lake  has  already  been 
deposited.  Utah  produced  more  than  400,000  barrels  of  salt  in 
1902,  and  253,829  (value  $321,301)  in  1904.  Deposits  of  salt  made 


316  PHYSIOGRAPHY 

by  lakes  and  inland  seas  which  are  now  extinct,  are  the  chief  sources 
of  sa'.t. 

Accessible  salt  deposits  and  "salt-licks"  determined,  or  helped 
to  determine,  the  location  of  numerous  early  trans- Allegheny  settle- 
ments, as  in  the  Blue  Grass  region  of  Kentucky. 

The  Climatic  Effects  of  Lakes 

The  great  number  of  lakes  in  the  northern  parts  of  the  United 
States  and  Europe  have  some  influence  upon  the  climate  of  the 
regions  in  which  they  occur.  They  increase  its  humidity  to  some 
slight  extent  at  least,  and,  since  water  is  heated  less  readily  than 
the  land  and  gives  up  its  heat  less  readily,  the  lakes  have  the 
effect  of  tempering  the  climate.  Until  they  freeze  over,  they  tend 
to  keep  the  temperature  of  their  surroundings  a  little  higher  than 
it  would  otherwise  be  in  the  autumn  and  early  winter,  and  to  reduce 
the  temperature  of  spring.  The  temperature  effects  of  lakes  are 
felt  chiefly  on  the  sides  toward  which  the  prevailing  winds  blow. 

Economic  Advantages  and  Disadvantages 

The  question  as  to  whether  lakes  are  beneficial  or  harmful  to 
mankind  may  be  looked  at  from  various  points  of  view. 

1.  The  Great  Lakes  serve  as  highways  and  make  cheap  trans- 
portation possible.  In  this  way  they  serve  a  good  purpose,  as  shown 
by  their  extensive  commerce.  2.  Many  cities,  like  Chicago,  draw 
their  water-supply  from  lakes.  A  city  located  as  Chicago  is  could 
not  readily  get  an  adequate  supply  from  any  other  source,  except 
at  far  greater  cost.  3.  Lakes  furnish  a  certain  amount  of  food 
material,  especially  fish.  4.  By  tempering  the  climate,  they  modify, 
to  some  slight  extent  at  least,  agricultural  pursuits.  Thus  the 
prevailing  westerly  winds  temper  the  climate  of  the  east  shore  of 
Lake  Michigan  in  such  a  way  as  to  make  it  favorable  for  fruit- 
growing, while  the  west  side  of  the  lake,  affected  by  winds  not 
tempered  by  the  lake,  is  not  favorable  for  this  industry.  In  these 
and  other  ways  the  lakes  seem  to  serve  mankind. 

On  the  other  hand,  it  is  to  be  remembered  that  such  a  body  of 
water  as  Lake  Michigan  occupies  some  22,450  square  miles  of  sur- 
face, much  of  which  would,  presumably,  have  been  good  farming 
land,  if  the  lake  basin  had  not  been  made.  The  value  of  such  an 


LAKES  AND  SHORES  317 

area  of  good  farming  land  might  offset,  or  more  than  offset,  the 
economic  advantages  of  the  lake. 

Small  lakes  are  of  little  consequence  as  highways,  and  the  modi- 
fications of  climate  which  they  effect  are  slight.  The  same  may 
be  said  of  marshes.  There  can  be  no  doubt  that  if  all  the  area 
occupied  by  lakes  were  cultivated  land  instead,  the  returns  would 
be  much  greater  than  those  which  now  accrue  from  any  uses  to 
which  the  lakes  are  put.  But  lakes  have  a  value  not  to  be  esti- 
mated in  dollars  and  cents.  They  beautify  the  landscape  and 
afford  the  means  for  rest  and  recreation  which  could  not  well  be 
spared.  The  actual  value  of  such  considerations  is  not  easy  of 
definite  estimate. 

The  advantages  which  a  primitive  people  may  derive  from  a 
location  upon  the  shores  or  islands  of  a  lake  are  suggested  by  the 
fact  that  the  earliest  European  civilization  arose  about  the  lakes 
of  Switzerland,  while  the  lakes  of  Mexico  and  Peru  were  the  seat 
of  the  ancient  civilizations  of  those  countries. 

THE  TOPOGRAPHIC  FEATURES  OF  SHORES 

In  connection  with  the  discussion  concerning  changes  now  tak- 
ing place  in  lakes,  reference  was  made  to  certain  topographic  fea- 
tures of  lake  shores,  because  their  development  has  a  bearing  on 
the  history  of  lake  basins,  and  so  on  the  life  of  the  lakes  them- 
selves; but  the  topic  is  of  such  importance  that  it  merits  more  than 
incidental  mention. 

The  topographic  features  developed  by  lakes  along  their  shores 
are  similar  to  those  developed  by  the  sea  along  its  coasts,  except 
that  the  latter  are  on  a  larger  scale.  A  discussion  of  the  develop- 
ment of  the  topographic  features  of  lake  shores  is  therefore  appli- 
cable, in  most  of  its  details,  to  the  shores  of  the  sea. 

Gradation  is  affecting  the  shores  of  oceans  and  lakes  every- 
where; diastrophism  is  affecting  them  in  many  places,  though 
not  universally,  at  least  not  to  such  an  extent  as  to  be  sensible 
from  year  to  year,  while  the  effects  of  vulcanism  on  shores  are 
very  limited  and  of  little  consequence  in  this  connection. 

Gradational  Changes  now  taking  Place  along  Shores 

Waves,  currents,  rivers,  winds,  glaciers,  ice  formed  along  the 
shore,  and  various  other  agencies  are  working  on  the  shores  of 


318  PHYSIOGRAPHY 

seas  and  lakes,  and  each  has  some  effect  on  the  coast-line.  Of  these, 
the  waves  and  the  movements  of  the  water  to  which  the  waves 
give  rise,  are  the  most  important. 

1.  Waves,  Undertow,  Shore  Currents.  The  top  of  a  wave  is  its 
crest,  and  the  depression  between  two  adjacent  crests  is  the 
trough.  The  vertical  distance  between  the  crest  and  the  bottom 
cf  the  trough  is  the  height  of  the  wave,  and  the  horizontal 
distance  between  two  adjacent  crests  is  the  length.  The  time  which 
it  takes  one  crest  or  one  trough  to  travel  the  length  of  the  wave 
is  the  period  of  the  wave.  In  the  open  sea,  storm  waves  often 
have  a  height  of  20  to  30  feet,  and  in  rare  cases  even  50  feet.  On 
the  shore  their  heights  may  be  much  greater,  as  we  shall  see.  The 
length  of  great  waves  may  be  as  much  as  1500  feet,  and  the  velocity 
as  much  as  60  miles  per  hour.  Such  lengths  and  velocities  are, 
however,  far  beyond  the  average. 

In  the  open  sea  wave  motion  does  not  involve  the  forward 
movement  of  the  water.  Each  particle  of  water  describes  a  curve, 
and  theoretically  comes  to  rest  at  the  point  whence  it  started, 
though  the  wave  form  moves  on.  Some  conception  of  the  motion 
involved  may  be  gained  from  a  waving  field  of  grain  or  grass,  where 
each  moving  stem  is  fixed  to  the  ground,  though  wave  after  wave 
crosses  the  field;  or  from  a  long  piece  of  rope  one  end  of  which  is 
fixed,  while  the  other  end  is  shaken  up  and  down.  Successive  waves 
travel  from  the  shaken  end  to  the  other  end. 

Fig.  340  gives  some  idea  of  the  nature  of  the  movement  of  the 
water  in  the  waves  of  the  open  sea. 


FIG.  340. — Diagram  to  illustrate  the  movement  of  water  in  waves.     The 
small  circles  represent  the  movement  of  water  particles. 

If  the  water  in  a  wave  moved  forward  at  the  velocity  at  which 
the  wave  form  travels,  the  sea  would  hardly  be  navigable. 

When  the  wind  is  very  strong,  the  top  of  a  wave  may  be  blown 
forward,  that  is,  the  wave  "breaks,"  and  so  has  a  motion  independ- 
ent of  the  true  wave  motion.  Even  when  the  waves  do  not  break, 
the  surface  water  is  slipped  along  to  some  extent  by  the  moving 
air. 

High  waves  in  the  ocean  are  often  called  "seas,"  and  when  a 
sailor  says  that  there  is  a  "high  sea,"  he  means  that  there  are  high 


LAKES  AND  SHORES  319 

waves.  The  destructiveness  of  waves  in  the  open  sea  depends 
quite  as  much  on  their  length  as  on  their  height.  With  a  given 
height,  the  longer  the  wave,  the  less  its  destruction. 

Waves  generated  by  a  storm  often  run  far  beyond  the  place 
where  they  were  started.  They  diminish  in  height,  but  keep 
their  velocity  and  their  length  if  the  water  is  deep  and  the  waves 
are  unobstructed  by  islands,  etc.  Waves  which  have  outrun  the 
storm  which  started  them  constitute  the  swell  or  the  ground-swell. 
In  the  case  of  great  hurricanes,  destructive  waves  are  sometimes 
felt  a  thousand  miles  from  the  storm.  This  was  the  case  on  the 
coast  of  New  Jersey  in  1889,  when  the  storm  was  to  the  south. 
As  a  result  of  storms  in  different  places,  the  open  sea  is  never 
altogether  quiet. 

As  a  wave  advances  from  the  open  sea  into  shallow  water,  it 
undergoes  notable  changes.  Where  the  water  is  so  shallow  that 
wave  motion  is  sensible  down  to  its  bottom,  the  wave  "drags" 
bottom.  The  velocity  and  the  length  of  the  wave  are  then 
diminished,  and  its  height  increased.  The  top  then  pitches  for- 
ward as  surf. 

In  strong  winds  and  in  shallow  water,  therefore,  there  is  a 
•distinct  forward  movement  of  some  of  the  water  of  a  wave.  Waves 
in  which  there  is  pronounced  forward  movement  are  sometimes 
•called  waves  of  translation. 

The  water  thrown  against  the  shore  in  the  wave  runs  back 
again,  and  this  from-shore  motion  is  the  undertow.  The  undertow 
tends  to  run  down  the  steepest  slope,  but  it  is  often  directed 
obliquely  by  incoming  waves.  Its  movement  is  checked,  too,  by 
every  incoming  crest. 

Where  waves  strike  a  shore  obliquely,  the  water  moves  more 
or  less  along  shore,  and  the  sum  of  these  movements  along  shore 
gives  rise  to  a  shore  or  littoral  current. 

The  waves,  the  undertow,  and  the  shore  currents  all  affect 
the  shore.  The  waves  erode  the  shore-line  in  some  places,  and 
all  of  these  movements  erode  the  bottom  in  some  places.  All  the 
sediment  acquired  by  erosion  is  deposited  sooner  or  later.  Whether 
the  movements  of  water  along  shores  erode  or  deposit,  they  affect 
the  outline  of  the  coast,  and  often  its  vertical  configuration. 

The  amount  of  motion  in  waves  diminishes  rapidly  downward, 
and  is  insensible  below  a  few  hundred  feet.  Submarine  structures, 
such  as  piers,  etc.,  are  rarely  disturbed  below  30  feet. 


s- 


320  PHYSIOGRAPHY 

The  erosive  work  of  waves.  The  force  of  the  wave  as  it  is 
hurled  against  the  shore  is  often  great.  The  surf  is  sometimes 
thrown  up  to  heights  of  more  than  100  feet  with  force  enough  to 
destroy  lighthouses  and  even  cliffs  of  rock.  Windows  of  the  Dunnet 
Head  lighthouse  on  the  coast  of  Scotland  are  said  to  have  been 
broken  at  heights  of  300  feet  above  sea-level  during  severe  gales. 
In  some  cases  the  bursting  of  doors  and  windows  during  such 
storms  appears  to  be  due  to  the  explosive  action  of  the  air  within 
the  buildings,  as  the  surf  dashed  against  them  falls  back.  It  has 
been  estimated  that,  in  exceptional  storms,  the  strength  of  waves 
on  the  exposed  coast  of  Britain  has  been  as  much  as  three  tons  per 
square  foot,  and  that  the  average  force  of  winter  waves  is  about 
one  ton  per  square  foot.  Such  waves  would  move  masses  of  rock 
tons  in  wreight.  It  is  clear,  therefore,  that  the  force  of  waves  is 
adequate  for  powerful  erosion. 

If  a  coast-line  were  regular,  but  composed  of  rock  of  unequal 
hardness,  it  would  not  be  likely  to  remain  regular,  so  far  as  wave 
erosion  is  concerned,  for  the  waves  would 
wear  the  weaker  rock  more  and  the  stronger 
rock  less.  The  result  would  be  the  develop- 
ment of  reentrants  on  the  weaker  rock, 
while  the  stronger  rock  would  remain  as 
projections  of  land  into  the  sea  (Fig.  341). 
Under  these  circumstances  the  irregularities 
of  the  coast  would  go  on  increasing,  so  far 
as  wave  erosion  is  concerned,  until  the 

reentrants   had   become  so  deep  that    the  Fl°;  341.— Diagram    to 

.       ,  illustrate  the  effect  of 

diminished  force  of  the  waves  in  them  would      wave  erosion  on  rocks 

wear  the  weaker   rock    at   those  points  no      °f  unequal  hardness. 

btarting  with  astraight 
faster  than   the   stronger   \vaves   wear  the      line,  indicated  by  the 

harder  rock   of   the   projecting   points   be-      dotted  line,  the  erosion 

of  the  waves  would  de- 
tween.      When   this   stage   is   reached,  the      velop  some  such  out  line 

shape  of   the  coast-line  is  stable,  so  far  as      as  shown :TF, weak  rock, 

and  S,  resistant  rock. 
wave  erosion  is  concerned.     Since  coast-lines 

are  made  of  stronger  and  weaker  rock  structures,  irregularities  of 
this  sort  are  constantly  in  process  of  development.  They  become 
greater  along  seacoasts  than  along  lake  shores,  because  the  waves 
of  seas  are  stronger  than  those  of  lakes. 

Where  a  coast  is  very  irregular,  especially  where  there  are  pro- 
jections of  land  into  the  sea,  the  waves  attack  projecting  points  of 


w 


s- 


w 


Sea 


LAKES  AND  SHORES 


321 


land  more  forcibly  than  they  attack  the  reentrants,  such  as  the 
heads  of  bays.  The  projecting  points  are  thus  worn  back  more 
than  the  heads  of  the  bays.  Where  a  coast  is  very  irregular,  there- 
fore, wave  erosion  tends  to  reduce  its  irregularities,  unless  the  pro- 
jecting points  are  of  rock  which  is  much  more  resistant  than  that 
of  other  parts  of  the  coast. 


FIG.  342. — Diagram  illustrating  high  sea  cliff.       It  shows  also  a  submerged 
terrace,  due  partly  to  wave-cutting  and  partly  to  building. 

We  conclude,  therefore,  that  wave  erosion  tends  to  develop 
small  irregularities  of  coast-line,  but  not  great  ones.  Their  extent 
is  dependent  upon  (1)  the  "fetch"  of  the  waves,  that  is,  the  dis- 
tance they  have  been  traveling,  (2)  the  strength  of  the  winds,  (3) 
the  depth  of  the  water,  (4)  the  exposure  of  the  coast  attacked,  (5) 
the  kind  and  abundance  of  the  tools  (such  as  gravel,  bowlders,  etc.) 
with  which  the  waves  work,  and  (6)  the  resistance  of  the  rock  against 
which  the  waves  beat,  the  resistance  being  determined  partly  by 
hardness  and  partly  by  structure. 


FIG.  343. — Diagram  showing  a  low  sea  cliff. 

Without  tools  to  work  with,  waves  would  be  relatively 
ineffective  against  hard  rock  which  had  no  bedding  or  jointing 
planes.  Thus  on  the  Outer  Hebrides,  barnacles  are  said  to  be  as 
abundant  after  a  storm  as  before,  where  gravel  and  stones  of 
suitable  size  for  the  waves  to  move  are  absent.  Rock  affected  by 
cleavage  planes,  whether  bedding  or  jointing,  may  be  effectively 
worn  by  waves,  irrespective  of  the  debris  which  they  move. 


322 


PHYSIOGRAPHY 


FIG.  344.— A  high  sea  cliff,  La  Jolla,  Cal. 


FIG.  345. — A  high  cliff  with  a  beach,  shore  of  Lake  Michigan. 
(U.  S.  Geol.  Surv.) 


PLATE  XXII 


Tenrv&ssee' 
Cave 


Fi<L  1.  PORTION  OF  THE  CALIFORNIA  COAST  NEAR  TAMALPAIS. 


MAKLN-   Co 
JfRANClScJ*^ 


L.H.  #'  Pt.Bonita 


FIG.  1. — A  coast  line  developed  chiefly  by  wave  erosion.     Scale  1  —  mile  per  inch. 
(Tamalpais,  Cal.,  Sheet,  U.  S.  Geol.  Surv.) 


-     ^^.-'A^^Qr  *>(:      •_3iUdfo.  >/^V  •    s 


Bass   PtX     pig. 2.  MASSACHUSETTS.  ./ 


FIG.  2.— An  island  tied  to  the  mainland  by  a 
"beach."  Scale  1  —  mile  per  inch.  (Boston 
Bay,  Mass ,  Sheet,  U.  S.  Geol.  Surv.) 


LAKES  AND  SHORES 


323 


Irregularities  developed  by  wave  erosion  are  extremely  numerous. 
Here  belong  very  many,  if  not  most,  of  the  small  projections  of 
high  land  into  the  sea.  Their  outlines  are  often  somewhat  angular 
(Fig.  1,  PI.  XXII).  Here,  too,  belong  the  islands  of  some  coasts,  es- 
pecially those  of  solid  rock,  many  of  which  have  been  isolated  from 


FIG.  346. — Steep  cliff  developed  by  waves;  Allen  Point,  Grand  Island,  Lake 
Champlam.     (Perry.) 

the  mainland  by  wave  erosion.     Such  islands  are  likely  to  be 
destroyed  in  time  by  the  same  processes  which  gave  them  being. 

The  cutting  of  the  waves  affects  the  vertical  as  well  as  the 
horizontal  configuration  of  the  shores.  Where  the  sea  is  advanc- 
ing upon  the  land,  steep  slopes,  called  sea  cliffs  (Figs.  342-346), 
are  developed.  Sea  cliffs  may  be  high  or  low,  according  to  the 
elevation  of  the  land  into  which  the  waves  cut.  Cliffs  are  of  fre- 
quent occurrence  along  seacoasts;  and  where  they  are  absent. 


324 


PHYSIOGRAPHY 


the  waves  are  not  cutting,  and  the  sea  is  not  advancing  on  the 
land,  or  at  least  not  as  a  result  of  its  own  cutting.  Slumping  often 
accompanies  wave  erosion. 


tic.  347. — Cross-section  of  a  beach.     (Gilbert.) 

The  sea  cliff  is  often  bordered  by  a  wave-cut  terrace  a  little 
below  the  surface  of  the  water  (Fig.  342).  The  area  of  this  terrace 
often  represents,  in  a  rough  way,  the  area  which  the  sea  has  gained 
from  the  land  by  wave-cutting. 


FIG.  348. — A  lake  beach  (barrier)   Griffins  Bay,  Lake  Ontario. 

Deposition  by  waves,  shore  currents,  etc.  Shore  waters  are 
aggradational  as  well  as  degradational.  The  material  cut  from 
the  land  by  waves,  or  brought  down  by  rivers,  is  shifted  about  by 
the  undertow  and  the  shore  currents,  but  it  must  ultimately  come 


LAKES  AND  SHORES 


325 


to  rest.  While  this  material  is  being  moved  about  by  the  shore 
waters,  it  constitutes  shore  drift,  whether  brought  in  by  rivers  or 
worn  from  the  coast  by  waves.  If  shore  drift  is  left  at  the  shore- 
line, it  makes  a  beach  (Figs.  348  and  349),  which  is  sometimes  de- 
nned as  the  area  of  sand,  gravel,  etc.,  between  high  and  low  tides. 
Deposits  of  gravel  and  sand  continuous  with  those  of  the  beach 
are  often  made  at  greater  depths,  the  material  being  carried  out  by 


FIG.  349. — A  barrier  beach,  shutting  in  a  marshy  tract  behind   it,  Lasells 
Island,  Penobscot  Bay,  A.e.     (Bastin,  U.  S.  Geol.  JSurv.) 

the  undertow  and  by  the  shore  currents  which  diverge  from  the 
coast-line.  It  is  sometimes  carried  out  and  deposited  at  the  outer 
edge  of  the  wave-cut  terrace  (Fig.  342),  and  it  is  sometimes  disposed 
as  a  terrace  along  shore  (Fig.  350)  where  there  is  no  wave-cut 
terrace. 

Waves  often  build  reefs  or  barriers  a  little  out  from  the  shore- 
line.   They  are  developed  near  the  line  of  breakers,  where  the  in- 


FIG.  350.— A  wave-built  terrace.     (Gilbert,  U.  S.  Geol.  Surv.) 

coming  wave  is  no  longer  able  to  carry  forward  the  bulk  of  the 
debris  which  it  is  moving  in  toward  the  shore.  The  undertow 
often  contributes  material  to  the  reef.  There  are  sometimes  sev- 
eral such  reefs  parallel  to  the  coast  and  to  one  another.  Bars, 
reefs,  etc.,  often  hinder  the  movements  of  ocean  vessels,  as  when 
they  close  the  entrances  of  harbors.  A  spit  which  does  not 
obstruct  the  entrance  to  a  harbor,  on  the  other  hand,  is  sometimes 
an  advantage,  since  it  breaks  the  force  of  the  incoming  waves  in 


326 


PHYSIOGRAPHY 


storms,  and  so  helps  to  form  a  harbor.    In  general,  reefs  discourage 
navigation. 

After  the  reef  is  developed,  waves  may  build  its  crest  above 
the  surface  of   the  water,  converting  it  into  land    (Fig.   351). 


FIG.  351. — Section  of  a  barrier.     (Gilbert,  U.  S.  Geol.  Surv.) 

Such  seems  to  have  been  the  origin  of  many  of  the  low,  narrow- 
belts  of  sandy  land  parallel  to  coasts,  with  marshes  and  lagoons- 
behind  them.  This  type  of  irregularity  is  illustrated  by  the  coast 


3ZL 


Cape  ffattei'a.s 


Fro.  352. — Map  showing  the  early  stages  in  the  simplification  of  a  shore- 
line, and  showing  that  at  this  stage  the  irregularities  are  increased. 

of  the  United  States  at  various  points  between  New  York  and 
Texas  (Fig.  352). 

Currents  along  the  shore  (littoral  currents)  shift  sediment  in 


LAKES'" AND  SHORES 


327 


the  direction  of  their  motion;  but  where  such  a  current  reaches  a 
bay,  it  does  not  commonly  follow  the  outline  of  the  bay.  It  tends 
rather  to  cross  its  debouchure  in  the  direction  in  which  it  was 
previously  moving.  Under  such  circumstances  it  tends  to  build 
an  embankment  of  gravel  and  sand  across  the  bay.  Such  em- 
bankments are  spits.  Currents  do  not  build  spits  above  the  water, 
but  waves  may  accomplish  this  result  by  washing  material  from 
their  slopes  up  to  their  tops  (Figs.  351-357).  They  may  thus 
become  land,  after  which  dunes  often  develop  on  them.  When 


FIG.  353.— Map  of  the  head  of  Lake  Superior.     (U.  S.  Geol.  Surv.) 

spits  cress  bays  they  become  bars  (Figs.  353  and  354).  Spits 
and  bars  are  often  hooked  (Fig.  355),  as  the  result  of  the  shift- 
ing of  the  currents  while  they  are  in  process  of  building. 

Spits  and  hooks  often  form  harbors,  and  so  have  determined 
the  location  of  numerous  settlements  and  towns.  A  great  hook 
makes  Provincetown  harbor,  where  the  Pilgrims  first  landed, 
while  the  harbor  whose  shores  they  finally  chose  for  their  settle- 
ment is  formed  by  a  large  spit.  A  hook-formed  harbor  upon  an 
otherwise  regular  coast  determined  the  location  of  Erie,  Pennsyl- 
vania. 

If  the  shore  drift  is  deposited  against  the  mainland,  it  may 
make  a  flat  extending  out  from  the  land  into  the  water.  A  coast- 


328 


PHYSIOGRAPHY 


line  developed  by  deposition  is  in  contrast  with  one  developed 
by  erosion,  for  the  former  has  no  sea  cliff. 

Land  areas  developed  from  reefs  and  spits  often  greatly  in- 
crease the  irregularity  of  the  coast-line  temporarily  (Fig.  352), 


FIG.  354. — Bar  joining  Empire  and  Sleeping  Bear  bluffs,  Lake  Michigan. 
(Gilbert,  U.  S.  Geol.  Surv.) 

but  they  really  represent  an  initial  stage  in  the  development  of 
regularity,  for  after  the  reefs  have  become  land,  the  lagoons  behind 
them  are  likely  to  be  filled  with  sediment,  organic  matter,  etc., 
and  converted  into  land  (Fig.  356).  The  sediment  which  con- 


FIG.  355. — A  recurved  spit,  Dutch  Point,  Grand  Traverse  Bay, 
Lake  Michigan.     (U.  S.  Geol.  Surv.) 

tributes  to  this  end  is  washed  down  from  the  land  or  blown  in. 
When  the  lagoon  is  filled,  the  shore-line  is  much  more  regular  than 
before,  but  the  first  effect  of  the  making  of  the  reef-land  is  to 
make  the  coast  more  irregular. 


LAKES  AND  SHORES 


329 


The  disposition  of  shore  deposition  to  simplify  coast-lines  is 
also  shown  in  another  way.  Deposits  are  sometimes  made  be- 
tween islands  near  the  shore  of  the  mainland,  and  the  mainland 
itself  (PI.  XXII  and  Fig.  357).  Thus  Nahant  island,  on  the  coast 
ot  Massachusetts,  and  the  Rock  of  Gibraltar,  on  the  coast  of  Spain, 


FIG.  356. — Sketch-map  of  a  part  of  the  New  Jersey  coast.  The  dotted 
belt  at  the  east  is  the  barrier  modified  by  the  wind.  The  area  marked 
by  diagonal  lines  is  the  mainland;  the  intervening  tract  is  marsh-land. 
The  numbers  show  the  depth  of  water  in  feet.  Scale;  J  mch  =  l  mile. 


have  been  "tied"  to  the  mainland  by  the  deposits  of  waves  and 
shore  currents.  While  this  tying  process  gives  rise  to  a  notable 
irregularity  of  the  mainland,  it  simplifies  the  outline  of  the  land 
areas  in  the  sense  that  it  unites  islands  to  mainland. 

2.  Rivers.  Rivers  erode  and  deposit  at  or  near  coasts.  The 
erosion  of  streams  has  little  effect  upon  the  coast-line,  for  a  river 
does  not  cut  below  sea-level  more  than  the  depth  of  its  own  water. 


330 


PHYSIOGRAPHY 


Working  alone,  therefore,  rivers   do   not  develop  bays   or  other 
similar  bodies  of  water  projecting  into  the  land. 

The  deposition  of  sediment  brought  down  to  coasts  by  streams 
is  of  more  consequence  in  modifying  the  outline  of  the  land.  This 
is  especially  the  case  where  deltas  are  built  into  lakes  or  seas.  At 
the  lower  end  of  the  Mississippi,  for  example,  a  great  delta  has 
been  built  out  into  the  Gulf  (Fig.  209).  The  great  irregularity 
which  the  delta  itselt  constitutes  has  smaller  irregularities  about 
its  borders.  Deltas  in  lakes  often  show  the  same  general  features 
on  a  smaller  scale.  The  forms  of  deltas  have  been  noted  (p.  200). 
Delta-land  is  always  low,  unless  affected  by  diastrophism,  or  by 
lowering  of  the  surface  of  the  water  in  which  it  was  built. 


FIG.  357. — Sheep  Island,  Penobscot  Bay,  Me.,  a  land-tied  island. 
(Bastin,  U.  S.  Geol.  Surv.) 

3.  Winds.      The  chief  effect  of  wind  along  the  shores  is  to 
blow  about  the  dry  sand.     The  sand  is  often  piled  up  into  con- 
siderable dunes,  as  we  have  seen,  but  the  shifting  of  the  sand  by 
the  wind  does  not  commonly  change  the  outline  of  the  land  area 
to  any  great  extent.     The  wind  often  piles  up  sand  on  low  bars 
and  on  low  coasts,  building  them  up  much  higher  than  they  were 
before,  even  though  it  does  not  change  the  position  of  the  coast- 
line.    Plate  V  shows  a  coast  where  the  land  has  been  built  up 
notably  by  wind-driven  sand.     At  Nag  Head,  N.  C.,  the  land  is 
said  to  have  gained  on  the  sea  350  feet  in  ten  years  as  a  result  of 
wind  deposits. 

4.  Glaciers.      Glaciers  descend  to  the  level  of  the  sea  in  some 
places,  as  in  Greenland  and  Alaska.     Where  this  is  the  case,  they 
usually  move  down  to  the  sea  through  valleys.     If  the  ice  is  thick, 
the  glaciers  gouge  out  the  valleys,  sometimes  to  great  depths 
below  the  level  of  the  sea. 


LAKES  AND  SHORES 


331 


When  glaciers  which  have  gouged  out  such  valleys  melt,  the 
lower  ends  of  the  valleys  are  filled  with  sea-water,  making  narrow 
bays,  or  fiords. 

This  is  the  explanation,  or  a  part  of  the  explanation,  of  many 


FIG.  358.— Alaska  fiords.     (C.  and  G.  Surv.) 

of  the  fiords  of  Norway,  Alaska  (Fig.  358),  Greenland,  and  Chile, 
and  some  other  coasts. 

Glaciers  which  descend  to  the  sea  deposit  their  drift  where  they 
end,  but  the  drift,  being  of  loose  material,  is  usually  soon  washed 
away  by  the  waves,  and  rarely  gives  rise  to  enduring  irregularities 
of  coast-line.  Drift-made  land  in  lakes  would  be  less  readily  swept 
Vway,  because  the  waves  are  weaker. 


332 


PHYSIOGRAPHY 


S  OUT  HERN 
ARGEN  T I NA 


FIG.  359. — Fiords  and  other  irregularities  on  the  west  coast  of  South  America. 


LAKES  AND  SHORES 


333 


5.  Shore  ice  is  another  agency  which  is  working  on  the  coast- 
lines, but  does  not  greatly  modify  their  outlines. 


EXTINCT  LAKES 

Many  former  lakes  have  become  extinct.  Extinct  lakes  are 
recognized  by  various  features.  If  a  lake  basin  became  extinct  by 
having  its  basin  rilled,  the  former  area  of  the  lake  is  marked  by  a 
flat  (Fig.  360)  covered  with  deposits  such  as  are  formed  in  lakes. 


FIG.  360. — A  part  of  the  flat  of  Lake  Agassiz,  Moorhead,  Minn.     (Goode.) 

These  deposits  may  be  of  gravel  or  sand  along  the  shores,  but  the 
materials  deposited  far  from  shore  are  fine.  Such  a  flat  is  a  lacus- 
trine plain.  A  lacustrine  plain  is  a  minor  type  of  plain,  and  may 
He  in  mountains,  on  plateaus,  or  on  plains  of  a  larger  type. 

If  a  lake  became  extinct  by  the  lowering  of  its  outlet  or  by 
evaporation,  the  old  bed  of  the  lake  would  be  less  flat,  might  even 
depart  much  from  flatness. 

The  former  borders  of  an  extinct  lake  are  often  marked  by 
various  shore  features,  such  as  deltas,  terraces,  beaches,  etc.;  while 
above  the  terraces,  in  places  at  least,  old  shore  cliffs  are  often 
found,  especially  if  the  lake  was  large.  Conspicuous  shore  features 
mark  the  former  borders  of  Lake  BonneviHe.  Some  of  them  are 
shown  in  Fig.  361.  The  lower  slope,  marked  by  terraces  developed 
about  the  shores  of  the  lake  in  relatively  recent  times,  is  in  strik- 
ing contrast  with  the  upper  slope,  the  topography  of  which  was 
developed  by  running  water.  The  topography  of  the  terraces  is 
young;  that  of  the  slopes  above,  much  more  advanced.  This 
relation  between  a  slope  of  older  topography  above  and  a  sur- 
face of  younger  topography  below,  has  been  called  a  topographic 
unconformity.  In  this  case,  the  lower  ends  of  the  ravines  and 


334 


PHYSIOGRAPHY 


valleys  were  filled  and  obliterated  by  the  deposits  along  the  shore 
of  the  lake,  the  water  of  which  stood  at  various  levels  at  various 
times. 

Shore  features,  less  conspicuous  than  those  about  Lake  Bonne- 
ville,  but  none  the  less  distinctive,  mark  the  borders  of  the  extinct 


FIG.  361. — Shore  of  former  Lake  Bonneville,  Wellsville,  Utah. 
(U.  S.  Geol.  Surv.) 

Lake  Agassiz  and  many  other  extinct  lakes.  They  also  appear 
about  many-  existing  lakes  well  above  their  present  shores,  thus 
showing  their  previous  higher  levels. 

All  shore  features  developed  by  lakes  are  likely  to  be  destroyed 
in  time  by  the  agents  of  degradation.  The  aridity  of  the  Great 
Basin  has  favored  the  preservation  of  the  shore  features  of  Lake 
Bonneville  and  Lake  Lahontan. 


LAKES  AND  SHORES 


335 


MAP  EXERCISES 

Maps  Showing  Lakes  and  Shores. 

I.  Study  the  following  maps,  group  by  group,  as  indicated  by  the  letters. 
in  preparation  for  conference: 


A.  Browns  Creek,  Neb. 
Great  Bend,  Kan. 

B.  St.  Louis,  Mo.— 111. 
Lancaster,  Wis. — la. — 111. 
Bodreau,  La. 

Bayou  de  Large,  La. 

C.  Ft.  McKinney.  Wyo. 
Greeley,  Colo. 
Granada,  Colo. 

D.  Mt.  Lyell,  Cal. 
Cloud  Peak,  Wyo. 
Chief  Mountain,  Mont. 

E.  Paradox  Lake,  N.  Y. 
Berne,  N.  Y. 
Webster,  Mass. 

F.  Pingree,  N.  D. 
Skanea teles,  N.  Y. 
Penn  Yan,  N.  Y. 
Hammondsport,  N.  Y. 
Cheian,  Wash. 
Methow,  Wash. 
Stehekin,  Wash. 


G.  White  Bear,  Minn. 
Minneapolis,  Minn. 

H.  Falmouth,  Mass. 

Marthas  Vineyard,  Mass. 
Nantucket,  Mass. 
Gay  Head,  Mass. 
Provincetown,  Mass. 

I.  Standingstone,  Tenn. 
Arredondo,  Fla. 

J.  Crater  Lake  Special,  Ore. 

K.  Boston  Bay,  Mass. 

(Also  maps  of  group  H.) 
Asbury  Park,  N.  J. 
Sandy  Hook,  N.  J. 
Atlantic  City,  N.  J. 
Southern  California,  Sheet  2. 
Tamalpais,  Cal. 
San  Mateo,  Cal. 
Erie,  Pa. 
Fairview,  Pa. 
Sodus  Bay,  N.  Y. 
Boothbay,  Me. 
Coast  Survey  Chart  103. 


Note. — Before  taking  up  other  details  in  the  case  of  any  map,  note 
its  position  in  the  country,  its  general  topography,  and  the  causes  which 
have  developed  its  topography.  Where  maps  are  adjacent,  it  is  well 
to  study  them  together;  e.g.,  the  New  York  maps  under  F,  the  Wash- 
ington maps  under  F,  some  of  the  Massachusetts  maps  under  H  and  K, 
and  two  of  the  New  Jersey  maps  under  K. 

II.  1.  Each  lake  shown  on  the  maps  presents  a  series  of  problems, 
among  which  are  the  following: 


336  PHYSIOGRAPHY 

a.  What  is  the  origin  (certain,  probable,  possible)  of  the  basin? 
6.  What  is  the  source  of  the  water-supply? 

c.  Does  the  map  indicate  (certainly,  probably,  possibly)  any 

changes  now  in  progress  about  the  lake? 

d.  Is  the  lake  likely  to  be  destroyed  soon?     If  so,  what  are 

likely  to  be  the  chief  factors  in  its  destruction? 

e.  What  inferences  may  be  made  from  the  map  as  to  the 

depth  of  individual  lakes?     What  measure  of  certainty 
or  uncertainty  attaches  to  the  inference? 
2.*  Classify  the  lakes  of  each  map  under  the  headings  indicated  on  pp. 

303-311. 

3.*  Find  as  many  types  as  possible  of  glacial  lakes  (see  p.  311),  speci- 
fying examples. 

4.  In  connection  with  each  map,  note  whether  there  are  areas  (cer- 
tain, probable,  possible)  which  were  once  lake  bottoms;  i.e., 
have  lakes  become  extinct  in  the  area  represented? 
5.*  Are  the  waters  in  the  ponds  along  the  shore  of  Marthas  Vineyard 

probably  fresh  or  salt?     Give  reasons. 
6.  In  studying  the  ponds  of  the  Standingstone  Sheet,  note  carefully 

the  contour  lines  and  the  drainage  about  them. 

III.  The  maps  of  group  K  are  designed  especially  to  illustrate  shore 
phenomena,  most  of  them  along  the  seashore.  Interpret  in  the 
light  of  pp.  320-329. 

1.  Indicate  what  parts  of  the  coasts  are  being  (or  have  recently 
been)  modified  by  (a)  wave  erosion  and  (6)  shore  deposition. 
Give  reasons  for  your  conclusions. 
2.*  In  general,  how  may  (a)  shore  deposition  and  (6)  wave  erosion 

be  inferred  from  topographic  maps  of  coasts? 
3.  From  any  good  map  or  model  of  the  United  States,  indicate 

where  (a)  erosion,  and  (b)  deposition  prevails. 
4.*  What  are  the  possible  explanations,  so  far  as  the  map  shows, 
of  the  marshes  on  the  bay  coast  of  the  San  Mateo  region? 

5.  Are  coastal  features  similar  to  those  shown  on  the  maps  of 

group  K  shown  on  the  shores  of  lakes  of  preceding  groups? 

6.  Make  a  careful  study  of  Coast  Survey  Chart  103,  noting  all 

the  processes  which  may  have  played  a  part  in  the  develop- 
•ment  of  the  coast-line. 

7.*  Is  there  evidence  on  the  Sodus  Bay  Sheet  of  change  of  rela- 
tive level  of  land  and  lake?    Give  reasons. 

8.  Interpret  the  steep  slope  just  south  of  the  N.  Y.,  C.  &  St.  L.  R.  R. 

9.  What  is  the  probable  meaning  of  the  low  ridge  extending  east 

from  Fairview? 

*  Answer  in  writing. 


LAKES  AND  SHORES  337 


REFERENCES 

1.  RUSSELL,  Lakes  of  North  America:  Ginn  &  Co. 

2.  TAYLOR,  Short  History  of  the  Great  Lakes,  in  Studies  in  Indiana  Geography. 

3.  CHAMBERLIN  AND  SALISBURY,  Geology,  Earth  History,  Vol.  Ill,  pp.  394- 
403.     See  also  index  of  same  volume  and  of  Volume  I. 

4.  GILBERT,   Topographic  Features  of  Lake  Shores,   in   5th   Ann.   Rept. 
U.  S.  Geol.  Surv.,  and  Mono.  I,  U.  S.  Geol.  Surv. 

5.  DILLER,   Crater  Lake,   etc.:  Prof.   Paper  No.   3,   U.   S.   Geol.    Surv.; 
Nat.  Geog.  Mag.,  Vol.  VIII,  pp.  33-48,  and  Am.  Jour.  Sci.,  Vol.  Ill,  1897, 
pp.  165-172. 

6.  RUSSELL,  Geography  of  the  Laurentian  Basin:    Bull.  Am.  Geog.  Soc., 
Vol.  XXX,  pp.  226-254,  and  Jour,  of  Geol.,  Vol.  I,  pp.  394-408. 

7.  TARR,    For  New  York  lakes,  see  Physical  Geography  of  New  York: 
The  Great  Lakes:   Bull.  Am.  Geog.  Soc.,  Vol.  XXXI,  pp.  101-117,   217-235, 
and  315-343;  and  Lake  Cayuga,  a  Rock  Basin:  Bull.  Geol.  Soc.  Am.,  Vol.  V, 
pp.  339-356. 

8.  UPHAM,  Glacial  Lakes  in  Canada:  Bull.  Geol.  Soc.  Am.,  Vol.  II,  pp.  243- 
274. 

9.  HARRINGTON,  Area  and  Drainage  Basin  of  Lake  Superior:   Nat.  Geog. 
Mag.,  Vol.  VIII,  pp.  111-120. 

10.  BRIGHAM,  Lakes:   a  Study  for  Teachers:  Jour,  of  Sen.  Geog.,  Vol.  I, 
pp.  65-72. 

11.  FENNEMAN,  Lakes  of  Southeastern  Wisconsin:   Bull.  8,  Wis.  Geol.  and 
Nat.  Hist.  Surv. 

12.  KEMP,  Physiography  of  Lake  George:    Annals  N.  Y.  Acad.  of  Sci., 
Vol.  XIV,  pp.  141-142,  and  Science,  Vol.  XIV,  p.  774. 

13.  MURDOCH,  Fall  of  Water  Level  in  Great  Salt  Lake:   Nat.  Geog.  Mag., 
Vol.  XIV,  1903,  pp.  75-77. 

14.  MILL,  Bathymetric  Survey  of  the  English  Lakes:   Geog.  Jour.,  Vol.  VI, 
pp.  46-73  and  135-166. 

15.  MURRAY,  Bathymetric  Survey  of  the  Fresh-water  Lochs  of  Scotland:  Scot. 
Geog.  Mag.,  Vol.  XV11,  p.  169;    Vol.  XIX,  pp.  449  and  561;  and  Vol.  XX, 
pp.  1,  169,  235,  449,  589,  and  628.    , 

EXTINCT  LAKES,  SHORE  FEATURES,  ETC. 

16.  RUSSELL,  Present  and  Extinct  Lakes  of  Nevada,  in  Physiography  of 
the  United  States:   Am.  Bk.  Co. 

17.  SHALER,  Sea-coast  Swamps  of  the  Eastern    United  States:   6th  Ann. 
Rept.  U.  S.  Geol.  Surv.;   also  Sea  and  Land:   Scribner's. 

18.  GILBERT,  Lake  Bonneville:  Mono.  I,  U.  S.  Geol.  Surv. 

19.  RUSSELL,  Lake  Lahontan:    Mono.  XI,  U.  S.  Geol.  Surv.;    and  Mono 
Lake:  8th  Ann.  Rept.  U.  S.  Geol.  Surv. 

20.  GEIKIE,  J.,  Earth  Sculpture,  Chapter  XV:   Putnam. 

21.  GEIKIE,  SIR  A.,  Scenery  of  Scotland,  Chapter  III:  Macmillan. 


CHAPTER  VII 


VULCANISM 

A  VOLCANO  is  a  vent  in  the  earth's  crust  out  of  which  hot  rock 
issues.  The  hot  rock  may  be  liquid  (called  lava)  and  may  flow 
out;  or  it  may  be  solid,  when  it  is  thrown  out  violently  in  pieces. 
If  the  vent  is  in  the  form  of  a  long  crack  or  fissure,  it  is  not  com- 
monly called  a  volcano. 

The  rock  material  which  comes  out  of  a  volcano  is  generally 
built  up  into  mounds  or  cones  (Fig.  362).  They  may  be  mere 


FIG.  362. — Fujiyama,  a  volcanic  cone  in  Japan. 

mounds  or  high  hills,  or  even  high  mountains.      The  cones  are 
often  called  volcanoes,  though  they  are  really  the  results  of  volcanic 

338 


VULCANISM 


339 


activity.     The  volcano  from  which  lava  flows  makes  a  cone  with 
low  slopes  (Fig.  363).     The  volcano  from  which  solid  matter  is 


flavna    Loa 


Seata     </  ffilet 


FIG.  363. — Profile  of  the  cone  of  Mauna  Loa.     Vertical  scale  same  as  hori- 
zontal.    (U.  S.  Geol.  Surv.) 

thrown  makes  a  cone  with  steeper  slopes  (Fig.  364).  Many 
volcanoes  send  out  both  liquid  rock  (lava)  and  solid  rock.  In 
this  case  both  may  be  issuing  at  about  the  same  time,  or  lava 
may  flow  out  at  one  time  and  solid  rock  be  thrown  out  at  another. 
Along  with  the  hot  rock,  quantities  of  gases  and  vapors,  some  of 
them  poisonous,  are  discharged.  So  long  as  a  volcano  is  active 


FIG.  364. — Typical  cinder  cone,  Clayton  Valley,  Cal.      (U.  S.  Geol.  Surv.) 

there  is  likely  to  be  a  hollow,  called  the  crater  (Figs.  365  and  366), 
in  the  summit  of  its  cone.  From  the  crater  an  opening  leads 
down  to  the  source  of  the  lava,  at  an  unknown  depth.  Craters 
vary  greatly  in  size.  Some  of  them  are  a  mile  or  more  across,  and 
some  but  a  small  fraction  of  a  mile.  The  sizes  and  shapes  of  the 
openings  leading  down  to  the  sources  of  the  lava  cannot  be  seen 
while  the  volcano  is  active,  but  they  doubtless  vary  much  in  size 
and  shape,  and  perhaps  in  length. 


340 


Volcanoes  exhibit  two  great  types  of  eruption.  These  are 
(1)  the  quiet  type  and  (2)  the  explosive  type.  In  the  former  the 
liquid  lava  rises  up  into  the  crater,  and  either  (a)  flows  over  its 
rim  or  (b)  breaks  through  the  cone  and  flows  down  its  sides.  In 


FIG.  365. — Panum  crater,  Cal.;   Lake  Mono  and  Paona  Island  in  the  dis- 
tance.    (U.  S.  Geol.  Surv.) 

the  latter  the  material  is  blown  out  by  explosions  from  within. 
In  this  case  the  material  may  be  either  liquid  or  solid  when  it  is 
thrown  out,  but  the  liquid  lava  cools  rapidly  in  the  air  and  be- 
comes solid  quickly.  Small  masses  of  liquid  lava  blown  out  of  a 
volcanic  vent  are  often  solid  when  they  fall,  after  even  a  few  sec- 
onds of  flight  through  the  air. 


FIG.  366. — Sketch  of  the  crater  of  the  cinder  cone  near  Lassen  Peak,  Cal., 
showing  the  peculiar  feature  of  two  rings.  The  funnel  is  240  feet  deep. 
(U.  S.  Geol.  Surv.) 

Some  volcanoes  discharge  quietly  at  one  time  and  explosively 
at  another,  and  in  some  there  is  some  measure  of  explosive  violence 
at  all  times,  accompanied  by  some  quiet  discharge  of  lava. 

From  the  follo"\fmg  accounts  of  a  few  active  volcanoes,  many 
of  the  features  of  volcanic  action  will  be  gathered. 


VULCANISM  341 


Examples  of  Active   Volcanoes 

Stromboli.  The  cone  of  this  volcano  is  an  island  4  or  5  miles 
in  diameter,  in  the  Mediterranean  Sea,  north  of  Sicily.  The  cone 
is  built  up  from  tho  bottom  of  the  sea,  and  is  about  a  mile  high, 
though  but  little  more  than  half  of  it  projects  above  the  water. 
About  1000  feet  below  its  top  there  is  an  opening  in  the  side  of 
the  mountain,  from  which  steam  issues  constantly.  At  a  dis- 
tance, the  condensed  water  vapor  looks  like  smoke. 

It  is  sometimes  possible  to  climb  up  to  the  opening  or  crater 
and  look  in.  The  floor  of  the  crater  is  then  seen  to  be  of  black 
rock  composed  of  hardened  lava.  There  are  cracks  in  the  floor, 
and  from  some  of  them  steam  puffs  out  somewhat  as  from  an 
engine.  In  other  cracks  liquid  lava  may  be  seen  to  be  boiling. 
Bubbles  form  in  it  and  burst,  much  as  bubbles  form  and  burst  in  a 
pot  of  boiling  mush.  When  they  burst,  fragments  of  the  lava 
of  which  the  bubbles  are  composed  are  hurled  hundreds  of  feet 
into  the  air,  and  fall  on  the  slopes  of  the  cone,  increasing  its  size. 

At  night  the  glowing  lava  in  the  cracks  of  the  crater  floor  lights 
up  the  clouds  which  hover  over  the  mountain.  For  this  reason 
Stromboli  is  known  as  "the  lighthouse  of  the  Mediterranean." 

The  eruptions  of  Stromboli  are  occasionally  so  violent  that  the 
roar  of  the  escaping  steam  may  be  heard  for  miles,  wrhile  the  ejected 
material  is  hurled  so  high  and  so  far  that  it  is  scattered  not  only 
over  the  entire  mountain,  but  into  the  surrounding  sea.  Stromboli 
is  an  example  of  a  volcano  which  is  at  the  present  time  constantly 
active. 

Stromboli  is  one  of  many  volcanoes  which  have  existed  in  this 
part  of  the  Mediterranean  Sea.  Some  of  the  others,  such  as  Etna, 
are  still  active,  while  others  are  dormant  or  extinct. 

Vesuvius.  Vesuvius  is  probably  the  best-known  volcano. 
Its  cone  is  a  mountain  about  4000  feet  high,  on  the  shore  of  the 
Bay  of  Naples,  about  10  miles  from  the  city  of  the  same  name. 
The  present  cone  of  the  volcano  (Fig.  367)  rises  within  the  half- 
destroyed  rim  of  an  older  and  much  larger  crater. 

Previous  to  79  A.D.  Vesuvius  was,  so  far  as  then  known,  only  a 
conical  mountain  in  whose  summit  was  a  deep  crater  three  miles 
in  diameter.  The  slopes  and  even  the  bottom  of  the  crater  were 
covered  with  vegetation.  In  that  year  a  most  destructive  ex- 


342  PHYSIOGRAPHY 

plosion  occurred,  and  blew  away  half  the  rim  of  the  old  crater. 
Much  of  the  rock  blown  out  was  broken  into  such  small  pieces 
as  to  constitute  dust  (often  called  volcanic  ash),  and  as  it  fell  on 
the  surrounding  country,  it  buried  and  destroyed  not  only  plants, 
but  even  cities.  Pompeii,  a  city  of  some  20,000  inhabitants,  was 
thus  buried,  locally  to  a  depth  of  25  to  30  feet,  and  about  2000  of 
its  people  were  killed.  During  this  eruption  there  were  no  streams 


FIG.  367. — Cinder  cone  forming  the  summit  of  Mt.  Vesuvius. 

of  lava.  Heavy  rains  accompanied  or  followed  the  eruption. 
Falling  on  the  volcanic  dust,  the  rains  gave  rise  to  devastating 
streams  of  hot  mud.  Herculaneum  was  overwhelmed  by  such  a 
stream,  perhaps  60  feet  deep  at  a  maximum.  The  present  cone  of 
Vesuvius  has  been  built  up  inside  the  remnant  of  the  rim  of  the 
older  cone  since  this  eruption. 

Since  the  outburst  of  79  A.D.,  Vesuvius  has  had  other  violent 
eruptions,  separated  by  periods  when  it  was  quiet  or  when  its 
activity  was  mild.  The  eruption  of  1631  was  especially  violent, 
destroying  18,000  lives.  The  emission  of  steam  and  volcanic 
dust  was  followed  by  outflows  of  lava,  some  of  which  reached  the 
sea.  Other  eruptions  of  importance  occurred  in  1737,  1794,  1822, 


VULCANISM  343 

and  1872.  For  several  months  before  the  principal  eruption  of 
1872  there  had  been  mild  eruptions,  during  which  steam  and  fine 
fragments  of  rock  matter  were  ejected  from  the  crater,  and  flows  of 
lava  issued  from  cracks  on  the  mountain-side.  The  activity  grad- 
ually increased  in  violence  until  April,  when  the  eruption  culminated. 
Two  huge  fissures  and  several  smaller  ones  opened  on  the  flanks 
of  the  cone,  and  from  them  great  streams  of  lava  flowed  into  the 
neighboring  valleys,  overwhelming  two  villages.  At  the  same 
time,  two  large  openings  were  made  at  the  summit,  from  which 
enormous  quantities  of  steam,  dust,  and  bomb-like  masses  of  molten 
rock  were  hurled  4000  feet  or  more  into  the  air,  with  a  noise  which 
could  be  heard  for  many  miles.  At  night  the  cloud  overhanging 
the  mountain  was  brightly  illuminated  by  the  glowing  lava  in  the 
crater.  Earthquakes  were  felt  throughout  the  entire  region. 
The  discharges  continued  with  great  violence  for  four  days.  After 
the  eruption  was  over,  two  craters  750  feet  deep,  with  nearly  vertical 
sides,  were  found  at  the  summit.  An  enormous  amount  of  loose 
material  had  accumulated  on  the  sides  of  the  mountain,  and  the 
lava  which  issued  from  the  fissures  lower  down  covered  a  large  area. 

When  Vesuvius  is  but  mildly  active  it  is  possible  to  climb  to 
the  rim  of  its  crater  and  look  in.  It  is  necessary  to  climb  up  on  the 
windward  side,  because  of  the  noxious  vapors  which  are  blown 
to  leeward.  Even  on  the  windward  side  it  is  necessary  to  be 
mindful  of  the  course  which  is  followed,  for  stifling  and  poisonous 
gases  are  pouring  out  of  numerous  little  vents.  Fortunately  the 
poisonous  gases  have  such  a  disagreeable  odor  that  they  are  readily 
detected. 

The  phenomena  which  may  be  seen  and  felt  on  the  mountain 
differ  from  time  to  time,  but  the  conditions  of  a  particular  day 
(in  June,  1887)  may  be  taken  as  fairly  characteristic.  Soon  after 
the  ascent  on  that  day  began,  rumbling  noises  were  heard,  accom- 
panied by  slight  tremors  or  quakings.  As  the  summit  was  ap- 
proached, the  noises  grew  louder,  and  the  shaking  of  the  moun- 
tain more  distinct,  until,  by  the  time  the  top  was  reached,  both 
noises  and  tremblings  were  nearly  continuous. 

From  the  rim  of  the  crater  it  could  be  seen  that  there  were 
three  places  where  the  floor  of  the  crater  was  not  crusted  over. 
In  these  openings,  the  lava  boiled  and  bubbled  like  thick  liquid 
in  huge  caldrons.  About  three  times  a  minute  there  were  ex- 
plosions within  these  openings,  which  shook  the  whole  top  of  the 


344 


PHYSIOGRAPHY 


mountain.  At  the  same  instant,  hundreds  of  fragments  of  the 
glowing  lava  were  shot  up  into  the  air.  After  rising  several 
hundred  feet,  these  fragments  fell;  but  they  were  so  scattered  as 
they  fell,  and  they  came  down  from  such  great  heights,  that  it 
was  not  difficult  to  avoid  them.  They  were  often  glowing-hot  as 
they  started  upward,  but  they  quickly  cooled  enough  to  stop  glow- 
ing, and  when  they  reached  the  surface  of  the  cone  they  were 


FIG.  368. — The  Cauliflower  cloud  above  Vesuvius,  April  7,  1906. 
(Jaggar,  Nat.  Geog.  Mag.) 

dark,  slag-like  pieces  of  rock,  though  not  always  thoroughly  solid. 
Some  of  the  material  ejected  was  in  very  small  fragments,  and 
some  of  it  in  pieces  weighing  scores  and  hundreds  of  pounds. 
Steam  and  many  ill-smelling  vapors  were  also  constantly  issuing 
from  the  crater.  The  water  vapor  which  issued  was  soon  condensed 
into  clouds  as  it  rose  and  cooled,  so  that  clouds  hung  over  the 
mountain. 

From  the  rim  of  the  crater  it  was  clear  that  the  explosions 
which  blew  out  the  lava  were  also  the  cause  of  the  noises  and  the 
quaking.  At  night  the  glowing  lava  of  the  uncrusted  openings  in  the 
bottom  of  the  crater  lighted  up  the  clouds  above,  most  brightly  dur- 
ing explosions,  when  hotter  lava  from  greater  depths  was  exposed. 


VULCANISM  345 

Vesuvius  was  again  disastrously  active  in  the  spring  of  1906, 
when  quantities  of  dust  and  flows  of  lava  were  sent  forth,  causing 
much  destruction  of  property  and  some  loss  of  life. 

Professor  Jaggar  has  described  the  conditions  late  in  April  as 
follows:  "The  lava-fields  of  1872  and  1898  were  found  buried  under 
5  or  6  inches  of  sand  and  dust,  which  formed  a  heavy  mantle,  but 
not  sufficient  to  wholly  disguise  the  slaggy  contortions  beneath. 
The  whole  cone  of  Vesuvius  became  cleared  of  clouds  in  the  course 
of  the  afternoon,  and  it  was  seen  to  be  covered  with  straight  sand- 


v  ; 


FIG.  369. — The  new  cone  of  Vesuvius,  shrouded  in  snow-white  ashes. 
(Jaggar,  Nat.  Geog.  Mag.) 

slides  of  whitish-gray  color,  which  occasionally  slipped  downward 
as  on  the  steeper  slopes  of  a  dune.  Pure  white  steam  boiled  up 
slowly  from  the  crater.  In  one  instance  it  burst  out  radially  over 
the  edge  of  the  crater,  showing  a  ring  on  the  border,  a  dome  of 
cumulus  above  and  within,  and  a  second  still  higher  outer  ring 
made  of  an  older  rain-cloud  which  had  been  punctured  and  pushed 
up  bodily.  The  effect  was  like  a  hat  on  the  mountain's  crown. 
At  night  the  cone  was  clear  and  entirely  without  luminosity." 

As  seen  from  the  top,  the  crater  wras  so  full  of  steam,  etc.,  that 
little  could  be  seen;  but  occasionally  "we  could  make  out  an  in- 
ward slope  of  35  or  more  degrees,  covered  with  hot  sand  and  broken 
rock  fragments,  terminated  about  120  feet  (vertically)  below  by 
jutting  ledges  which  appeared  to  be  precipitous.  Beyond  was 


346 


PHYSIOGRAPHY 


steam  and  sulphurous  heat  and  obscurity.  The  ledges  fumed  in 
places.  No  noise  could  be  heard  above  the  howling  of  the  wind. 
The  curvature  of  the  crater  edge  was  irregular  with  embayments, 
and  it  showed  much  irregularity  in  height.  We  could  not  see 
the  opposite  side  of  the  caldron,  but  from  the  curvature  it  was 
estimated  that  the  crater  could  not  be  less  than  from  one-fourth  to 
one-half  mile  in  diameter — unusually  large  for  Vesuvius." 

The  history  of  the  recent  eruption  is  summed  up  by  the  same 
author   as   follows:   "In   May,  1905,  lava   flowed   from  a  split 


FIG.  370.— Vesuvius  in  1906.     (Hobbs.) 

in  the  northwest  side  of  the  cone  and  continued  in  active  motion 
throughout  the  year.  It  ceased  flowing  at  the  time  when  the  pres- 
ent eruption  opened  a  new  vent  on  the  south  side  of  the  cone. 
On  April  4,  1906,  a  splendid  black  '  cauliflower '  cloud  rose  from  the 
crater.  On  April  4,  5,  6,  and  7  lava  mouths  opened  along  the  south- 
ern rift  above  mentioned,  first  500  feet  below  the  summit,  then  1300 
feet  lower,  and  finally  600  feet  lowrer  still,  all  in  the  same  radial 
line.  The  lowest  mouth  was  more  than  half-way  down  the  moun- 
tain, and  from  this  orifice  came  the  destructive  streams.  It  should 
be  borne  in  mind  that  these  flows  are  not  floods  of  lava  which 
cover  the  whole  slope  of  the  mountain,  but  relatively  narrow, 
snake-like  trickles,  none  the  less  deadly  when  they  push  their  way 
through  a  closely  built  town.  The  molten  rock  crusted  over  and 
cracked,  making  a  tumble  of  porous  bowlders  at  its  front. 


VULCANISM 


347 


"At  8  P.M.,  April  7,  a  column  of  dust-laden  steam  shot  up  four 
miles  from  the  crater  vertically.  The  cloud  snapped  with  inces- 
sant lightnings.  New  lava  mouths  opened,  and  the  flows  moved 
forward,  crushing  and  burning  and  swallowing  parts  of  Boscotrecase, 
the  stream  forking  so  as  to  spare  some  portions  of  the  town.  Mean- 
time torrents  of  ashes  fell  on  Ottajano,  on  the  opposite  side  of  the 
volcano,  and  many  roofs  collapsed  and  lives  were  lost.  At  the 
observatory  Dr.  Matteuci  and  his  colleagues  were  obliged  to  re- 


FIG.  371. — The  ruins  of  Ottajano.     The  roofs  have  fallen  in  under  the  load 

of  ashes. 

treat,  as  the  observatory  was  rocking  violently  and  heavy  stones 
were  falling.  .  .  . 

"Boscotrecase  was  ruined  wholly  by  lava;  Ottajano  by  falling 
gravel.  Boscotrecase  is  traversed  in  two  places  by  the  clinkery 
lava  stream,  and  in  some  cases  houses  were  literally  cut  in  two. 
The  stream  of  lava  had  forked  about  a  spur  of  the  mountain,  leav- 
ing the  higher  land  with  its  vineyards  untouched.  The  lower  land 
with  its  town  was  invaded.  There  is  so  little  timber  in  the  Italian 
masonry  construction  that  the  uninvaded  part  of  the  town  was  not 
burned  at  all.  At  Ottajano  the  roofs  fell  in  under  the  weight  of 
sand  and  gravel.  The  roofs  were  largely  flat  or  slightly  sloping  tiled 


348 


PHYSIOGRAPHY 


affairs.  The  ash  and  lapilli  reached  a  depth  of  three  feet  on  level 
surfaces.  The  roofs  carried  the  walls  with  them  in  many  cases, 
but  there  was  no  significant  "earthquake.  There  was  no  fire,  de- 
structive lightning,  nor  strong  wind.  The  persons  who  perished 
were  all  found  in  the  houses,  where  the  sole  cause  of  death  was  en- 
tombment in  the  ruins." 

Like  Stromboli,  Vesuvius  is  situated  in  a  region  where  there 
have  been  other  volcanoes,  some  of  which  have  been  active  within 
historic  times. 


B 

FIG.  372. — Krakatoa  after  the  eruption.    A,  as  seen  from  the  southwest, 
and  B,  from  the  north.     (Kept,  of  the  Roy.  Soc.) 

Krakatoa.  One  of  the  most  violent  and  destructive  volcanic 
explosions  of  which  there  is  historical  record  was  that  of  1883, 
in  Krakatoa,  a  volcanic  island  in  the  Strait  of  Sunda,  between 
Sumatra  and  Java. 

Previous  to  the  great  eruption,  the  island  had  been  shaken  by 
earthquakes  and  minor  explosions  for  some  years.  On  the  morn- 
ing of  the  27th  of  August  there  was  a  series  of  terrible  explosions, 
the  sound  of  which  was  heard  in  southern  Australia,  2200  miles 
away.  About  two-thirds  of  the  island  was  blown  away  (Fig.  372), 
and  the  sea  is  now  1000  feet  deep  where  the  centre  of  the  mountain 
formerly  stood.  Enormous  sea-waves  were  formed,  which  traveled 
half-way  around  the  earth.  On  the  shores  of  the  neighboring 
islands  the  water  rose  50  feet,  causing  great  destruction.  More 
than  36,000  persons  perished,  mostly  by  drowning,  and  295  villages 
were  wholly  or  partially  destroyed.  The  sky  over  the  island  and 


VULCANISM 


349 


the  bordering  coasts  became  black  as  night  from  the  clouds  of  dust. 
It  was  estimated  that  steam  and  dust  were  shot  up  into  the  air  17 
to  23  miles.  The  explosion  produced  great  air-waves  which  traveled 
three  and  more  times  around  the  earth.  Its  passage  was  recorded 
by  barometers  in  all  parts  of  the  world.  The  dust  ejected  during 
this  explosion  has  been  noted  already  (p.  57). 

Over  a  circle  10  to  12  miles  from  the  centre  of  Krakatoa,  the 
sea  bottom  outside  the  crater  was  built  up  10  to  12  feet.     Along 

a  line  to  the  west,  the  depth  of  the  water  was  increased. 

• 

/          \ 


FIG.  373. — A.  Probable  outline  of  the  great  crater  ring  of  the  Krakatoa 
volcano  after  the  ancient  paroxysmal  outbursts.  The  dotted  line  indi- 
cates the  mass  which  was  blown  away. 

B.  Probable  outline  of  the  Krakatoa  volcano  after  the  great  crater  indi- 
cated by  the  dotted  line  had  been  filled  up  by  growth  of  numerous  small 
cones  within. 

C.  Form  of  Krakatoa  in  historical  time,  after  the  formation  of  the  great 
lateral  cone  of  Rakata  and  the  growth  of  other  cones  within  the  great  crater. 

D.  Outline  of  the  crater  of  Krakatoa  as  it  is  now.     The  dotted  lines 
indicate  the  parts  blown  away  by  the  outburst  of  1883  and  the  change  in 
form  of  the  flanks  by  the  fall  of  ejected  matter.      (Kept,  of  the  Roy.  Soc.) 

The  cause  of  this  awful  explosion  was  probably  the  same  as 
that  of  the  milder  eruptions  of  Stromboli,  that  is,  the  sudden  es- 
cape or  explosion  of  superheated  steam. 

Something  of  the  conjectured  history  of  this  volcano  is  shown 
by  Fig.  373,  which  is  suggestive  of  the  changes  undergone  by 
volcanic  cones.  The  explanation  beneath  the  figure  gives  its 
interpretation. 


350 


PHYSIOGRAPHY 


There  are  many  other  volcanoes,  living  and  dead,  in  the  vicinity 
of  Krakatoa. 

Mont  Pelee  and  Soufriere.  The  volcano  of  Mont  Pelee  is 
situated  on  the  island  of  Martinique  (Fig.  376),  one  of  the  Lesser 


Forsaken  ( 


Lang  I. 


FlO.  374. — Krakatoa  Island  and  surroundings  before  the  eruption  of  1883. 
The  numbers  indicate  the  depth  of  the  water  in  fathoms. 

Antilles,  at  the  eastern  border  of  the  Caribbean  Sea.  Its  cone 
descends  by  steep  slopes  to  the  sea  on  all  sides  but  the  south,  where 
it  is  bordered  by  a  plain  on  which,  prior  to  the  eruption  of  1902, 


Forsak 


1  |  Lang  I. 


50 


FIG.  375. — Krakatoa  Island  and  surroundings  after  the  eruption  of  1883. 
The  numbers  indicate  the  depths  of  the  water  in  fathoms. 

stood  the  city  of  St.  Pierre,  with  a  population  of  about  26,000. 
The  crater  of  Pelee  was  half  a  mile  in  diameter,  and  its  floor  2000 
feet  below  the  highest  part  of  the  crater  rim.  This  rim  was  inter- 
rupted at  the  southwest  by  a  deep  gash,  through  which  a  stream 


VULCANISM 


351 


FIQ.  376. — Sketch-map  of  Martinique.     (Nat.  Geog.  Mag.) 


Pte.de  Macouba 


Cap  St.ttartin 


LEGEND : 

•  Cnter 

•  Hud  Cnter 
'•'   FumarolM 

Annihilation  Lint 

Singe  Lln« 

Ash  Line 


FIG.  377. — Map  of  that  part  of  Martinique  devastated  by  the  volcanic  out 
burst  of  1902.     (Hill.  Nat.  Geog.  Mag.) 


352 


PHYSIOGRAPHY 


flowed.      In  the  crater  there  was  formerly  a  lake,  but  it  is  said  to 
have  been  dry  for  about  half  a  century. 

Previous  to  the  eruption  of  1902,  Pelee  had  had  two  periods  of 
moderate  activity  within  historic  times,  namely,  in  1762  and  in 
1851.  Neither  was  destructive  to  life.  From  1851  to  1902  the 
volcano  slumbered.  In  the  later  part  of  April  of  the  latter  year 
activity  was  renewed  by  (1)  the  discharge  of  steam,  vapors,  and 
ashes,  some  of  which  were  thrown  1300  feet  above  the  top  of  the 
mountain,  and  (2)  by  the  opening  of  three  vents  in  the  basin  of 
the  old  crater.  By  April  25  sulphurous  vapors  had  become  so 


FIG.  378.— Mt.  Pelee.     (Am.  Mus.  Nat.  Hist.) 

abundant  that  horses  dropped  dead  in  the  streets  of  St.  Pierre,  and 
a  little  later  the  traffic  of  the  streets  was  obstructed  by  the  volcanic 
dust  or  "ashes."  On  May  5  the  mud  which  had  accumulated  in 
the  basin  of  the  crater  broke  out  and  flowed  down  the  valley, 
overwhelming  a  factory  and  destroying  a  number  of  lives.  Dur- 
ing these  early  stages  of  activity  there  were  numerous  earthquakes, 
and  all  cables  from  Martinique  were  broken.  Detonations  like 
the  report  of  artillery  were  heard  even  300  miles  away. 

On  May  8  the  activity  of  the  volcano  reached  its  climax.  On 
that  day  a  heavy  black  cloud  swept  down  through  the  gash  in  the 
crater  rim  over  the  plain  to  the  southwest,  and  two  minutes  later 
struck  the  city  of  St,  Pierre,  five  miles  distant.  The  city  was  at 
once  demolished.  Buildings  were  thrown  down,  statues  hurled 
from  their  pedestals,  and  trees  torn  up.  Explosions  were  heard  in 


VULCANISM 


353 


FIG.  379. — Successive  stages  of  the  dust-cloud  of  the  eruption  of  Mt.  Pele"e, 
December  16,  1902.     (La  Croix.) 


354 


PHYSIOGRAPHY 


St.  Pierre  as  the  cloud  reached  it,  and  the  city  burst  into  flames, 
fired  either  by  the  heat  of  the  gases  or  the  red-hot  particles  of 
rock  which  the  gases  carried.  A  few  moments  later  a  deluge  of 
rain,  mud,  and  stones  fell,  continuing  the  destruction.  With  very 
few  exceptions,  the  entire  population,  increased  to  some  30,000  by 
refugees  from  the  surrounding  country,  was  wiped  out  of  existence. 
Study  of  the  region  after  the  eruption  showed  that  the  cloud 
was  probably  composed  of  steam,  sulphurous  vapors,  and  dust. 


FIG.  380. — Outside  of  southern  rim  of  crater  of  Pelee.     The  serrate  edge 
is  due  to  landslides.     (Hovey,  Am.  Mus.  Nat.  Hist.) 

It  is  estimated  to  have  had  a  temperature  of  1400°  to  1500°  F. 
(800°  C.).  The  gases  were  heavier  than  air,  and  so  swept  along  the 
ground  instead  of  rising.  They  may  also  have  been  kept  down 
by  the  clouds  of  steam  and  ashes  thrown  out  just  before  the  out- 
burst of  the  destructive  gases.  Combustible  gases  seem  not  to 
have  been  abundant,  for  the  vegetation  and  thatched  roofs  in  the 
path  of  the  blast  were  not  burned,  but  only  dried  and  withered. 
The  bodies  of  the  victims  were  scorched,  burned,  or  scalded.  Ex- 
cept in  the  axis  of  the  blast,  the  clothing  of  the  bodies  was  un- 
burned,  though  the  flesh  beneath  was  burned  and  scalded.  The 
chief  causes  of  death  seem  to  have  been  suffocation  by  the  noxious 
vapors  and  gases  and  the  great  heat.  Minor  causes  were  blows 
from  stones  thrown  from  the  volcano,  burns  from  hot  stones,  dust, 
and  steam,  cremation  in  burning  buildings,  etc. 


VULCANISM 


355 


FIG.  381. — Great  rocks  thrown  out  by  the  eruption  of  August  30,  1902. 
(Hovey,  Am.  Mus.  Nat.  Hist.) 


FIG.  382. — St.  Pierre  after  the  eruption  of  Mt.  Pelee,  which  is  seen  in  the 
distance.     (Hovey,  Am.  Mus.  Nat.  Hist.) 


356 


PHYSIOGRAPHY 


Other  eruptions  occurred  on  May  20,  26,  June  6,  July  9,  and 
August  30.  The  first  of  these  was  similar  in  character  and  violence 
to  that  of  May  8,  and  destroyed  such  portions  of  the  town  as  had 
been  spared  by  the  first  eruption.  The  blast  of  August  30  took  a 
path  somewhat  different  from  that  of  the  earlier  ones,  and  dev- 
astated a  number  of  villages  in  the  vicinity  of  St.  Pierre,  adding 
about  2000  to  the  list  of  human  victims.  Clouds  of  steam  and 
ashes  were  thrown  to  heights  of  6  and  7  miles. 


FIG.  383. — Spine  of  Mt.  Pelee.     The  spine  rose  about  1210  feet  above  the 
crater  rim.     (Hovey,  Am.  Mus.  Nat.  Hist.) 

The  great  crater  of  Mont  Pelee  is  now  occupied  by  a  cone  of 
fragmental  material  and  some  lava.  This  cone  now  overtops  the 
crater  rim,  and  terminates  in  a  spire  which  rises  hundreds  of  feet 
above  the  shallow  crater  which  occupies  the  apex  of  the  cone,  and 
out  of  which  it  was  thrust.  Unlike  the  cone,  the  spire  consists 
of  solid  rock.  It  is  believed  to  be  the  lava  which  filled  the  vent, 
and  which  was  pushed  up  by  the  expansive  forces  beneath.  The 
spire  is  reported  to  be  rapidly  crumbling. 

An  interesting  case  of  sympathetic  action  was  shown  by  a  vol- 
cano (Soufriere)  on  the  island  of  St.  Vincent  (Fig.  385),  about  90 


VULCAXISM 


357 


miles  south  of  Martinique.     After  two  days  of  premonitory  symp- 
toms the  first  eruption  of  the  Soufriere  occurred  on  May  7.     The 


Ekng  S«o.          / 
700  m  / 


FIG.  384. — Cross-section  through  the  northern  part  of  Mt.  Pelee,  showing 
the  growth  of  the  spine.     (Hovey,  Am.  Mus.  Nat.  Hist.) 


FIG.  385. — Sketch-map  of  the  Island  of  St.  Vincent,  showing  the  zones  of 
devastation.  On  the  black  area  the  devastation  of  life  was  nearly 
complete;  in  the  checked  area,  slight.  (Russell,  Nat.  Geog.  Mag.) 

eruption  was  similar  to  that  of  Mont  Pelee,  but  as  there  was  no  con- 
siderable city  in  the  path  of  the  steam-cloud,  the  loss  of  life  was 


358 


PHYSIOGRAPHY 


much  smaller,  about  1350.    The  discharges  from  the  vent  were  not 
confined  and  directed  by  a  valley  so  definitely  as  those  of  Mont 


FIG.  386. — The  Soufriere,  St.  Vincent.     (Hovey,  Am.  Mus.  Nat.  Hist.) 

Pelee;   hence  they  spread  over  a  larger  area,  with  less  violence. 
A  later  eruption,  on  May  18,  preceded,  by  a  short  period,  an 


FIG.  387. — Ash-filled  gorge  of  the  Rabaka,  St.  Vincent. 
(Hovey,  Am.  Mus.  Nat.  Hist.) 

outburst  of  Mont  Pelee,  and  another,  on  September  3,  followed  a 
great  eruption  of  the  sister  volcano. 

From  both  centres  of  activity  the  dust  driven  out  was  carried 
long  distances.     On  St.  Vincent  it  formed  beds  50  and  60  feet  thick 


VULCANISM 


359 


in  some  places.    There  were  no  lava-flows  in  connection  with  any 
of  these  eruptions. 


FIG.  388. — An  eruption  of  steam  from  the  ashes  of  the  Walliban  Valley 
(Hovey,  Am.  Mus.  Nat.  Hist.) 


FIG.  389. — Ridge  of  Bunker  Hill  on  the  Richmond  estate,  St.  Vincent. 
Shows  the  devastation  of  trees  and  the  accumulation  of  dust  on  the 
crest  of  the  ridge.  (Hovey,  Am.  Mus.  Nat.  Hist.) 

Earthquake  tremors  felt  in  China  on  May  8  are  supposed  to  have 
been  connected  with  the  violent  eruption  of  that  date.    This  is 


360 


PHYSIOGRAPHY 


FIG.  390. — The  Soufriere   in  eruption.     Ruins  of  Walliban   sugar-factory 
in  the  foreground.     (Photograph  by  Wilson.) 


Fia.  391. — A  river  of  mud  pouring  from  La  Soufriere;  the  steam  is  rising 
from  hundreds  of  points  in  the  hot  stream.     (Russell.) 


VULCANISM 


361 


the  only  case,  with  the  exception  of  Krakatoa,  in  which  tremors 
are  known  to  have  been  transmitted  through  the  centre  of  the 
earth  to  the  opposite  side.  Earthquake  shocks  were  felt  in  Vene- 
zuela on  August  30. 


FIG.  392.— Map  of  Hawaii.     (U.  S.  Geol.  Surv.) 

Hawaiian  volcanoes.  The  eruptions  of  the  volcanoes  thus  far 
described  are  more  or  less  violent;  but  in  the  Hawaiian  Islands 
there  are  volcanoes  whose  eruptions  are  relatively  quiet.  Mauna 
Loa  is  the  largest  of  the  four  volcanic  cones  whose  united  mass 
forms  the  island  of  Hawaii,  which  is  80  miles  across.  Mauna 
Loa  rises  14,000  feet  above  the  sea.  So  far  as  known,  almost  the 


362 


PHYSIOGRAPHY 


whole  island  is  made  up  of  volcanic  materials.  The  highest  point 
of  the  island  is  about  14,000  feet  above  the  sea,  but  the  island  has 
been  built  up  from  the  sea  bottom  by  the  lava  poured  out  from  the 
craters,  and  since  the  water  about  the  island  is  about  16,000  feet 
deep,  the  volcanic  pile,  the  top  of  which  is  the  island,  is  really 
about  30,000  feet  high.  This  is  about  the  height  of  the  highest 
mountain  above  sea-level. 

The  crater  of  Mauna  Loa  (Fig.  392)  is  3  miles  long,  2  miles  wide, 
and  about  1000  feet  deep — a  very  large  crater.  When  the  volcano 
is  not  active,  it  is  possible  to  descend  into  the  crater  and  to  walk 


FIG.  393. — View  of  crater  of  Kilauea.     (U.  S.  Geol.  Surv.) 

about  on  its  hard  but  hot  floor.  Cracks  and  other  openings  are, 
however,  generally  present,  and  give  evidence  of  the  hot  liquid 
rock  beneath. 

Before  an  eruption  the  floor  of  the  crater  rises,  and  lakes  of 
lava  appear  in  the  enlarged  openings  in  it.  At  intervals,  foun- 
tains of  lava  may  rise  from  the  lakes,  sometimes  to  heights  of  sev- 
eral hundred  feet.  Finally  the  eruption  occurs;  but  the  lava  does 
not  usually  flow  over  the  rim  of  the  crater.  It  generally  comes 
out  through  fissures  which  open  on  the  side  of  the  mountain,  some- 
times far  from  the  top.  Through  them  the  liquid  lava  spouts, 
sometimes  for  hundreds  of  feet,  into  the  air,  and  then  flows  down 
the  sides  of  the  mountain  in  streams.  Such  streams  are  some- 
times half  a  mile  in  width,  and  flow  for  50  miles.  The  lava  streams 
are  somewhat  like  mountain  glaciers  in  form.  Their  rate  of  ad- 
vance is,  however,  much  faster  than  that  of  glaciers,  though  much 
slower  than  that  of  rivers.  The  lava  flows  faster  at  first,  and  more 
slowly  as  it  becomes  cooler.  Residents  in  the  cities  below  go  out  at 
intervals  when  the  volcanoes  are  discharging,  to  see  how  the  streams 


VULCANISM 


363 


of  lava  are  coming  on,  and  whether  they  are  likely  to  descend 
so  far  as  to  endanger  life  and  property  in  the  settled  regions  below. 
As  the  lava  streams  reach  natter  ground,  they  spread  out,  and 
the  lava  may  collect  in  hollows,  forming  pools  and  lakes  which 
soon  harden.  The  lava  occasionally  falls  over  cliffs  (Fig.  395), 
sometimes  into  the  sea. 


FIG.  394.— The  crater  of  Kilauea.     (U.  S.  Geol.  Surv.) 

After  it  becomes  hard  the  surface  of  a  lava-flow  may  be  nearly 
smooth  (Fig.  396),  but  it  is  often  rough.  It  may  be  ropy  (Fig.  397) 
or  clinkery  (Fig.  398).  The  ropiness  is  due  to  movement  of  the 
surface  lava  after  it  is  partially  hardened.  The  clinkery  surface 
is  due  to  the  breaking  up  of  the  hardened  crust  of  the  lava  stream. 

As  the  lava  flows  out,  the  lava  lake  in  the  crater  at  the  summit 
subsides,  and  great  masses  of  the  floor  of  the  crater,  formerly  held 
up  by  the  lava  below,  sink. 

During  the  eruptions  of  the  Hawaiian  volcanoes  little  steam 


364 


PHYSIOGRAPHY 


FIG.  395. — Lava  falling  over  cliffs,  Kilauea.      (H.  M.  S.  Challenger  Kept.) 


FIG.  396. — Relatively  smooth  lava  surface  near  the  Jordan  craters, 
Malheur  Co.,  Ore.     (U.  S.  Geol.  Surv.) 


VULCANISM 


365 


is  discharged,  and  there  are  no  showers  of  dust  or  cinders,  no  loud 
rumbling  or  explosive  reports,  and  earthquakes  are  rare.     The 


FIG.  397. — Ropy  surface  of  lava,  Mauna  Loa,  flow  of  1881.      (Calvin.) 


Fro.  398. — Clinkery  lava,  Cinder  Buttes,  Idaho.     (U.  S.  Geol.  Surv.) 

eruption  may  continue  for  months  at  a  time  with  so  little  dis- 
turbance that  only  persons  in  the  vicinity  are  aware  of  it. 


366 


PHYSIOGRAPHY 


Hawaii  is  one  of  a  chnin  of  volcanic  islands,  400  miles  long. 
Mauna  Loa,  therefore,  like  the  other  volcanoes  studied,  is  one  of  a 
considerable  number  in  its  region. 

Common  phenomena  of  an  eruption.  From  the  preceding 
descriptions  the  essential  features  of  eruptions  may  be  gleaned. 
In  the  explosive  type  of  eruption,  rumblings  and  earthquake  shocks 


FIG.  399. — The  volcano  of  Colima,  Mex.,  in  an  active  condition,  March  24, 

1903.     (Arreola.) 

due  to  explosions  within  the  throat  of  the  volcano  often  occur  for 
weeks  or  months  previous  to  a  violent  outbreak.  As  the  explo- 
sions become  violent,  ashes,  cinders,  and  bombs  are  shot  forth  and 
fall  upon  the  sides  of  the  cone,  while  the  summit  of  the  mountain 
is  shaken.  The  clouds  of  condensed  steam  and  dust  rising  from 
the  crater  darken  the  sky,  and  torrents  of  rain,  falling  upon  the 
fine  dust,  form  rivers  of  hot  mud.  Liquid  lava  may  or  may  not 
accompany  the  discharge  of  dust,  cinders,  etc.  In  the  quiet  type 
of  eruption,  the  lava  rises  in  the  crater  and  occasionally  overflows 
its  rim;  but  more  commonly  a  crack  is  opened  in  the  side  of  the 


VULCAN  ISM  '367 

cone  by  an  earthquake  shock,  or  by  the  pressure  of  the  molten 
rock  within,  and  the  lava  issues  below  the  top. 

There  is  little  or  no  burning  in  a  volcano,  for  there  is  little  or 
nothing  to  burn.  There  is  therefore  no  smoke.  What  appears  as 
smoke  is  mostly  condensed  water  vapor  (cloud),  often  blackened 
by  the  dust. 

The  Products  of  Volcanoes 

The  materials  which  come  out  of  volcanoes  are  partly  solid, 
partly  liquid,  and  partly  gaseous.  The  dust,  the  cinders,  and  the 
larger  pieces  of  rock  are  solid,  the  flowing  lava  is  liquid,  while  the 
number  of  vapors  and  gases  which  issue  is  large. 

Lava.  All  the  liquid  rock  which  issues  from  a  volcano  is 
lava.  The  term  is  also  applied  to  the  rock  formed  when  the  liquid 
lava  becomes  solid  on  cooling. 

Lava  never  flows  so  freely  as  water,  and  it  is  sometimes  very 
stiff  or  viscous.  The  distance  to  which  it  flows  depends  on  its 
amount,  on  the  slope  of  the  surface  over  which  it  flows,  and  on  its 
liquidity.  The  greater  the  amount  of  lava,  the  steeper  the  slope 
on  which  it  flows,  and  the  more  fluid  it  is,  the  farther  it  will  flow. 

As  lava  flows  its  upper  surface  cools  and  hardens.  The  sur- 
face of  a  lava  stream  may  thus  become  solid,  while  the  interior  is 
still  fluid.  The  fluid  part  may  then  break  out  at  the  side  or  end 
of  the  hardened  shell  and  flow  away,  leaving  the  hollow  crust. 
On  further  cooling  the  shell  contracts  and  cracks,  and  sometimes 
caves  in.  Sometimes  the  hardened  surface  is  broken  by  the  move- 
ment of  the  fluid  lava  below,  and  the  solid  fragments,  displaced 
and  upturned  by  the  moving  liquid,  give  the  surface  a  jagged 
appearance  (Fig.  398).  In  1872  the  Modoc  Indians  of  south- 
eastern Oregon,  from  their  nearly  inaccessible  retreat  among  the 
lava-beds,  waged  a  warfare  which  was  for  some  time  successful 
against  the  United  States  troops. 

Lava  takes  on  various  forms  as  it  becomes  solid.  If  it  hardens 
under  little  pressure,  as  at  the  surface,  the  gases  and  vapors  which 
it  contains  expand,  and  it  is  converted  into  a  sort  of  rock  froth. 
If  the  lava  solidifies  quickly,  without  becoming  frothy,  it  makes 
volcanic  glass  or  obsidian.  If  the  lava  cools  slowly  under  pressure, 
the  substances  of  which  it  is  composed  crystallize  into  the  form  of 
various  minerals.  The  kinds  and  proportions  of  the  minerals 
depend  upon  the  composition  of  the  lava. 


368  PHYSIOGRAPHY 

Cinders,  ashes,  etc.  The  fragmental  materials  which  are  blown 
r  out  of  a  volcano  are  often  nothing  more  than  portions  of  lava  which 
solidified  before  ejection,  or  during  their  flight  in  the  air.  They 
may  be  large  or  small.  Masses  of  rock  tons  in  weight  are  some- 
times thrown  out,  and  from  such  masses  there  are  pieces  of  all 
sizes  down  to  minute  dust  particles. 

The  dust  is  often  transported  great  distances  from  the  volcano. 
Being  relatively  light,  it  is  thrown  far  into  the  air  and,  caught 
by  the  winds,  its  particles  are  shifted  incredible  distances,  as  already 
noted.  While,  therefore,  the  fluid  lava  and  the  larger  fragmental 
materials  ejected  from  the  volcano  stay  near  the  vent,  the  fine 
materials  are  scattered  broadcast. 

Gases  and  vapors.  The  gases  and  vapors  which  issue  from 
volcanoes  are  of  many  kinds.  Among  the  commoner  ones  are 
those  of  water  (H2O),  carbon  dioxide  (CO2),  chlorine  (Cl),  hydro- 
chloric acid  (HC1),  sulphur  dioxide  (SO2),  and  hydrogen  sulphide 
(H2S) ;  but  with  these  more  important  ones  there  are  many  others. 
Some  of  the  gases  are  poisonous,  and,  as  in  the  case  of  Pelee,  their 
temperature  is  sometimes  so  high  as  to  be  destructive  to  life. 

Number,  Distribution,  etc. 

Number.  The  number  of  volcanoes  is  not  easily  determined. 
Various  reasons  make  such  determination  difficult.  In  the  first 
place,  it  is  often  impossible  to  say  whether  a  quiet  volcano  is  dor- 
mant or  extinct.  If  the  former,  it  should  be  counted;  if  the  latter, 
it  should  not.  Again,  the  vent  of  a  volcano  often  changes.  In- 
stead of  discharging  lava  through  a  single  crater,  it  may  dis- 
charge through  several  subordinate  vents,  more  or  less  closely 
associated  with  the  main  one.  There  may  be  differences  of  opinion 
as  to  whether  these  several  vents  should  be  regarded  as  separate 
volcanoes.  For  this  and  other  reasons,  the  number  of  active  vol- 
canoes is  not  capable  of  definite  statement.  According  to  the 
more  common  estimates  there  are  between  300  and  400.  Some- 
thing like  two-thirds  of  them  are  on  islands,  and  the  remainder  on 
the  continents.  There  may  be  many  in  the  sea  which  are  not 
known,  for  volcanoes  in  the  deep  sea  might  not  be  readily  detected. 

Distribution.  The  general  distribution  of  active  volcanoes 
is  shown  in  Fig.  400.  Many  of  them  are  arranged  in  belts,  and 
within  the  belts  they  are  sometimes  in  lines.  The  most  marked 


VULCANISM 


369 


& 


is 

§  -s 


370  PHYSIOGRAPHY 

belt  nearly  encircles  the  Pacific  Ocean,  as  with  a  girdle  of  steaming 
vents.  This  belt  may  be  said  to  begin  with  the  volcanic  islands 
south  of  South  America,  and  includes  the  numerous  vents  in  the 
Andes  and  in  the  mountains  of  Central  America  and  Mexico.  The 
belt  widens  in  the  western  part  of  the  United  States,  where  the 
volcanoes  are  extinct,  but  narrows  again  in  Alaska  and  the  Aleu- 
tian Islands.  On  the  west  side  of  the  Pacific,  the  volcanoes  form 
a  well-marked  belt  with  many  active  vents  through  Kamchatka, 
Corea,  Japan,  the  Philippine  Islands,  New  Guinea,  New  Hebrides, 
and  New  Zealand.  A  branch  belt  includes  the  volcanoes  in  the 
islands  of  Java  and  Sumatra.  The  volcanoes  of  the  West  Indies 
are  sometimes  considered  as  an  eastern  branch  of  the  same  belt. 
Volcanoes  are  also  numerous  in  the  Mediterranean  Sea,  and  there 
are  not  a  few  which  cannot  be  regarded  as  parts  of  any  well- 
defined  belt. 

Most  volcanoes  are  in  the  sea  or  near  it.  Not  a  few  of  them 
are  in  mountain  regions,  but  it  is  by  no  means  true  that  all  moun- 
tain regions  have  them.  Not  a  few  are  on  ridges  or  swells  on  the 
sea  bottom,  or  on  ridges  or  swells  which  rise  above  the  sea.  Such, 
for  example,  are  the  West  Indian  volcanoes.  While  the  volcanoes 
which  are  on  the  continents  are  on  the  whole  near  the  shores,  they 
are  not  all  near  shores,  nor  do  they  occur  along  the  borders  of  all 
continents.  There  is  an  active  volcano  in  Africa  700  miles  from 
the  sea,  and  there  are  fresh  cones  of  extinct  volcanoes  500  to  800 
miles  from  the  sea  in  Arizona,  Colorado,  and  Thibet.  It  cannot  be 
said,  therefore,  that  nearness  to  "the  sea  or  mountain  ridges  are  con- 
ditions necessary  for  volcanoes. 

Many  of  the  active  volcanoes  lie  near  the  line  where  the  con- 
tinental plateau  descends  to  the  oceanic  basins.  Perhaps  this  is 
the  most  significant  feature  of  their  distribution. 

Volcanoes  are,  on  the  whole,  not  notably  more  abundant  in 
one  latitude  than  in  another.  At  any  rate,  they  have  a  wide  range 
in  latitude. 

The  data  which  are  now  in  hand  seem  to  point  to  the  general 
conclusion  that  volcanoes  on  land  are  commonly  associated  with 
lands  which  have  been  recently  warped.  It  is  conceived  that 
these  movements  of  the  surface  have  some  effect  upon  the  pressures 
and  temperatures  of  the  deeper  zones  beneath  them,  and  that 
these  variations  of  pressure  and  temperature  are  among  the  con- 


VULCANISM  371 

ditions  necessary  for  the  extrusion  of  lava  from  beneath  the 
surface. 

Historical.  Volcanoes  have  existed  throughout  the  history 
of  the  earth,  so  far  as  this  history  is  now  known,  even  back  to  the 
earliest  ages;  but  volcanic  processes  do  not  seem  to  have  been 
equally  active  at  all  times.  There  seem  to  have  been  periods  of 
great  volcanic  activity,  alternating  with  much  longer  periods  of 
much  less  activity.  There  is  no  knowledge,  however,  that  vul- 
canism  ever  ceased  altogether  at  any  time. 

While  vulcanism  seems  to  have  been  continuous,  but  more  or 
less  periodic  in  its  violence,  the  sites  of  volcanic  activity  have 
shifted  from  time  to  time,  and  the  areas  where  they  now  exist 
are  not  the  areas  where  they  existed  in  former  times. 

What  is  now  known  of  vulcanism  seems  to  indicate  that,  gen- 
erally speaking,  a  volcano  has  a  beginning,  runs  a  given  course, 
and  dies.  The  vulcanism  of  a  given  region  appears  to  have  a 
similar  course. 

It  appears  also  that  the  phase  of  vulcanism  sometimes  changes 
in  a  given  region.  In  some  volcanic  regions  fissure  eruptions 
came  early  in  the  course  of  the  volcanic  history.  As  activity  de- 
clined, fissure  eruptions  gave  place  to  volcanoes,  and  the  volcanoes 
became  less  and  less  active,  and  finally  extinct. 

Even  after  vulcanism  proper  ceases,  associated  phenomena 
are  continued.  Thus  in  the  Yellowstone  National  Park  there 
are  numerous  geysers,  hot  springs,  and  other  vents  out  of  which 
hot  vapors  issue.  Such  phenomena  probably  represent  the  last 
phases  of  volcanic  activity  in  the  region. 

IGNEOUS  PHENOMENA  NOT  STRICTLY  VOLCANIC 

Fissure  eruptions.  Lava  sometimes  rises  to  the  surface  through 
great  fissures  instead  of  through  the  relatively  small  vents  of 
volcanoes.  From  such  fissures  floods  of  lava  spread  over  the 
surrounding  country  sometimes  for  hundreds  of  miles.  Such  lava 
floods  once  occurred  in  Oregon,  Washington,  and  Idaho,  where,  by 
successive  flows,  the  pre-existing  hills  and  valleys  were  buried,  and 
a  vast  plateau  200,000  square  miles  or  more  in  extent  was  built  up 
(Fig.  401).  Locally,  the  nearly  level  surface  of  the  lava  plateau 
meets  the  mountains  along  its  border,  somewhat  as  the  sea  meets 
the  land,  while  islands  of  older  rock  rise  above  it. 


372 


PHYSIOGRAPHY 


In  this  lava  plateau  the  Snake  River  (Fig.  26)  has  excavated 
a  great  canyon  4000  feet  deep  in  some  places,  and  15  miles  wide. 
The  walls  of  the  canyon  show  the  structure  of  the  plateau.  They 
show,  among  other  things,  the  edges  of  the  successive  lava-flows, 
sometimes  separated  by  beds  of  sediment,  with  soils  in  which  the 
roots  and  trunks  of  trees  are  still  preserved.  These  beds  of  sedi- 


FIG.  401. — Lava-flows  of  the  northwestern  part  of  the  U.  S. 

ment,  and  these  soils,  show  that  long  periods  of  time  elapsed  be- 
tween successive  lava-flows.  At  one  point  in  the  walls  of  the 
canyon,  a  peak  of  older  rock  rising  2500  feet  above  the  river  is 
buried  by  1500  feet  of  lava.  A  rugged  mountain  region  was  here 


FIG.  402. — A.  Ideal  cross-section  of  a  laccolith  with  accompanying  sheet  and 
dikes.  B.  Ideal  cross-section  of  a  group  of  laccoliths.  (Gilbert,  U.  S. 
Geol.  Surv.) 

transformed  into  a  plateau  by  the  lava  floods.  A  part  of  the 
plateau  has  since  been  deeply  dissected  by  streams,  parts  still 
remain  nearly  plane,  parts  have  been  broken  into  blocks  which 
have  been  tilted  into  mountain  ridges,  while  still  other  parts  have 
been  arched  up  into  great  dome  mountains.  The  Blue  Mountains 


VULCAN  ISM 


373 


of  Oregon  are  the  most  conspicuous  example  of  doming.  Badger 
Mountain  of  Washington  is  an  elongate  dome  or  anticline. 

An  older  lava  plateau  of  still  greater  size  occurs  in  India.  Ow- 
ing to  its  greater  age,  its  nearness  to  the  sea,  and  the  humid  climate, 
it  is  more  dissected  than  the  Oregon  plateau.  This  lava  has  in 
some  areas  weathered  so  as  to  form  a  soil  of  great  fertility,  to 
which  the  Deccan  owes  its  fame  as  a  cotton-growing  country. 
Prominent  hills  of  lava  along  the  dissected  edges  of  the  flows  have 
frequently  served  as  natural  forts  of  great  strength  in  the  wars 
of  the  country.  Other  dissected  lava  plateaus  are  found  on  the 
north  coast  of  Ireland  and  the  west  coast  of  Scotland.  Some  of  the 
islands  off  the  coast  of  Scotland  are  remnants  of  an  old  lava  plateau. 

Fissure  eruptions  have  occurred  in  Iceland  within  historic 
times.  In  1783  such  flows  took  place  from  a  fissure  20  miles  or 
so  in  length.  The  lava  spread  out  in  sheets  on  both  sides  of  the 
fissure,  advancing  in  the  valleys  farther  than  on  the  uplands 
between  them.  In  this  respect  the  lava-flows  resemble  the  move- 
ment of  glacier  ice. 

While  fissure  eruptions  of  lava  sometimes  build  up  plateaus 
or  raise  the  level  of  the  plains  on  which  they  spread,  they  do  not 
commonly  give  rise  to  mountains;  but  mountains  are  sometimes 
developed  from  them,  as  they  are  dissected  by  stream  erosion. 

Intrusions  of  lava.  Lava  is  sometimes  intruded  from  below 
into  the  crust  of  the  lithosphere,  without  rising  to  the  surface. 


FIG.  403. — Diagram  of  a  bysmalith. 

In  such  cases  the  surface  strata  may  be  arched  up  over  the  in- 
trusion, making  domes  which  sometimes  reach  the  size  of  moun- 
tains. Such  mountains  (Figs.  402,  404),  of  which  the  Henry 
Mountains  of  Utah  are  examples,  are  called  laccoliths.  If  the 
roof  of  the  intrusion  is  faulted  up  instead  of  being  arched  up,  the 


374 


PHYSIOGRAPHY 


YHENRYMOUNTAINS 


FIG.  404. — Relief-map  of  the  Henry  Mountains.    (Gilbert,  U.  S.  Geol.  Surv.) 


375 


intrusion  is   called  a  bysmalith   (Fig.   403).     Intrusions  of  very 
great  size  are  batholiths.     Lava  is  sometimes  intruded  between  beds 


FIG.  405. — Diagrammatic  representation  of  the  relations  of  igneous  rock 
to  stratified  rock.  The  igneous  rocks  represented  in  black  have  been 
forced  up  from  beneath. 

of  stratified  rock  in  sheets  or  sills  (Fig.  405).  Lava  is  also  some- 
times forced  into  cracks  of  the  rock,  solidifying  there  as  dikes 
(a,  Fig.  405). 

Causes  of  Vulcanism 

The  causes  of  vulcanism  are  somewhat  outside  the  province  of 
physiography,  but  it  may  be  stated  that  the  lava  of  volcanoes  does 
not  appear  to  come  from  a  liquid  interior,  and  the  lavas  from  ad- 
jacent vents  do  not  appear  to  come  from  a  common  reservoir  of 
liquid  rock.  This  is  suggested  by  the  fact  that  adjacent  vents 
frequently  discharge  different  sorts  of  lava,  and  that  the  lava  in 
adjacent  craters  often  stands  at  very  different  heights  at  the  same 
time. 

The  great  pressure  which  exists  in  the  interior  of  the  earth 
because  of  the  weight  of  the  overlying  parts,  insures  a  high  tem- 
perature to  the  interior.  The  heat  thus  developed  is  continually 
working  its  way,  in  one  way  or  another,  to  the  outer  portions  of  the 
earth.  It  passes  out  by  conduction  everywhere,  and  locally,  where 
conditions  favor,  small  amounts  of  rock  may  become  liquid.  This 
liquid  rock  then  works  its  way  to  the  surface  or  toward  it.  Ac- 
cording to  this  view,  the  extrusion  of  lava  is  to  be  looked  upon  as 
one  phase  of  the  passage  of  interior  heat  to  the  surface. 

In  the  explanation  of  volcanoes,  two  things  are  to  be  accounted 
for:  (1)  the  liquid  lava  and  the  heat  necessary  for  its  production, 
and  (2)  the  force  which  brings  it  to  the  surface. 

Lava  is  to  be  regarded  as  a  solution  of  mineral  matter  in  min- 
eral matter,  rather  than  as  melted  rock.  The  solution,  however, 
takes  place  only  at  high  temperatures.  Various  views  have  been 


376  PHYSIOGRAPHY 

entertained  as  to  the  source  of  the  heat  necessary  to  cause  minerals 
to  dissolve  in  one  another.  These  views  may  be  grouped  into  two 
classes:  (1)  those  according  to  which  the  heat  is  primary,  that  is, 
that  the  interior  of  the  earth  has  been  hot  always,  or  that  it  has 
been  hot  since  the  earth  attained  its  present  size;  and  (2)  those 
according  to  which  the  heat  which  liquefies  the  rock  is  secondary, 
and  developed  in  rock  (relatively  near  the  surface)  \vhich  was  once 
cool.  Some  of  the  hypotheses  of  volcanic  action  based  on  these 
views  may  be  considered  briefly. 

1.  It  was  formerly  thought  that  the  whole  interior  of  the  earth 
might  be  liquid,  and  that  the  volcanic  vents  were  connected  with 
this  liquid  interior.     This  view  was  based  on  certain  familiar  facts. 
Deep  mines  and  borings  of  all  sorts  show  that  the  temperature 
increases   with   increasing   depth.     The    rate    of   increase   varies 
widely  from  1°  for  17  feet  to  1°  for  more  than  100  feet.     The  average 
rate  of  increase  is  commonly  stated  as  about  1°  for  every  50  to  60 
feet;   but  if  the  estimate  be  based  on  the  records  of  those  deep 
mines  and  other  borings  which  seem  to  afford  the  most  reliable 
data,  the  rate  is  more  nearly  1°  for  100  feet,  down  to  the  greatest 
depths  yet  penetrated.     It  is  to  be  remembered,  however,  that 
the  deepest  excavations  are  but  little  more  than  a  mile  in  depth, 
and  that  most  excavations  on  which  the  generalizations  are  based 
are  much  shallower.      If  the  heat  increases  at  the  average  rate 
of  1°  per  100  feet,  a  temperature  of  3000°  would  be  reached  at  a 
depth  of  about  60  miles.      Such  a  temperature  would  be  enough 
to  liquefy  rocks  at  the  surface,  but  we  are  not  to  conclude  that  rocks 
are  liquid  at  this  depth  even  if  the  temperature  is  3000°.      At 
this  depth,  the  pressure  due  to  the  overlying  rock  is  enormous. 
Rock  expands  when  it  is  liquefied,  and  the  pressure  at  this  depth 
may  be  enough  to  prevent  expansion,  and  so  to  prevent  general 
liquefaction.     There  are  many  reasons  for  believing  that,  though 
the  temperature  of  the  interior  of  the  earth  is  very  high,  the  rock 
is  still  solid.     The  fundamental  element  of  the  hypothesis,  that  all 
volcanoes  start  from  a  common  liquid  center,  is  therefore  believed 
to  be  wrong. 

2.  It  has  been  suggested  that  there  is,  at  some  depth  beneath 
the  surface,  a  liquid  layer  below  the  solid  crust  and  above  a  great 
solid  centre.     This  hypothesis  does  not  seem  to  be  well  supported, 
and  does  not  seem  to  meet  the  objections  to  the  hypothesis  first 
mentioned. 


VULCANISM  377 

3.  Another  view  has  been  that  while  the  earth  is  virtually  solid, 
it  is  solid  in  spite  of  its  internal  temperature,  and  that  if  the  pres- 
sure were  lessened  at  some  point  beneath  the  surface,  the  hot  rock 
would  expand  and  become  liquid.  The  pressure,  it  is  conceived, 
would  be  lessened  where  the  outer  part  of  the  earth  is  folded  up, 
as  in  some  mountains.  This  hypothesis  has  found  much  favor, 
but  it  does  not  seem  to  account  for  some  of  the  fundamental  facts 
connected  with  volcanoes,  such  as  their  distribution. 

The  hypotheses  that  the  heat  involved  in  volcanic  action  is 
secondary,  seek  to  explain  the  heat  (1)  by  means  of  the  crushing 
of  rock  such  as  sometimes  takes  place  when  beds  of  rock  are  folded, 
or  (2)  by  chemical  action  between  the  elements  of  the  rocks,  or 
between  these  elements  and  water  which  descends  from  the 
surface.  These  hypotheses  have  little  acceptance  at  the  present 
time. 

No  one  of  the  preceding  hypotheses,  nor  all  combined,  seem 
to  adequately  explain  vulcanism,  and  no  hypothesis  which  seems 
altogether  satisfactory  has  been  put  into  definite  form.  It  seems 
possible  (1)  that  the  local  formation  of  liquid  lava  is  a  process 
which  is  constantly  but  slowly  going  on  in  the  deep  interior, 
perhaps  where  the  rock  material  is  more  readily  soluble  than  the 
average;  and  (2)  that  the  liquid  rock  is  continually  finding  its 
way  to  the  surface,  faster  and  in  greater  quantities  at  some  times 
than  at  others.  The  regions  where  the  crust  is  least  stable,  that  is, 
where  there  is  movement,  are  the  regions  most  likely  to  afford  the 
lava  a  place  of  escape,  for  it  is  in  such  places  that  it  is  weakest. 

The  principal  forces  involved  in  the  extrusion  of  lavas  are  ap- 
parently two,  (1)  gravity  and  (2)  the  expansive  and  explosive 
force  of  the  vapors  and  gases  contained  in  the  lavas,  especially 
water  vapor. 

Lava  beneath  the  surface  would,  if  lighter  than  the  solid  rock 
above,  tend  to  find  its  way  to  the  surface,  or,  more  strictly,  the 
heavier  rock  above  would  tend  to  sink  down,  squeezing  out  the 
lighter  liquid  rock  beneath.  This  has  probably  been  an  important 
factor — perhaps  the  most  important  factor — in  the  eruption  of 
some  volcanoes  and  in  some  fissure  eruptions.  If  at  the  same 
time  the  region  concerned  is  affected  by  lateral  pressure,  this 
pressure  might  help  to  squeeze  out  the  liquid  lava.  Pressure 
from  above  or  from  the  sides  seems  to  be  the  principal  factor 
involved  in  the  extrusion  of  lavas  in  quiet  eruptions.  Gases  and 


378  PHYSIOGRAPHY 

vapors  in  the  lava  tend  to  expand  it,  especially  as  pressure  is 
relieved,  and  so  tend  to  diminish  its  specific  gravity. 

In  the  case  of  violent  eruptions  the  gases  and  vapors,  espe- 
cially water  vapor,  appear  to  play  a  principal  part.  Even  in  these 
cases,  however,  it  is  probable  that  gravity  is  the  principal  factor 
in  getting  the  lava  up  near  to  the  surface,  and  that  the  vapors 
and  gases  come  into  effective  function  only  as  the  surface  of  the 
lithosphere  is  approached. 

The  source  of  the  vapors  which  issue  from  volcanoes  is  a  matter 
about  which  there  is  much  difference  of  opinion.  Among  the 
vapors  which  escape  from  volcanoes  there  are  those  which 
might  have  been  derived  from  sea-water.  From  this  fact  it  was 
inferred  that  sea-water  had  access  to  the  sources  of  the  lava.  It 
is  now  thought,  however,  that  water  probably  does  not  descend 
more  than  five  or  six  miles  beneath  the  surface  of  the  lithosphere, 
for  below  some  such  depth,  pores  and  cracks,  without  which  water 
cannot  descend,  do  not  exist.  It  seems  certain  that  the  sources 
of  the  lava  are  much  deeper,  and  it  therefore  seems  improbable 
that  descending  water,  either  from  the  sea  or  from  the  land,  reaches 
the  sources  of  vulcanism. 

It  seems  probable  that  lava  from  depths  far  below  all  ground- 
water  is  forced  up  to  within  a  short  distance  of  the  surface  before 
coming  into  contact  with  water.  In  its  passage  through  the  outer 
part  of  the  crust,  which  contains  water,  the  lava  doubtless  con- 
verts water  into  steam;  and  the  steam  thus  produced  is  possibly 
an  important  factor  in  the  rise  of  the  lava  through  the  outermost 
portion  of  the  earth's  crust.  But  there  is  the  best  of  reason  for 
believing  that  lava  brings  up  vapors  and  gases,  and  among  them 
water  vapor,  from  much  greater  depths.  Such  gases  and  vapors 
must,  it  would  seem,  have  been  long  within  the  earth.  It  is  prob- 
able, indeed,  that  some  of  them  are  now  reaching  the  surface  of 
the  earth  for  the  first  time.  If  this  be  true,  they  are  to  be  looked 
upon  as  original  constituents  of  the  earth. 

Topographic  Effects  of  Volcanic  Action 

Many  volcanoes  build  up  great  cones,  some  of  them  mountain- 
high,  as  already  indicated.  The  first  stages  of  growth  have  some- 
times been  observed. 

Volcanic   cones.      In  1538  a  small  volcano    appeared  on  the 


VULCANISM  379 

north  shore  of  the  Bay  of  Naples,  and  built  up  a  cone  440  feet  high 
and  half  a  mile  in  diameter  at  its  base  in  a  few  days.  Its  crater 
was  more  than  400  feet  deep.  The  development  of  the  volcano 
was  preceded  by  earthquakes,  which  were  felt  in  the  same  regions 
for  two  years  before  the  volcano  was  formed. 

In  1770  the  volcano  Izalco  in  Central  America  broke  out  in 
the  midst  of  a  plain  which  was  then  a  cattle-ranch.  Since  that 
time  it  has  built  up  a  symmetrical  cone  about  3000  feet  high,  with 
steep  slopes.  In  the  earlier  part  of  its  history,  lava-flows,  attended 
with  streams  of  cinders,  etc.,  were  of  frequent  occurrence.  For 
many  years  no  lava  has  flowed  out,  though  the  volcano  has 
remained  active,  discharging  explosively.  Earthquakes  and  rum- 
blings preceded  the  original  eruption. 

In  January,  1880,  a  volcano  broke*  out  in  Lake  Ilopango,  San 
Salvador,  Central  America.  The  eruption  continued  more  than 
a  month,  heating  the  waters  of  the  lake,  killing  the  fish,  and  form- 
ing a  cone-shaped  island  rising  160  feet  above  the  lake,  which  was 
600  feet  deep.  A  violent  earthquake  occurred  in  this  region  a 
few  months  before  the  birth  of  the  volcano.  After  the  earth- 
quake the  water  of  the  lake  sank  35  feet. 

Early  in  the  last  century  a  volcanic  island  (Graham  Island) 
arose  in  the  Mediterranean,  between  Sicily  and  Africa,  where  the 
water  had  been  800  feet  deep.  In  1831  a  ship  near  the  place  felt 
earthquake  shocks.  In  July  a  sea-captain  reported  that  he  saw 
a  column  of  water  60  feet  high  and  800  yards  in  diameter  rising 
from  the  sea,  and  soon  afterward  a  column  of  steam  which  rose 
1800  feet.  A  few  days  later  there  was  a  small  island  12  feet  high 
where  the  disturbance  had  been,  and  in  its  centre  there  was  a 
crater,  from  which  eruptions  were  seen  to  be  taking  place.  By 
the  end  of  the  month  the  island  was  50  to  90  feet  high  and  f  oi 
a  mile  in  circumference.  On  August  4  it  was  200  feet  high  and 
3  miles  in  circumference.  Activity  soon  ceased,  and  early  in  1832 
the  island  had  been  destroyed  by  the  waves.  This  volcano  was 
short-lived,  as  was  the  island  which  it  built. 

Volcanoes  have  recently  built  up  islands  off  the  coast  of  Alaska. 
In  1795  such  an  island  appeared  about  40  miles  west  of  Unalaska. 
In  1872  this  island  was  850  feet  above  the  sea,  but  had  no  crater. 
In  1883  another  island  appeared  close  by,  and  was  later  joined  to 
the  first.  In  1884  it  was  500  to  800  feet  high. 

Great  mountains,  as  well  as  small  ones,  are  often  formed  bv 


380 


PHYSIOGRAPHY 


FIG.  406. — Mt.  Shasta,  a  typical  volcanic  cone  furrowed  by  erosion,  but 
retaining  its  general  form.     (U.  S.  Geol.  Surv.) 


FIG.  407. — A  part  of  the  "crater"  of  Coon  Butte,  Ariz.  The  "butte"  is 
only  the  rim  built  up  about  the  "crater"  by  the  material  blown  out. 
(R.  T.  Chamberlin.) 


VULCANISM  381 

volcanoes.  Thus  Mt.  Rainier  in  Washington  (Fig.  408),  Mt. 
Hood  in  Oregon  (Fig.  227),  Mt.  Shasta  in  California  (Fig.  406), 
and  the  San  Francisco  Mountain  in  Arizona  (Fig.  411),  as  well 
as  numerous  other  high  and  well-known  mountain  peaks,  were 
built  up  by  volcanoes.  The  volcanoes  themselves  have  been 
dead  for  long.  Rainier,  Hood,  and  Shasta  are  all  so  high  that, 
in  spite  of  their  origin,  snow-fields  and  even  glaciers  are  found  on 
them. 

Many  small  islands,  and  some  large  ones,  such  as  Iceland,  are 
due  chiefly  or  wholly  to  the  building  up  of  volcanic  cones  which 
have  their  foundations  on  the  ocean  bottom.  The  Aleutian  Islands, 
the  Kurile  Islands,  and  many  of  the  islands  of  Australasia  were 
formed  in  the  same  way.  Among  the  latter  are  the  famous  Spice 
Islands  (Moluccas)  so  important  in  connection  with  the  early 
history  of  America. 

By  the  making  of  cones,  volcanoes  become  an  important  factor 
in  shaping  the  surface  of  the  lithosphere.  The  number  of  volcanic 
cones  which  have  assumed  mountain  size  on  land  is  large,  and 
the  number  in  the  sea  still  larger;  but  in  spite  of  their  great  num- 
ber, their  aggregate  area  is  relatively  slight.  The  total  area  of 
mountainous  lands  developed  by  volcanoes  is  but  a  fraction  of  the 
area  of  mountain  land  developed  in  other  ways. 

Intrusions  of  lava  may  give  rise  to  mounds,  mountains,  or  even 
plateau-like  swells  (laccoliths,  bysmaliths,  batholiths),  as  already 
indicated. 

In  Arizona  (near  Canyon  Diablo)  there  is  a  crater-like  depression 
with  a  distinct  rim  about  it,  composed  of  the  material  which  was 
blown  out  of  the  depression.  The  rim  is  high  enough  to  be  seen 
from  a  great  distance,  and  is  known  as  Coon  Butte  (Fig.  407).  There 
is  no  lava  about  the  butte,  and  it  cannot  therefore  be  called  a 
volcano.  Apparently  a  great  explosion  beneath  the  surface  blew 
out  a  large  quantity  of  rock  from  the  crater-like  depression,  form- 
ing the  high  rim.  The  formation  of  this  depression  may  perhaps 
be  looked  upon  as  a  preparatory  step  for  a  volcano.  If  so,  the 
process  was  arrested  before  a  volcano  was  developed.1 

1  The  hypothesis  was  advanced  long  since  by  Gilbert  that  this  so-called 
butte,  with  its  accompanying  depression,  was  due  to  the  fall  of  a  large 
meteorite.  Mr.  Gilbert  abandoned  this  hypothesis,  after  somewhat  full 
investigation,  but  it  has  been  revived  recently,  in  modified  form,  by  Fair- 
child  (Bull.  G.  S.  A.,  Vol.  XVIII,  p.  493.) 


382  PHYSIOGRAPHY 

Destruction  of  volcanic  cones.  1.  A  volcanic  cone  may  be 
partially  destroyed  by  a  violent  explosion,  as  in  the  cases  of  Kra- 
katoa  and  Vesuvius,  already  cited.  Enormous  depressions,  called 
calderas,  several  miles  in  diameter  and  hundreds  of  feet  deep,  may 
be  developed  in  this  way  from  previous  cones  and  craters. 

A  volcanic  cone  may  be  undermined  by  the  withdrawal  of  the 
liquid  lava  in  the  core  of  the  mountain,  if  it  escapes  at  a  lower 
level.  The  entire  summit  of  the  mountain  may  then  sink  and  be 
engulfed,  thus  forming  a  caldera.  Crater  Lake,  Oregon  (p.  305), 
occupies  a  caldera  in  the  stump  of  a  great  volcanic  cone  (Figs. 
333-336).  There  are  several  large  calderas  in  the  Azores,  and  the 
floors  of  some  of  them  are  the  sites  of  villages. 

2.  Volcanic  cones  are  also  destroyed  by  the  less  violent  proc- 
esses of  weathering  and  erosion.  The  destruction  of  Graham 
Island  by  the  waves  has  already  been  cited.  Wind  and  rain  attack 
volcanic  cones  as  soon  as  they  are  formed,  but  their  results  are  not 
conspicuous  until  the  volcano  is  extinct  and  the  cone  stops  grow- 
ing. Cones  composed  largely  of  cinders,  etc.,  are  worn  away  with 
comparative  ease,  while  those  of  lava  resist  longer.  Glaciers  fre- 
quently aid  in  their  degradation.  Among  the  many  extinct 
volcanic  peaks  in  the  western  part  of  the  United  States,  it  is  pos- 
sible to  find  illustrations  of  various  stages  in  the  process  of  destruc- 
tion. Only  those  of  relatively  recent  origin  still  show  their  un- 
modified or  but  slightly  modified  forms  All  volcanic  cones 
except  those  of  recent  origin  have  lost  their  craters  and  the 
symmetry  of  slope  which  they  probably  once  possessed. 

Examples  of  fresh  cones.  In  Arizona,  California  (Fig.  364), 
Idaho,  Oregon,  and  elsewhere  in  the  United  States,  there  are 
volcanic  cones  so  recently  formed  that  they  have  suffered  little 
destruction.  Their  conical  forms  have  been  little  disfigured  by 
erosion,  and  their  surface  materials  appear  to  have  been  but 
little  changed  by  weathering.  Cones  of  similar  freshness  are  found 
in  other  lands,  as  in  the  Auvergne  (France). 

Examples  of  worn  cones.  Mt.  Shasta  (Fig.  406),  in  northern 
California,  is  a  volcanic  cone  which  rises  2  miles  above  its  base,  17 
miles  in  diameter,  to  a  height  of  14,350  feet  above  the  sea.  It  is 
partly  of  lava  and  partly  of  fragmental  material.  Its  upper 
slopes  are  steep  and  furrowed  with  ravines.  About  2000  feet 
below  the  summit  on  the  west  side  is  a  fresher  and  therefore 
younger  cone,  known  as  Shastina,  with  a  crater  in  its  top.  Re- 


VULCANISM  383 

mains  of  more  than  20  smaller  cones  also  occur  on  the  lower  flanks. 
Near  the  base  are  several  lava-fields  which,  from  the  roughness  of 
their  surfaces  and  the  absence  of  soil,  are  known  to  have  been 
formed  since  the  glaciers  occupied  the  summit.  The  fact  that  the 
Sacramento  River  has  cut  a  narrow  gorge  100  feet  deep  across 
one  of  them  proves,  however,  that  the  last  eruption  was  many 
years  ago.  Mt.  Shasta  is  a  good  example  of  a  volcano  which  has 
suffered  some  erosion,  but  about  which  evidence  of  recent  erup- 
tions has  not  been  destroyed. 

The  great  changes  which  Shasta  has  undergone  are  made  clear 
by  the  fact  that  the  once  hot  mountain  is  now  the  home  of  several 
glaciers,  which,  moving  over  its  slopes,  are  helping  to  waste  it 
away. 

Mt.  Rainier  (Fig.  408)  is  another  splendid  mountain  developed 
by  a  former  volcano.  Various  features  of  the  mountain  show 


FIG.  408.— Mt.  Rainier,  Wash. 

that  a  second  period  of  activity  followed  a  long  period  of  quiescence 
in  the  history  of  this  snow-capped  mountain.  Hot  vapors  still 
issue  from  some  small  vents  in  the  mountain,  though  the  dis- 
charge of  rock  material  ceased  long  ago.  The  mountain  is  snow- 
covered  and  the  home  of  several  glaciers. 

Mt.  Hood  (Fig.  227),  one  of  the  peaks  of  the  Cascade  Range, 
has  been  eroded  more  than  Rainier.  Only  a  part  of  the  wall  of  its 
summit  crater  remains,  and  its  sides  are  deeply  furrowed  by  ra- 
vines, which  are  separated  by  sharp,  jagged  ridges,  and  precipices 
hundreds  of  feet  in  height.  Nevertheless,  sulphurous  fumes  are 
still  escaping  from  openings  in  the  rocks,  even  on  the  snow-clad 
summit. 


384  PHYSIOGRAPHY 

The  Marysville  Buttes.  This  circular  cluster  of  hills  (Figs. 
409  and  410),  10  miles  in  diameter,  rises  1700  to  2000  feet  above 
the  level  of  the  Sacramento  River  in  California.  The  buttes  are 
composed  of  lava  with  an  outer  layer  of  fragmental  material  (or 
tuff).  The  volcanic  cone,  which  probably  once  rivaled  Vesuvius, 


FIG.  409 — Marysville  buttes  in  contour.     (U.  S.  Geol.  Surv.) 

has  been  dissected  into  a  group  of  hills  with  jagged  and  fantastic 
outlines.  So  deeply  have  the  roots  of  the  mountain  been  laid  bare 
that  the  broken  and  distorted  layers  of  sedimentary  rock  through 
which  the  lava  was  erupted  are  exposed. 

The  San  Francisco  Mountain  (Fig.  411)  is  another  example 
of  a  volcanic  mountain  partially  destroyed  by  erosion.  The 
form  of  the  old  cone  can  be  but  imperfectly  known.  Numerous 
minor  volcanoes  existed  about  the  main  ore  after  the  latter  be- 


VULCANISM 


385 


came  extinct.     It  is  said  that  the  number  of  fresh  volcanic  cones 
in  this  vicinity  is  more  than  300.     Many  of  them  are  so  young  that 

they  show  little  sign  of  erosion. 

..-  "?#.•  ••  ••  ••  .••» .;" 

Indirect  Topographic  Effects  of  Vulcanism 

Volcanic  necks.  When  a  volcano  becomes  extinct,  the  throat, 
or  passage  from  the  interior,  may  be  filled  with  hardened  lava. 
This  may  be  of  much  more  resistant  rock  than  the  rest  of  the  cone. 
The  cone  may  in  time  be  worn  away,  but  the  plug,  transformed 


FIG.  410. — Marysville  volcanic  cone.     (Photograph  of  model  by 
Newsome.) 

into  a  hill,  may  still  mark  the  site  of  the  former  volcano.  These 
volcanic  necks  or  plugs  are  sometimes  conspicuous.  East  of  the 
Mt.  Taylor  plateau,  in  central  Mexico,  a  number  of  them  rise  by 
precipitous  slopes  800  to  1500  feet  above  their  surroundings. 
Massive  intrusions  of  lava  have  the  same  effect  (Fig.  171). 

Intrusions  of  lava  may  give  rise  to  topographic  features  of 
importance  after  erosion  has  affected  the  regions  where  they 
occur,  for  the  hardened  lava  or  igneous  rock  is  often  harder  than 
its  surroundings.  Dikes  often  give  rise  to  ridges  (Fig.  412).  Sills 
also,  if  they  have  been  tilted  notably  from  a  horizontal  position, 
give  rise  to  ridges  which  may  be  so  high  as  to  be  called  moun- 
tains. The  Palisade  Ridge  of  the  Hudson  (Fig.  413)  and  most  of 
the  mountains  of  the  Connecticut  River  Valley  are  illustrations. 
A  sheet  of  lava  poured  out  on  the  surface  and  subsequently  buried 


386 


PHYSIOGRAPHY 


VULCANISM 


387 


by  sediment  may  have  the  same  effect  on  topography  as  a  sill. 
The  Watchung  Mountains  of  New  Jersey  are  the  projecting  edges 


FIG.  412. — A  dike  isolated  by  erosion,  Spanish  Peaks  region,  Colo. 
(U.  S.  Geol.  Surv.) 


FIG.  413.— The  Palisade  Ridge. 


of  extrusive  sheets   of  lava,   once   buried   beneath  sedimentary 
rocks,  then  tilted,  and  later  isolated  by  erosion.    Sills  and  extrusive 


388 


PHYSIOGRAPHY 


sheets  of  lava  may  also  give  rise  to  buttes,  mesas,  rock  terraces, 
etc. — indeed,  to  all  the  topographic  forms  which  result  from  the 
erosion  of  rock  of  unequal  hardness  (p.  163). 

Columnar  structure.  Hardened  lava  sometimes  assumes  a 
columnar  structure  (Figs.  414  and  415).  This  is  probably  the  result 
of  the  contraction  incident  to  cooling.  The  surface  of  the  homo- 
geneous lava  contracts  about  equally  in  all  directions  on  cooling. 
The  contractile  force  may  be  thought  of  as  centering  about  equi- 
distant points.  About  a  given  point  the  least  number  of  cracks 

B 


FIG.  414. — A.  Columnar  structure  in  basalt,  Giants  Causeway,  Ireland. 
B.  Columnar  structure  on  a  larger  scale. 

which  will  relieve  the  tension  in  all  directions  is  three  (A,  Fig.  415). 
If  these  radiate  symmetrically  from  the  point,  the  angle  between 
any  two  is  120°,  the  angle  of  the  hexagonal  prism.  Similar  radi- 
ating cracks  from  other  centers  complete  the  columns  (B,  Fig.  415). 
A  five-sided  column  would  arise  from  the  failure  of  the  cracks  to 
develop  about  some  one  of  the  points  (C,  Fig.  415). 

Mud  Volcanoes 

Mud  volcanoes  have  some  features  in  common  with  volcanoes 
and  some  in  common  with  geysers,  while  in  others  they  depart 
from  both.  Like  volcanoes  and  geysers,  they  are  eruptive,  but, 
as  the  name  implies,  their  discharge  is  mud,  instead  of  lava  or 
water.  The  general  conditions  which  seem  to  be  necessary  for  their 
existence  are  (1)  sufficient  heat  beneath  the  surface,  presumably  at 


VULCAXISM 


389 


a  relatively  slight  depth,  to  produce  steam,  and  (2)  a  surface  layer 
of  earthy  material,  which  when  moist  becomes  mud.     The  steam 


A  B  C 

FIG.  415. —  Diagrams  to  illustrate  the  formation  of  columns  in  basalt: 
A.  The  first  stage  i§  the  development  of  a  hexagonal  column.  B.  The 
completion  of  a  hexagonal  column.  C.  A  pentagonal  co.umn. 


escaping  through  the  mud  forces  some  of  it  out,  building  up  small 
cones  which  simulate  volcanic  cones  in  form,  though  not  in  con- 
stitution. They  never  reach  great  size. 


FIG.  416. — Mud  cones    Yellowstone  National  Park.     (Fairbanks.) 


Like  geysers,  mud  volcanoes  occur  in  regions  of  present  or 
relativelv  recent  vulcanism,  for  the  most  part.    They  are  some- 


390  PHYSIOGRAPHY 

times  violently  explosive,  and  sometimes  not.  Some  of  them 
erupt  at  infrequent  intervals,  and  some  nearly  continuously. 

The  "paint-pots"  (Fig.  416)  of  the  Yellowstone  Park  belong  to 
the  same  category,  though  from  them  there  is  little  discharge,  and 
they  do  not  build  considerable  cones. 

When  quantities  of  gas  escape  from  beneath  the  surface  through 
wind,  eruptions  somewhat  like  those  of  mud  volcanoes  may  take 
place,  even  in  the  absence  cf  heat. 

MAP  EXERCISE 

I.  Study  the  following  maps  showing  volcanic  mountains,  in  prepara- 
tion for  the  confrenece: 

1.  Marys ville,  Cal. 

2.  Mt.  Lyell,  Cal. 

3.  Mt.  Shasta,  Cal. 

4.  San  Francisco  Mountain,  Ariz. 

5.  Crater  Lake,  Ore. 

Note, — If  models  of  any  of  these  mountains  are  available,  study  them 
in  connection  with  the  maps.  Photographs  of  the  mountains  appear  in 
some  of  the  works  of  the  reference  list,  and  these  may  well  be  studied 
in  connection  with  the  maps. 

For  Crater  Lake  and  its  surroundings,  see  reference  10,  and  for 
Mt.  Shasta,  reference  7,  at  end  of  chapter. 

II.  1.  Make  a  special  study  of  the  topography  of  the  volcanic  mountains 
to  determine  how  far  their  present  configuration  is  the  result 
of  the  original  cone-building,  and  how  far  the  result  of  subse- 
quent erosion. 
2.  Are  craters  in  original  or  modified  form  shown  on  any  of  the  maps? 

REFERENCES 

1.  Standard  text-books  on  Geology. 

2.  RUSSELL,  Volcanoes  0}  Xorth  America:   Macmillan. 

3.  JUDD,  Volcanoes:  Appleton. 

4.  BONNET,  Volcanoes:  G.  P.  Putnam's  Sons. 

5.  Recent  Eruptions  in  the  West  Indies.     HEJLPRIN,  Mt.  PeUe  and  the 
Tragedy  of  Martinique,  Lippincott;    HILL,  Nat.  Geog.  Mag.,  Vol.  XIII,  pp. 
223-267;     RUSSELL,  Nat.  Geog.  Mag.,   Vol.  XIII,  pp.   267-285,  415-435; 
HOVEY,  Nat.  Geog.  Mag.,  Vol.  XIII,  pp.  444-459,  Am.  Jour.  Sci.,  Vol.  XIV, 
1902,  pp.  319-358,  and  Bull.  Am.  Mu?.  Nat.  Hist.,  Vol.  XVI,  pp.  333-372. 

6.  Rept.  of  the  Roy.  Soc.  on  The  Eruption  of  Krakatoa:   Tiiibner  &  Co. 

7.  DILLER,  Mt.  Shasta,  in  Physiography  of  the  United  States:  Am.  Book  Co. 


VULCAN  ISM 


391 


8.  BUTTON,  Hawaiian  Volcanoes:    4th  Ann.  Kept.  U.  S.  Geol.  Surv. ; 
and  Mt.  Taylor  and  the  Zuni  Plateau:    6th  Ann.  Rept.  U.  S.  Geol.  Surv. 

9.  GILBERT,  Geology  of  the  Henry  Mountains:  U.  S.  Geol.  Surv. 

10.  DILLER  and  PATTON,  Geology  and  Petrography  of  Crater  Lake  Park, 
Part  I:    Professional  Paper  3,  U.  S.  Geol.  Surv. 


FIG.  417.— Mt.  Wrangell,  Alaska.     (U.  S.  Geol.  Surv.) 


CHAPTER  VIII 
CRUSTAL  MOVEMENTS.     DIASTROPHISM 

. SECULAR  CHANGES 

IN  many  places  the  coastal  lands  appear  to  have  recently 
risen  from  the  sea,  while  in  others  coastal  tracts  appear  to  have 
been  recently  submerged.  The  most  obvious  evidence  of  the 
apparent  rise  is  found  in  the  beaches  and  other  shore  features 
now  well  above  sea-level,  and  the  most  obvious  evidence  of 
sinking  is  found  in  the  drowned  lower  ends  of  rivers  (p.  173). 
These  relative  changes  of  level  of  the  land  are  best  seen  along  the 
sea-shore,  because  the  sea-level  is  the  place  from  which  land  ele- 
vations are  measured.  Movements  of  the  outer  portions  of  the 
solid  part  of  the  earth  are  also  in  progress,  or  have  recently  taken 
place,  far  from  coasts,  but  they  are  not  so  readily  detected,  and 
are  therefore  less  well  known.  Some  of  them  have  been  referred 
to  incidentally  in  other  connections  (p.  174).  The  changes  of  level 
are  in  general  so  slow  that  no  motion  is  seen  from  day  to  day,  or 
even  from  year  to  year.  All  that  is  seen  is  the  result  of  changes 
which  have  been  going  on  slowly  for  centuries. 

Movements  of  the  earth's  crust  (outer  parts  of  the  lithosphere) 
were  first  inferred  from  various  phenomena  along  the  shores. 
They  were  subsequently  demonstrated  by  careful  measurements 
which  have  shown  not  only  the  fact  of  movement,  but  in  some 
cases  its  rate. 

It  is  to  be  especially  noted  that  beaches  or  other  shore  marks 
above  the  present  level  of  the  sea  do  not  necessarily  mean  that  the 
land  has  risen.  They  might  mean  depression  of  the  sea-level 
instead,  but  in  either  case  they  mean  increased  emergence  of  the 
land.  Similarly,  the  lower  ends  of  valleys  would  be  drowned  by 
the  rise  of  the  sea  just  as  effectively  as  by  the  sinking  of  the  land; 
but  in  either  case  there  has  been  a  depression  of  the  land  relative 

392 


CRUSTAL  MOVEMENTS.    DIASTROPHISM  393 

to  sea-level.  In  some  situations  and  relations  it  may  be  possible 
to  say  whether  it  is  the  land  or  the  sea  surface  which  has  changed 
its  position;  but  in  general  it  is  best  to  think  of  the  changes  as  rela- 
tive. 

Evidences  of  (Relative)  Elevation  of  Land 

1.  Human  structures.     In  certain  regions  which  have  been  long 
inhabited,  structures  which  were  once  at  sea-level  are  now  above  it. 
Thus  on  the  island  of  Crete,  in  the  Mediterranean  Sea,  the  remains 
of  old  docks  are  in  some  places  as  much  as  27  feet  above  the  water. 
This  is  the   more  extraordinary  since  other  parts  of  the  same 
island  have  sunk  so  as  to  submerge  human  structures,  the  ruins 
of  which  are  still  visible  beneath  the  water. 

2.  Rocks.     Several  rocks  in  the  Baltic  Sea  which  within  his- 
toric time  were  at  sea-level,  or  so  little  below  it  as  to  be  dangerous 
to  navigators,  are  now  well  above  the  water.     One  is  said  to  have 
risen  about  60  feet  in  50  years.     From  an  inscription  supposed  to 
be  about  500  years  old  in  Lake  Maelar  (near  the  Baltic),  the  land 
at  that  point  is  inferred  to  have  risen  about  13  feet  in  the  50  years 
preceding  1730. 

3.  Measurements.    Changes  of  level  were  recognized  long  ago 
in  Scandinavia,  and  they  excited  so  much  interest  that  marks 
were  made  on  the  shores  at  different  places  and  kept  under  obser- 
vation for  a  period  of  years  in  order  to  determine  the  rate  of  change 
of   level   between   land  and  water.     These  observations  showed 
that    the  coast  was  gradually  becoming    higher  relative  to  the 
Baltic. 

In  recent  times  it  has  been  found  that  the  larger  part  of  the 
coast  of  Scandinavia  is  rising  relative  to  sea-level,  but  that  it  is 
rising  at  unequal  rates,  and  that  the  southern  extremity  is  sink- 
ing. In  some  localities  the  rate  of  rise  has  been  determined  to  be 
about  2 \  feet  per  century. 

4.  Organic  remains ;  fossils.     Another  line  of  evidence  pointing 
to  the  rise  of  coasts  is  found  in  the  shells,  tests,  etc.,  of  marine  animals 
found  above  sea-level.     Thus  barnacle  shells  are  sometimes  found 
above  the  surface  of  the  water,  attached  to  the  rocks  where  they 
grew.     There  is  no  escape  from  the  conclusion  that  the  sea-level 
has  sunk,  or  the  land  risen,  to  the  extent  of  their  height  above  sea- 
level.     Beds  of  marine  shells  accumulated   beneath   the  sea   are 
also  sometimes  found  above  sea-level.     Such  shells  are  conclusive 


394  PHYSIOGRAPHY 

of  the  relative  rise  of  the  land,  in  case  they  are  known  to  have  been 
deposited  by  the  sea-water.  The  evidence  from  unattached  shells 
must,  however,  be  carefully  scrutinized,  since  birds  and  other 
animals  frequently  carry  marine  shells  inland  and  up  to  con- 
siderable heights. 

Beds  containing  sea-shells  which  were  certainly  deposited  be- 
neath the  sea  are  now  found  above  the  water  at  various  points,  as 
along  the  coast  of  Sweden,  and  at  some  points  in  North  Greenland, 
where  they  occur  up  to  heights  of  100  to  200  feet.  The  shells 
here  are  so  fresh  that  in  some  cases  they  are  still  covered  with  the 
epidermis.  The  sand  in  which  they  are  imbedded  is  frozen  during 
a  large  part  or  all  of  the  year,  and  the  low  temperature  undoubtedly 
keeps  the  organic  matter  from  decay.  Darwin  long  ago  found 
shells,  etc.,  along  the  west  coast  of  South  America  up  to  eleva- 
tions of  1300  feet  above  sea-level.  On  the  coast  of  Peru  a  reef 
of  corals  of  modern  species  is  said  to  be  found  at  an  elevation  of 
nearly  3000  feet.  On  the  coast  of  the  New  Hebrides,  coral  reefs 
occur  up  to  2000  feet,  and  on  the  coast  of  Cuba  up  to  heights  of 
1000  feet  or  more. 

5.  Raised  beaches,  etc.     Raised  beaches  and  terraces  along  the 
shore  are  also  evidences  of  change  of  level.     Such  features  are 
found  along  many  parts  of  the  northern  coast  of  western  Europe 
and  eastern  North  America,  about  the  West  Indies,  on  the  Cali- 
fornia coast,  and  in  many  other  places.     Along  certain  coasts,  for 
example  those  of  California  and  Scotland,  towns  are  situated  on 
these  elevated  terraces,  and  wragon-roads  and  railroads  follow  them 
for  considerable  distances. 

One  of  the  significant  facts  concerning  the  elevated  beaches  and 
other  shore  features  is  that  they  are  no  longer  horizontal.  The 
island  of  Crete,  already  cited,  affords  an  illustration,  and  the  coast 
of  Scandinavia  another. 

6.  Sea  cliffs.    Sea  cliffs  (Fig.  418)  developed  by  wave-cutting 
are  sometimes  found  above  the  elevated  shore  terraces. 

7.  Sea  caves.     Waves  sometimes  develop  sea  caves  at  water 
level.    Caves  developed  in  this  way  are  now  sometimes  found  at 
levels  considerably  above  the  sea.     Such  caves  occur  on  the  coast 
of  Scotland  up  to  levels  of  100  feet. 

All  these  phenomena  are  evidences  that  the  land  has  risen 
relative  to  the  sea,  in  many  places,  in  recent  times. 


PLATE  XXIII 


A  section  of  the  California  coast,  showing  lands,  near  the  coast,  which  have 
recently  emerged.  Scale  1—  mile  per  inch  (Oceanside,  Cal.,  Sheet,  U.  8. 
Geol.  biirv.) 


PLATE  XXIV 


Cushetunk  and  Round  Mountains,  New  Jersey ;  examples  of  isolated  moun- 
tains left  by  the  removal  of  less  resistant  surroundings.  Scale  1 —  mile 
per  inch.  (High  Bridge  Sheet,  U.  S.  Geol.  Surv.) 


CRUSTAL  MOVEMENTS.    DIASTROPHISM  395 

Evidences  of  Relative  Depression 

Evidences  of  the  sinking  of  land  are,  in  the  nature  of  the  case, 
less  readily  seen,  because  they  are  for  the  most  part  beneath  the 
water. 

1.  Human  structures.  It  has  already  been  noted  that  at  the 
east  end  of  the  island  of  Crete  ancient  buildings  are  submerged. 


FIG.  418. — Wave-cut  terraces.     West  side  of  Ojai  Valley,  Cal.     (Arnold.) 

Certain  portions  of  the  coast  of  Greenland  have  likewise  been  sink- 
ing recently,  for  various  human  structures  on  low  coasts  have 
sunk,  and  still  stand  beneath  the  water.  The  southern  end  of 


FIG.  419. — Wave-cut  terraces.    Bottle  and  glass.    St.  Vincent. 
(Hovey,  Am.  Mus.  Nat.  Hist.) 

Scandinavia,  as  already  noted,  has  been  sinking  recently,  while 
the  rest  of  the  peninsula  appears  to  have  been  rising.  "At  Malmo 
one  of  the  present  streets  is  over-flooded  by  the  waters  of  the  Baltic 
when  the  wind  is  high,  and  excavations  made  some  years  ago  dis- 
closed an  ancient  street  at  a  depth  of  eight  feet  below  the  present 
one." 


396 


PHYSIOGRAPHY 


2.  Submerged  forests.  Along  some  coasts  there  are  evidences  of 
submerged  forests.  This  is  the  case,  for  example,  at  some  points 
north  of  Liverpool,  England  (Fig.  420).  Here,  when  the  tide  is 


FIG.  420. — Stumps  laid  bare  on  the  beach  at  low  tide.     Lease  we,  Cheshire. 

Eng.     (Ward.) 

out,  numerous  stumps  may  be  seen  standing  on  the  beach  where  the 
trees  once  grew.  Since  trees  of  the  varieties  represented  by  these 
stumps  do  not  grow  in  salt  water,  there  is  no  alternative  but  to 
conclude  that  the  land  where  they  grew,  once  dry,  has  since  sunk 


FIG.  421. — Masses  of  peat  and  tide  marsh  sod,  broken  up  by  storm  waves 
and  washed  ashore  near  Sea  Isle  City,  N.  J.     (Knapp,  N.  J.  Geol.  Surv.) 

below  the  level  of  high  water.  On  the  coast  of  New  Jersey 
stumps  have  been  found  seven  feet  below  sea-level  at  low  tide. 
Old  marsh-lands  are  submerged  beneath  the  sea  along  some 
shores.  From  them  the  strong  waves  of  storms  sometimes 
wrench  large  masses  of  peat  and  toss  them  up  on  the  shore. 


CRUSTAL  MOVEMENTS.     DIASTROPHISM 


397 


This  was  the  case  at  Cape  May  in  New  Jersey  in  a  violent  storm 
but  a  few  years  ago  (Fig.  421). 

3.  Submerged  valleys.  Some  river  valleys  on  land  are  con- 
tinuous with  valleys  in  the  shallow  sea  bottom  far  out  beyond  the 
coast-line  (Fig.  422).  Such  submerged  valleys  indicate  that  the 
surface  in  which  they  are  cut  was  land  when  they  were  excavated, 
and  that  subsequent  sinking  has  submerged  them.  The  numerous 
bays  along  the  eastern  coast  of  the  United  States,  especially  between 
New  York  and  the  Carolinas,  indicate  recent  sinking  of  the  land, 


FIG.  422. — The  submerged  valley  which  is  believed  to  be  the  continuation 
of  the  Hudson  Valjey.  The  position  of  the  valley  is  indicated  by  the 
contours.  Depths  in  fathoms.  (Data  from  C.  and  G.  Survey.) 

enough  to  carry  the  lower  ends  of  the  former  valleys  below  sea- 
level,  thus  converting  them  into  bays.  Submerged  valleys  of  this 
sort  occur  in  many  parts  of  the  earth,  and  show  that  submergence 
of  coastal  lands  is  now  taking  place,  or  has  recently  taken  place, 
along  many  coasts. 

4.  An  Italian  temple.  One  of  the  most  striking  cases  of  change 
of  level  appears  to  involve  both  upward  and  downward  move- 
ment. On  the  shore  of  Italy,  near  Naples,  are  the  ruins  of  an  old 
temple.  From  inscriptions  it  is  known  to  have  been  above  water 
as  late  as  235  A.D.  In  1749  several  columns  of  the  temple  were 


398  PHYSIOGRAPHY 

found  still  standing.  Their  bases  were  buried  to  a  depth  of  12  feet 
in  sediment  deposited  by  the  sea.  For  9  feet  above  the  sediment, 
the  columns  were  filled  with  holes  bored  by  marine  animals.  It 
is  inferred,  therefore,  that  between  the  years  235  and  1749,  the  land 
on  which  the  temple  stood  sank  until  the  water  stood  21  feet  above 
the  bases  of  the  columns,  and  then  rose  again  so  that  its  floor  was 
above  sea-level. 

7s  it  the  Land  or  the  Sea  which  Changes  its  Level  f 

It  is  clear  that  if  the  outside  of  the  lithosphere,  commonly  called 
the  crust  of  the  earth,  is  subject  to  warping,  some  parts  rising  and 
some  sinking,  the  observed  phenomena  of  coasts  could  be  readily 
explained;  and,  in  most  discussions  of  the  changes  of  relative  level 
of  land  and  sea,  it  is  commonly  assumed  that  the  land,  rather  than 
the  sea,  is  the  element  which  changes.  The  validity  of  this  assump- 
tion has,  however,  been  questioned,  and  with  reason.  The  alter- 
natives are  several,  namely:  (1)  Is  it  the  sea  rather  than  the  land 
which  changes  its  level?  Or  (2)  do  both  land  and  sea  change  their 
levels?  In  the  latter  case  (a)  does  each  rise  and  fall,  or  (6)  does 
one  rise  and  fall  while  the  other  rises  or  falls  only?  Certain  gen- 
eral considerations  will  make  it  clear  that  some  of  these  alternatives 
are  not  tenable. 

Other  conditions  remaining  constant,  the  sea-level  would  be  de- 
pressed everywhere  if  depressed  at  one  point,  because  all  oceans 
are  connected.  In  this  case,  all  coasts  would  appear  to  rise.  Simi- 
larly, if  the  sea-level  rose  at  one  point,  it  would  seem  that  it  should 
rise  everywhere,  in  which  case  all  coasts  should  appear  to  be  sink- 
ing at  the  same  time.  Since  some  coasts  seem  to  be  rising  while 
others  seem  to  be  sinking,  it  is  clear  that  the  changes  of  level  of 
the  sea  surface,  taken  alone,  do  not  explain  the  observed  phe- 
nomena. It  does  not  follow,  however,  that  such  changes  may  not 
be  one  of  the  elements  involved  in  the  explanation  of  the  observed 
phenomena. 

Without  discussing  all  the  other  alternatives  separately,  the 
principles  involved  in  the  problem  may  be  readily  understood. 

Let  us  suppose  the  sea-level  to  be  lowered  at  all  points,  as  it 
would  be  by  the  sinking  of  the  bottom  of  any  one  of  the  ocean 
basins,  and  let  us  suppose  further  that  the  continents  sink  at  the 
same  time.  Under  this  general  conception  various  cases  arise. 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


399 


(1)  If  the  lands  are  lowered  as  much  as  the  sea-level,  and  lowered 
equally  everywhere,  the  relation  of  coasts  to  sea-level  would  not 
be  changed.  (2)  If  the  land  were  lowered  more  than  the  sea,  and 
lowered  equally  everywhere,  the  old  coast-lines  would  all  be  sub- 
merged. (3)  If  the  land  were  lowered  less  than  the  sea,  and 
lowered  equally  everywhere,  all  coasts  would  appear  to  have  risen. 
(4)  If  different  parts  of  the  coasts  sank  unequally,  those  parts 
which  sank  less  than  the  sea  would  appear  to  have  risen,  those 
which  sank  as  much  as  the  sea  would  not  have  changed  their 
position  relative  to  sea-level,  and  those  which  sank  more  than  the 
sea  would  appear  to  have  sunk. 

All  these  relations  are  illustrated  by  Figs.  423  to  425.      The 
sea-level,  which  in  Fig.  423  is  at  AB,  is  represented  in  Fig.  424  as 


FIG.  423. — Diagram  showing  a  coastal  tract  rising  above  sea-level. 


having  sunk  from  A'  to  Bf.  In  Fig.  425  the  sea-level  of  Fig.  423  is 
represented  as  having  sunk,  but  the  land  of  the  old  coast-line  has 
sunk  more  than  the  sea  from  A"  to  C,  as  much  as  the  sea  at  C,  and 


FIG.  424. — Diagram  showing  the  same  coast  as  Fig.  423,  after  the  sea-level 
has  been  lowered  uniformly.     The  land  appears  to  have  risen. 

less  than  the  sea  from  C  to  B".     From  A"  to  C  the  coast  seems  to 
have  sunk,  from  C  to  B"  it  seems  to  have  risen. 

It  appears,  therefore,  that  all  apparent  risings  and  sinkings 
along  coasts  might  be  explained  by  unequal  sinking  of  land  while 
the  sea-level  is  being  lowered;  but  it  does  not  follow  that  this  is 
necessarily  the  true  explanation.  Existing  phenomena  about 
coasts  may  be  explained  on  the  supposition  that  coastal  lands 
rise  locally  and  sink  locally,  as  well  as  on  the  supposition  that  they 
sink  only.  They  may  be  equally  well  explained  on  the  supposi- 
tion that  the  sea-level  sometimes  rises  and  sometimes  sinks,  at  the 
same  time  that  the  coasts  are  being  warped,  in  some  places  up  and 
in  some  places  down.  On  the  whole,  it  seems  probable  that  the 


400 


PHYSIOGRAPHY 


sea-level  does  change,  sometimes  rising  and  sometimes  sinking, 
and  that  coastal  lands  are  warped  upward  in  some  places  and 
downward  in  others,  and  that  observed  phenomena  involve  all 
these  movements. 

Current  theories  of  the  origin  and  history  of  the  earth  all  pro- 
ceed on  the  assumption  that  the  earth  is  a  cooling  and  therefore  a 


0 


FIG.  425.- — Diagram  showing  the  same  area  as  the  preceding.  The  sea  has 
sunk  as  much  as  in  Fig.  424,  but  the  land  at  the  left  has  also  sunk,  and 
has  sunk  more  than  the  sea-level  has.  At  the  left,  therefore,  the  land 
seems  to  have  sunk  and  at  the  right  it  seems  to  have  risen,  while  at  one 
point,  C,  it  appears  to  have  neither  risen  nor  sunk. 

contracting  body.  If  this  be  true,  it  is  clear  that  depressions  rather 
than  elevations  of  the  surface  must  be  the  rule,  and  that  such 
elevations  of  the  surface  of  the  lithosphere  as  take  place  are  in- 
cidental to  the  general  lowering  of  surface  which  results  from 
contraction. 

We  may  now  inquire  into  the  causes  which  make,  or  which  may 
make,  the  surface  of  the  sea  rise  and  fall,  and  also  into  the  causes 
which  make,  or  may  make,  the  surface  of  the  lithosphere  rise  and 
fall  locally. 

Why  the  Sea-level  Changes 

Sedimentation.  Rivers  carry  a  large  amount  of  sediment 
from  the  land  to  the  sea  each  year  (p.  154).  This  material,  de- 
posited on  the  sea  bottom,  tends  to  fill  the  ocean  basins.  The 
result  must  be  rise  of  the  sea-level.  The  detritus  worn  from  the 
shores  by  waves,  blown  from  the  land  by  winds,  and  brought  to 
the  sea  by  glaciers,  is  likewise  deposited  beneath  the  water  and 
produces  the  same  result.  The  material  extracted  from  the  sea- 
water  by  plants  and  animals,  and  deposited  on  its  bottom  when 
they  die,  likewise  helps  to  raise  the  surface  of  the  sea,  for  the  space 
occupied  by  shells,  etc.,  exceeds  the  reduction  of  volume  suffered 
by  the  water  when  the  material  of  the  shells  was  extracted  from 
it.  Rise  of  the  surface  of  the  sea  due  to  sedimentation  would  be 
universal. 


CRUSTAL  MOVEMENTS.     DIASTROPHISM  401 

The  rise  of  the  sea  due  to  sedimentation  is  extremely  slow,  too 
slow  to  be  obvious  from  year  to  year,  or  perhaps  even  in  a  lifetime. 
But  if  existing  lands  were  base-leveled,  the  resulting  rise  of  sea- 
level  would  be  hundreds  of  feet,  enough  to  submerge  a  very  con- 
siderable part  of  the  existing  land,  and  a  much  larger  part  of  the 
land  as  it  would  be  after  base-leveling.  There  is  reason  to  be- 
lieve that  great  areas  have  been  nearly  base-leveled  in  the  past. 
Degradation  of  the  land  and  aggradation  of  the  sea  bottom  may, 
therefore,  have  been  very  important  factors  in  the  repeated  sub- 
mergence of  great  areas  of  land  in  past  ages. 

It  is  presumed  that  evaporation  from  the  sea  is  balanced  by 
rainfall  and  by  the  inflow  of  rivers.  If  this  is  the  case,  evapora- 
tion and  precipitation  do  not  affect  its  level.  But  if  great  quanti- 
ties of  water  evaporated  from  the  sea  were  to  be  retained  on  the 
land  in  the  form  of  ice,  as  it  was  in  the  glacial  period,  the  result 
would  be  a  lowering  of  the  sea-level.  The  melting  of  the  ice,  on 
the  other  hand,  would  cause  the  sea-level  to  rise.  The  ice  of  the 
glacial  period  was  of  such  quantity  that  its  withdrawal,  on  the  one 
hand,  and  its  return,  on  the  other,  would  have  influenced  the 
surface  of  the  sea  sensibly. 

Submarine  volcanic  extrusions.  Extrusions  of  lava  beneath 
the  sea  likewise  cause  its  surface  to  rise,  as  would  also  laccolithic 
and  other  intrusions  beneath  its  bottom. 

Diastrophism.  While  sedimentation  and  vulcanism  have 
surely  caused  changes  in  the  level  of  the  sea,  it  is  not  believed  that 
they  have  been  the  only  causes  which  have  produced  such  changes; 
neither  is  it  believed  that,  in  the  long  course  of  time,  they  have 
been  the  causes  of  the  most  profound  changes. 

The  progressive  cooling  of  the  earth  has  resulted  in  its  pro- 
gressive shrinking  since  the  time  when  it  attained  its  growth  and 
its  maximum  heat.  This  shrinking  must  have  resulted  in  the 
deformation  of  its  outer  part,  for  the  same  reason  that  the  skin 
of  an  apple  wrinkles  when  its  juice  evaporates.  As  the  cooling 
and  contraction  are  constant,  it  appears  that  slow  warpings  of  the 
crust  may  also  be  constant;  but  it  appears  also  that  the  rigidity 
of  the  earth  may  be  such  that  its  outer  parts  are  able  to  withstand 
for  a  time  the  strain  set  up  by  contraction.  As  the  strain  accumu- 
lates, it  ultimately  overcomes  the  resistance,  and  the  outer  part 
of  the  earth  yields.  If  the  yielding  results  in  the  sinking  of  the 
ocean  basin,  the  surface  of  the  water  is  drawn  down,  and  the  sur- 


402  PHYSIOGRAPHY 

rounding  lands  seem  to  rise,  unless  they  sink  as  much  as  the  surface 
of  the  sea  does  at  the  same  time.  The  lowering  of  the  sea  surface, 
because  of  the  sinking  of  the  sea  bottom,  is  probably  the  most 
fundamental  single  cause  of  the  apparent  rise  of  land. 

The  periodic  emergences  of  the  continents,  alternating  with 
periodic  submergences  in  the  course  of  geological  history,  are 
perhaps  to  be  thus  explained.  Periodic  submergences,  on  the 
other  hand,  might  be  explained  by  the  sinking  of  the  continental 
segments  of  the  earth,  or  by  such  sinking  combined  with  the  proc- 
esses already  referred  to  which  cause  the  rise  of  the  sea. 

Why  the  Land  Changes  Level 

The  reasons  assigned  for  changes  of  level  of  the  lithosphere 
beneath  the  sea  have  equal  force  when  applied  to  the  land.  It 
is  probable  that  the  continents  sink  more  in  the  course  of  ages  than 
they  rise,  but  that  they  sink  less  than  the  ocean  basins.  A  local  rise 
of  the  surface  may  result  from  a  more  general  sinking.  A  de- 
pression of  ocean  basins,  for  example,  may  crowd  up  the  conti- 
nental area  between,  and  the  same  principle  may  be  applicable  to 
smaller  areas.  Again,  in  volcanic  regions,  the  intrusion  of  lava 
may  cause  the  surface  to  rise,  as  over  a  laccolith,  and  the  bringing 
of  the  hot-rock  material  near  to  the  surface  heats  the  surface  rocks 
and  perhaps  expands  them  enough  to  cause  sensible  rise  of  the 
surface. 

We  are  to  conclude,  therefore,  that  the  apparent  rise  of  the 
land  along  coasts  is  probably  due  in  part  to  the  sinking  of  the  sea, 
in  part  to  the  lesser  sinking  of  the  coastal  lands,  as  compared  with 
the  sea,  and  in  part  to  the  actual  rise  of  the  land  itself. 

Changes  of  Level  in  the  Interiors  of  Continents 

General  facts.  Changes  of  level  are  perhaps  as  common  in 
the  interiors  of  continents  as  along  coasts,  though  less  easily  de- 
tected. There  are  raised  beaches  about  many  lakes,  as  about  the 
Great  Lakes  and  Great  Salt  Lake  (Fig.  361).  Raised  beaches 
about  lakes  result  from  the  lowering  of  the  level  of  the  lakes,  either 
by  the  cutting  down  of  their  outlets  or  by  evaporation.  They 
do  not,  therefore,  indicate  a  rise  of  the  land.  In  both  cases  cited, 
however,  the  old  shore-lines  should  remain  horizontal.  But  about, 
many  lakes  the  old  shore-lines  are  not  level,  as  they  must  have 


CRUSTAL  MOVEMENTS.    DIASTROPHISM  403 

been  when  formed.  Some  parts  of  one  of  the  old  shore-lines  about 
Lake  Bonne ville  (p.  314)  are  300  feet  higher  than  other  parts 
formed  at  the  same  time.  An  old  shore-line  about  the  east  end  of 
Lake  Ontario  is  more  than  400  feet  above  the  lake,  while  the  same 
shore-line  traced  westward  passes  beneath  the  water  at  the  west 
end  of  the  lake.  Similar  phenomena  are  found  about  the  shores  of 
all  the  Great  Lakes,  though  the  departure  from  horizontality  is 
not  everywhere  so  great.  Such  deformed  shore-lines  show  that  the 
surface  about  the  lake  basins  has  warped  since  the  old  shore-lines 
were  formed. 

The  former  shore-lines  of  many  smaller  lakes  and  of  some  ex- 
tinct lakes  are  also  well  known,  and  they  tell  the  same  story. 
This  is  notably  the  case  with  the  shore-lines  of  the  extinct  Lake 
Agassiz  (p.  282). 

Changes  of  level  are  still  in  progress.  The  accurate  observations 
and  measurements  of  recent  years  have  shown  that  the  area  of  the 
Great  Lakes  is  being  tilted  upward,  relatively,  to  the  northeast,  and 
downward,  relatively,  to  the  southwest.  The  rate  has  been  shown 
to  be  less  than  six  inches  per  hundred  miles  per  century. 

Extent.  So  wide-spread  are  the  evidences  of  changes  of  level 
that  it  may  be  said  that,  within  regions  so  situated  as  to  furnish 
evidence,  more  of  the  earth's  surface  has  been  sinking  or  rising  in 
recent  times,  than  has  been  standing  still. 

This  general  statement  seems  to  point  to  great  instability  of 
the  earth's  crust,  but  it  should  be  supplemented  by  the  statement 
that  these  changes  go  on,  as  a  rule,  with  extreme  slowness  and, 
in  general,  probably  without  violence.  The  amount  of  move- 
ment is,  perhaps,  to  be  reckoned  in  small  fractions  of  an  inch  per 
year,  more  commonly  than  in  larger  terms.  At  times  and  places, 
however,  it  is  probable  the  movements  have  been  more  rapid,  but 
even  in  these  cases  it  is  not  to  be  supposed  that  the  movements  were 
always  violent. 

The  instability  of  the  earth's  exterior  is  believed  to  indicate  that 
it  is  not  in  perfect  adjustment  to  the  interior,  and  that  the  con- 
tinued lack  of  adjustment  is  the  result  of  the  continued  shrinking  of 
the  earth  as  a  whole. 

Ancient  changes  of  level.  Old  shore-lines  and  all  features 
connected  with  ocean  shores  are  destroyed  in  time  by  erosion;  but 
the  evidence  of  movements  which  took  place  so  long  ago  that  no 
traces  of  shore-lines  remain,  is  firm.  Thus  layers  of  rock  which 


404  PHYSIOGRAPHY 

were  once  deposited  as  sediment  (sand,  mud,  etc.)  beneath  the 
sea  are  now  found  over  great  areas,  far  above  sea-level.  Most  of 
the  solid  rock  beneath  the  Mississippi  basin,  for  example,  was  de- 
posited as  sediment  beneath  the  sea.  The  land  has  emerged, 
perhaps  because  the  sea-level  has  been  drawn  down  by  the  sink- 
ing of  the  sea  basin.  In  the  Appalachian  Mountains,  rocks  simi- 
larly formed  are  found  up  to  altitudes  of  a  few  thousand  feet.  In 
the  Rocky  Mountains  they  occur  as  high  as  10,000  feet  or  even 
more.  In  the  Andes  Mountains  they  are  found  in  limited  areas 
up  to  16,000  feet  or  more,  and  in  the  Himalaya  Mountains  at 
still  greater  heights.  In  these  extreme  cases,  at  least,  it  seems 
probable  that  there  has  been  an  actual  rise  of  the  crust,  that  is, 
that  the  outside  has  been  bent  up,  or  thrust  up  locally,  so  as  to 
be  farther  from  the  centre  of  the  earth  than  when  in  its  former 
position. 

Future  changes  of  level.  Not  only  have  changes  of  level 
between  land  and  sea  been  taking  place  for  untold  ages,  but  they 
are  likely  to  continue.  The  wear  of  the  land  and  the  transfer  of 
sediment  to  the  sea  tends  to  raise  the  sea-level,  as  already  noted 
(p.  400).  This  tends  to  increase  the  area  of  the  sea  and  to  corre- 
spondingly diminish  the  area  of  the  land.  In  the  past  there  seem 
to  have  been  occasional  sinkings  of  the  ocean  basins,  increasing 
their  capacity  and  drawing  down  the  level  of  the  sea,  thus  caus- 
ing the  continents,  as  a  whole,  to  appear  to  rise,  and  such  changes 
are  likely  to  occur  in  the  future,  so  far  as  can  now  be  seen.  On 
the  average,  the  lowering  of  the  sea-level,  due  to  the  subsidence 
of  its  basins,  has  probably  been  greater  than  the  rise  of  the  sea- 
level,  due  to  sedimentation  from  the  land.  The  result  is  that  as 
the  continents  have  been  brought  low  by  wind  and  water  and 
ice,  they  have  been  renewed,  periodically,  by  the  sinking  of  the 
sea. 

In  this  general  sequence  of  events  appears  to  lie  the  explana- 
tion of  the  fact  that  though  rain  and  river  and  ice  erosion  tend  to 
bring  the  land  to  base-level,  and  though  wave  erosion  tends  to 
reduce  it  even  below  sea-level,  the  land  is  not  destroyed,  and  is  not 
even  completely  reduced  to  base-level. 

As  the  great  ocean-basin  segments  of  the  earth  settle  down, 
it  seems  possible  that  the  smaller  continental  segments  between 
may  sometimes  be  wedged  up,  and  perhaps  warped  and  deformed 
at  the  same  time.  In  this  process  may  lie  the  explanation  of  many 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


405 


mountains,  plateaus,  and  plains,  the  physiographic  features  of  the 
second  order. 

Crustal  Deformation 

The  foregoing  discussion  of  changes  of  level  has  implied  a 
measure  of  deformation  of  the  outside  of  the  solid  part  of  the  earth. 


FIG.  426. — Open  anticlinal  fold,  near  Hancock,  Md.     (U.  S.  Geol.  Surv.) 

This  deformation  sometimes  takes  the  form  (1)  of  gentle  warping, 
sometimes  (2)  of  folding,  and  sometimes  shows  itself  (3)  in  faulting. 

Warping  and  folding.  The  warping  may  be  gentle,  resulting  in 
slight  arching  or  tilting  of  the  beds  of  rock,  or  it  may  be  so  great 
that  the  arches  grade  into  folds  (Figs.  426  and  427).  Most  rock 
strata  of  the  land  are  at  least  gently  deformed,  and  folding  is  com- 
mon in  many  mountain  regions,  and  in  some  plains  which  were 
once  high,  but  which  have  been  brought  low  by  erosion. 

Warping  and  folding  give  rise  to  great  topographic  features, 
but  in  most  mountains  of  folded  rock,  the  present  topography  is 
the  result  of  erosion  rather  than  of  the  original  folding.  The 
structure  of  the  rock  resulting  from  the  folding  has,  in  many 
cases,  determined  or  greatly  influenced  the  topography  which  has 
resulted  from  erosion. 


406 


PHYSIOGRAPHY 


Faulting.  At  many  times  and  in  many  places  portions  of 
the  earth's  surface  have  sunk  or  risen  along  a  plane  of  fracture, 
as  shown  by  Figs.  429  and  430.  Such  movements  are  faults.  Fig. 
428  represents  a  normal  or  gravity  fault,  and  Fig.  429  a  reversed 
or  thrust  fault.  The  former  implies  tension  when  it  was  made, 
and  the  adjustment  takes  place  under  the  control  of  gravity. 
The  latter  implies  lateral  thrust,  and  the  adjustment  takes  place 
under  the  control  of  such  pressure. 

Faults  of  both  types  are  common,  but  reversed  faults  are 


FIG.  427. — Closed  anticlinal  fold,  near  Levis  Station,  Quebec. 
(U.  S.  Geol.  Surv.) 

common  only  in  regions  where  the  rock  strata  have  been  folded. 
Fig.  429  suggests  the  relation  between  such  a  fault  and  a  fold. 
A  fold  which  is  not  faulted  sometimes  passes  into  a  reversed  or 
thrust  fault.  Normal  faults  may  also  grade  into  folds,  especially 
monodinal  folds  (Fig.  430).  Faulted  blocks  of  the  earth's  crust  are 
sometimes  tilted.  They  may  be  of  such  size  and  so  displaced  as 
to  give  rise  to  mountains,  basins,  etc.  (Figs.  331  and  433).  Nor- 
mal faulting  has  taken  place  on  a  great  scale  in  the  plateau  region 
of  the  West,  between  the  Rocky  Mountains  and  the  Sierras,  and 
many  of  the  more  striking  topographic  features  of  that  region, 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


407 


including  numerous  mountain  ranges  and  lines  of  cliffs  (Fig.  433), 
are  the  result  of  such  movements.  Cliffs  or  mountain  slopes  due 
to  faulting  are  called  fault-scarps.  The  steep  slopes  at  the  borders 
of  the  continental  segments  are  probably  fault-scarps  in  some 


FIG.    428.  —  Diagram    of    a 
normal  fault. 


FIG.  429. — Diagram  of  a  reversed  or 
thrust  fault.     (U.  S.  Geol.  Surv.) 


cases  (p.  13).     Great  faults  are  probably  not  developed  by  a  single 
movement,  but  by  repeated  displacements  along  the  same  plane. 

Though  faults  are  common  phenomena,  only  those  of  recent 
times  now  show  themselves  in  the  topography  of  the  surface. 


FIG.  430. — Fault  passing  into  a 
monoclinal  fold. 


FIG.  431. — A  branching  fault.    (Powell, 
U.  S.  Geol.  Surv.) 


Fault-scarps  of  earlier  ages  have  been  obliterated  by  erosion, 
though  the  faults  still  show  themselves  in  the  structure  of  the  rock. 
Faults  have  determined  the  positions  of  valleys  in  many  cases, 
so  that  present  topographic  features  are  often  closely  associated 
with  them,  even  though  the  fault-scarps  have  vanished. 


408 


PHYSIOGRAPHY 


Earthquakes 

Definition.  Earthquakes  are  tremors  or  quakings  of  the 
earth's  surface,  due  to  causes  which  cannot  be  referred  to  human 
agencies.  The  passing  of  a  railway-train  causes  the  surface  near 
the  track  to  vibrate,  and  this  vibration  is  often  enough  to  be  felt 
in  adjacent  buildings.  In  this  case,  the  cause  is  readily  under- 
stood, and  the  shaking  is  not  called  an  earthquake;  but  an  equal 


FIGS.  432^434. — Diagrams  to  illustrate  the  history  of  a  fault-scarp.  Fig.  432 
shows  an  unfaulted  block  with  a  line  of  cliffs  due  to  the  superior  hard- 
ness of  one  formation.  Fig.  433  shows  the  same  faulted,  and  with  a 
pronounced  fault-scarp.  Fig.  434  shows  the  fault-scarp  partly  worn 
down.  (Huntington  and  Goldthwait.) 

amount  of  quaking,  due  to  causes  which  were  not  known,  would 
be  called  an  earthquake,  especially  if  felt  over  a  considerable  area. 

Strength  and  destructiveness.  Earthquakes  vary  much  in 
strength.  Some  are  so  gentle  that  they  can  barely  be  felt;  others 
are  so  violent  that  buildings  are  overthrown,  crevasses  opened  in 
the  surface  of  the  land,  and  masses  of  rock  loosened  from  cliffs 
and  precipitated  into  the  valleys  below.  Earthquakes  some- 
times disturb  the  waters  of  the  seas,  causing  destructive  sea  waves. 

Besides  quakings  which  are  sensible,   there  are  many  earth 


CRUSTAL  MOVEMENTS.    DIASTROPHISM  409 


FIG.  435. — Monument 
disturbed  by  earth- 
quake. (Falb.) 


FIG.  436. — A  chapel  in  Kasina  injured  in 
an  earthquake  of  November  9,  1880. 
(Wahner.) 


FIG.  437. — Horizontal  and  vertical  displacement  during  an  earthquake. 
Bengal-Assam  earthquake  of  July  12,  1897.  (From  Button's  Earth- 
quakes, by  permission  of  G.  P.  Putnam's  Sons.) 


410 


PHYSIOGRAPHY 


tremors  so  slight  as  not  to  be  felt.     They  are  known  only  by  means 
of  delicate  instruments  set  up  for  the  purpose  of  recording  all 


FIG.  438. — Craterlets  observed  after  the  Calabrian  earthquake  of  1783. 

(Sieberg.) 

vibrations  of  the  surface.  The  -  number  of  such  tremors  too 
slight  to  be  generally  noticed  is  much  greater  than  the  number  of 
earthquakes  strong  enough  to  be  felt. 

Although  violent  earthquakes  are  sometimes  very  destructive 


FIG.  439. — Great  sea-wave  on  the  coast  of  Ceylon.     (Sieberg.) 

to  buildings  and  to  life,  the  amount  of  movement  of  the  surface  is 
usually  so  slight  as  to  be  measured  in  millimetres  (a  millimetre  is 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


411 


about  -2*5  of  an  inch)  rather  than  in  inches  or  feet.     Bodies  on  the 
surface  often  move  much  more  than  the  solid  crust.     The  relation 


Phase  of  Vibrations 


Scale  of  Minutes 


FIG.  440. — Seismograph  of  earthquake  in  Punjab,  India,  April  4, 1905,  show- 
ing the  actual  amount  of  movement.     (De  Montessus  de  Ballore.) 

is  illustrated  by  the  fact  that  a  blow  on  the  floor  may  cause  a  ball 
which  rests  upon  it  to  bound  up  inches  or  even  feet,  though  the 
floor  itself  moves  but  a  small  fraction  of  an  inch. 

While  earthquakes  are  among  the  most  disastrous  and  appal 


FIG.  441. — The  bending  of  railway  track  in  India,  earthquake  of  1897. 

(Oldham.) 

ling  of  natural  phenomena,  so  far  as  human  affairs  are  concerned, 
those  of  historic  times,  at  least,  have  left  few  important  marks  on 


412 


PHYSIOGRAPHY 


the  surface  of  the  earth.  Their  destructiveness  to  human  life 
comes  largely  from  the  fall  of  buildings  and  from  the  "great  sea 
waves "  caused  by  the  earthquakes.  Destruction  of  life  results 
from  the  advance  of  these  waves  upon  a  low  coast  which  is  densely 
populated.  In  the  Lisbon  earthquake  of  1755  a  wave  60  feet  high 
swept  upon  the  shore  and  destroyed  some  60,000  human  lives. 


FIG.  442.— Fault  in  Japan,  1891.     (Koto.) 

Vessels  in  harbors  have  been  swept  in  by  waves  and  left  high  and 
dry  above  the  water-level. 

Examples.  Some  of  the  principal  features  of  earthquakes 
may  be  brought  out  by  the  study  of  a  few  striking  examples. 

1.  On  October  28,  1891,  an  earthquake  on  the  main  island 
of  Japan  opened  a  fissure  traceable  for  over  40  miles.  The  ground 
on  one  side  of  this  fissure  sank  2  to  20  feet  (a  fault)  below  that 
on  the  other.  At  the  same  time,  the  east  wall  of  the  fissure  was 
pushed  horizontally  about  13  feet  northward.  In  some  places  the 
cracking  of  the  rock  "showed  itself  at  the  surface  as  a  cracked 
ridge,  like  the  track  of  a  mole  just  below  the  surface."  Several 
tracts  of  land  became  lakes,  one  on  the  depressed  side  of  the 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


413 


fissure,  the  others  in  hollows  formed  by  the  shocks.  Fig.  443 
shows  the  distribution  of  earthquakes  in  Japan  from  1883  to  1902. 
2.  On  the  evening  of  August  31,  1886,  the  city  of  Charleston, 
South  Carolina,  was  disturbed  by  an  earthquake  which  was 
felt  over  a  large  part  of  the  United  States.  Strange  noises  and 


FIG    443. — Distribution  of  earthquakes  in   Japan,    1885-1892.^     (Davison. 
From  Button's  Earthquakes,  by  permission  of  G.  P.  Putnam  s  Sons.) 

slight  tremblings  of  the  earth  had  been  noted  for  several  days 
previous  to  the  destructive  quaking,  but  they  excited  no  great 
alarm.  About  ten  o'clock  in  the  evening  of  the  fateful  day,  a 
low  rumbling  sound  was  heard,  which  rapidly  deepened  into  an 
awful  roar.  The  slight  trembling  of  the  ground  increased  until 
it  became  destructively  violent.  The  motion  then  subsided 


414 


PHYSIOGRAPHY 


slightly,  but  increased  again  in  intensity  and  then  died  away.  The 
violent  disturbance  lasted  70  seconds.  A  second  shock,  almost 
as  severe  as  the  first,  occurred  eight  minutes  later,  and  six  or  seven 
more  or  less  severe  ones  were  felt  before  morning,  and  slight  trem- 
ors occurred  at  intervals  until  the  following  April.  During  the 
shocks,  buildings  swayed,  chimneys  were  thrown  down,  walls 


105°       100°        95°        90°        85°         80°          75°          70  J         65s 


FIG.  444. — Isoseismal  curves  (that  is,  curves  connecting  points  having  the 
same  amount  of  disturbance)  of  the  Charleston  earthquake.  (Button 
U.  S.  Geol.  Surv.) 

were  cracked,  houses  moved  from  their  foundations,  railroad-tracks 
displaced  sidewise  and  the  rails  bent,  and  trees  disturbed  in  the 
ground.  Numerous  fissures  were  formed  in  the  earth,  and  from 
some  of  them  streams  of  water,  mud,  and  sand  were  forced  out. 
Hardly  a  large  building  in  the  entire  city  but  was  damaged,  and 
27  persons  were  killed,  chiefly  by  falling  masonry.  The  people 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


415 


fled  in  terror  from  their  homes,  and  for  several  days  and  nights  a 
large  part  of  the  population  camped  in  the  public  parks. 

Outside  the  vicinity  of  Charleston  the  earthquake  shock  was 
less  violent,  but  the  quaking  was  felt  over  an  area  of  between 


Tli*  two  epicentral  tract*  are  indicated  by  arbitrart 
iwseigmal  curvet;  tint  heavy  line  being  tfa  index. 

SCALE    OF  MILES     . 


012  i  6  8  10 


FIG.  445.— Epicentral  tracts  (i.e.,  tracts  over  the  centers  of  disturbance)  of  the 
Charleston  earthquake,  with  isoseismal  lines.     (Button,  U.  S.  Geol.  burv.J 

2,000,000  and  3,000,000  square  miles.  It  was  felt  earliest  near 
Charleston,  and  later  at  increasing  distances  from  the  city  (Fig. 
444).  There  were  two  centers  of  disturbance  (Fig.  445),  and 
the  earthquake  wave  spread  at  the  rate  of  about  150  miles  per 
minute. 


416  PHYSIOGRAPHY 

There  was  no  volcano  near  Charleston,  and  this  earthquake 
appears  to  have  been  altogether  independent  of  any  volcanic 
eruption. 

3.  In  1822  and  again  in  1835  the  coast  of  Chile  was  shaken  by 
earthquakes  for  1200  miles.  In  both  years  the  shocks  continued 
for  several  months.  When  they  were  over,  it  was  found  that 
the  coast-lands  had  been  elevated  2  to  4  feet.1  In  1835  a  volcano 


FIG.  446. — Wreck  of  the  Charleston  earthquake.     (U.  S.  Geol.  Surv.) 

broke  out  beneath  the  sea  (San  Fernandez  Island)- at  the  time  of 
the  earthquake  shocks  on  the  coast,  and  many  of  the  Andean  vol- 
canoes were  active. 

The  same  region  was  again  shaken  disastrously  in  August,  1906, 
causing  great  destruction  of  life  and  property  in  some  of  the 
principal  cities  cf  the  country. 

4.  In  1819  a  part  of  the  delta  of  the  Indus  River  experienced  a 
series  of   shocks  lasting  four  days.     During  the  disturbance  an 
area  some  2000  square  miles  in  extent  subsided  so  as  to  be  covered 
by  the  sea,  while  a  neighboring  belt,  50- miles  long  and  16  miles 
wide,  rose  about  10  feet. 

5.  The  earthquake  of  Kangra,  in  the  same  country,  April,  1905, 
affected  an  area  of  1,625,000  square  miles,  and  killed  about  20,000 
people.     In  this  case,  the  vibrations  spread  from  two  centers,  and 

1  This  statement  has  been  disputed,  and  the  records  which  bear  on  the 
point  seem  to  be  less  perfect  than  could  be  desired. 


CRUST AL  MOVEMENTS.     DIASTROPHISM  417 

traveled  at  the  rate  of  about  two  miles  per  second.     The  same 
region  had  been  shaken  in  1897. 


FIG.  447. — A  large  craterlet  formed  during  the  Charleston  earthquake.     Hun- 
dreds of  them  were  formed  near  Summerville,  S.  C.     (U.  S.  Geol.  Surv.) 


FIG   448. — Sand-cones  and  craterlets  observed  after  the  Achaischen  earth- 
quake of  1861.     (Schmidt.) 

6.  A  series  of  earthquake  shocks,  lasting  from  1811  to  1813,  af- 
fected the  Mississippi  Valley  just  below  the  mouth  of  the  Ohio. 


418  PHYSIOGRAPHY 

Many  fissures  were  formed  in  the  deposits  of  the  flood  plain  of 
the  Mississippi,  and  some  of  them  remained  open  for  years.  Parts 
of  the  flood  plain  sank,  and  the  sunken  portions  gave  rise  to  marshes 
and  lakes,  some  of  which  still  remain.  The  trunks  of  the  drowned 
trees  are  in  some  cases  yet  standing  above  the  water  of  these  lakes 
and  marshes.  The  Orleans,  the  first  steamboat  built  west  of  the 


FIG.  449. — Map  showing  the  intensity  of  earthquakes  in  Italy. 
(Baratta  and  Gerland.) 

Appalachians,  narrowly  escaped  destruction  at  New  Madrid  during 
this  earthquake. 

7.  At  about  the  same-  time,  1812,  nearly  10,000  persons  were 
killed  in  a  violent  earthquake  which  destroyed  Caracas,  Venezuela. 

8.  Earthquakes  have  been  most  destructive  in  southern  Italy. 
Some  20,000  lives  were  lost  here  in  1688;  43,000  in  1693,  and  32,000 
in  1783;  in  all  about  100,000  in  a  single  century. 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


419 


9.  On  April  18,  1906,  there  was  a  destructive  earthquake  on  the 
coast  of  California,  in  and  about  San  Francisco.  Many  buildings 
were  injured  by  the  earthquake  and  some  practically  destroyed, 
both  in  San  Francisco  and  elsewhere.  Fire  broke  out  at  various 
points  in  San  Francisco  after  the  shocks,  and  as  the  quaking  had 


Fro.  450. — Chart  of  epicentra  and  outer  limits  of  sensibility  of  the  earth 
quakes  of  the  eastern  Mediterranean,  from  1846  to  1870.  (After  J. 
Schmidt.  From  Button's  Earthquakes,  by  permission  of  G.  P.  Putnam's 
Sons.) 

cut  off  the  city's  supply  of  water,  none  was  available  for  fighting 
the  flames,  and  a  large  part  of  the  city  was  burned.  This  earth- 
quake, the  most  disastrous  in  North  America  in  historic  times, 
was  caused  by  a  horizontal  fault  of  eight  to  twenty  or  more  feet, 
which  was  promptly  traced  some  300  miles.  Figs.  451  to  459  show 
some  of  the  phenomena  of  this  seismic  disturbance. 


420 

Earthquakes  starting  beneath  the  sea.  Earthquakes  some- 
times seem  to  start  beneath  the  sea,  and  to  spread  thence  to  the  land. 
The  record  of  the  accompanying  changes  beneath  the  sea  is  rarely 
distinct,  but  in  a  few  cases  some  facts  are  known  about  them. 


FIG.  451. — Map  showing  the  position  of  the  San  Francisco  earthquake  fault. 
The  line  north  of  Point  Arena  is  quite  uncertain.     (Gilbert.) 

This  is  especially  the  case  with  reference  to  some  of  the  earthquakes 
which  have  occurred  about  the  coast  of  Greece,  for,  in  a  number  of 
cases,  cables  have  been  broken,  and  soundings  taken  when  they 
were  repaired  gave  some  indication  of  what  had  happened.  In 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


421 


FIG.  452  A. — An  "earth  flow,"  or  landslide,  which  occurred  during  the  Cali- 
fornia earthquake,  several  miles  west  of  the  fault  line  east  of  Half  Moon 
Bay.  (Photo,  by  Dudley.) 


FIG.  452  B. — Characteristic  surface  appearance  of  the  fault  line,  south  end 
of  Tomales  Bay,  Cal.     (Photo,  by  Newsom.) 


PHYSIOGRAPHY 


one  case,  the  soundings  from  the  bow  and  the  stern  of  the  vessel 
which  repaired  the  cable  show  differences  of  more  than  1500  feet 


FIG.  453. — Deformed  railway,  Seventh  and  Mission  Streets, 
San  Francisco.     (Lindgren,  U.  S.  Geol.  Surv.) 


FIG.  454. — A  fissure  on  East  Street,  San  Francisco,  near  the  water  front. 
"Made  "  ground.     (Lindgren,  U.  S.  Geol.  Surv.) 

in  the  depth  of  the  water,  where  the  bottom  had  been  nearly  level 
when  the  cable  was  laid. 


CRUSTAL  MOVEMENTS.     DIASTROPHISM 


423 


FIG.  455. — The  fault  line  two  miles  southeast  of  Portola,  Cal. 
some  vertical  displacement  at  this  point. 


There  was 


FIG.  456. — A  broken  (now  mended)  and  offset  fence.   That  in  the  foreground 
was  formerly  in  line  with  the  single  length  directly  behind  the  man. 


424 


PHYSIOGRAPHY 


The  earthquake  wave.  An  earthquake  usually  spreads  over  the 
surface,  somewhat  as  a  water  wave  spreads  from  a  center.  Hence 
we  have  come  to  speak  of  the  earthquake  wave.  The  actual  move- 


FlG.  457. — A  water-pipe  buckled  out  of  the  ground  by  the  earthquake. 
Alpine  road,  Portola  Valley,  Cal.     (Photo,  by  Dudley.) 


Fio.  458.— County  Bridge,  Pajaro  River,  Chittenden   Cal 
(Photo,  by  Dudley.) 

ment  is  a  wave,  but  the  wave  is  unlike  that  of  water,  though  there 
are  some  points  of  similarity.  The  center  of  an  earthquake  wave 
may  be  a  line,  a  belt,  or  a  point,  and  in  many  cases  it  is  not  readily 
located. 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


425 


Points  distant  from  the  center  feel  the  earthquake  shock  later 
than  those  nearer  to  it.  Lines  drawn  upon  the  surface  connecting 
points  where  a  given  earthquake  wave  is  felt  at  the  same  time  are 
coseismic  lines  (Fig.  462).  These  lines  are  rarely  circles,  but  they 
are  usually  closed  curves,  and  are  often  irregular.  Their  irregu- 
larity appears  to  be  due  to  the  fact  that  the  wave  motion  spreads 
faster  in  some  directions  than  in  others.  This  is  another  way  of 
saying  that  the  rocks  in  some  parts  of  the  earth's  crust  transmit 
the  motion  faster  than  others.  If  a  circular  metal  plate  were  set 


FIG.  459. — Tree  uprooted  by  the  earthquake. 
(Photo,  by  Dudley.) 


Searsville  road,  Cal. 


vibrating  at  its  center,  the  vibrations  would  spread  from  the 
center  in  all  directions  at  about  the  same  rate,  and  would  reach  the 
border  of  the  plate  at  about  the  same  time.  But  if  the  plate  were 
made  of  sectors  of  different  material,  one  sector  being  of  steel, 
another  of  hard  wood,  and  another  of  cork,  the  vibrations  started 
at  the  center  would  pass  outward  through  these  sectors  with  dif- 
ferent velocities,  and  would  extend  to  different  distances  in  a 
given  time.  The  more  porous  and  open  the  medium,  the  less  the 
distance  which  the  vibrations  would  travel  before  being  com- 
pletely deadened. ...  The  principle  illustrated  here  has  some  applica- 
tion in  earthquakes.- 


426 


PHYSIOGRAPHY 


FIG.  460  A. — Track  of  electric  railway,  be-       FIG.  460  B. — A  street  railway  on  loose 
tween    South   San    Francisco   and    San  ground.     Union  Street,  near  Pierce 

Bruno  Point.     (Photo,  by  Moran.)  Street.     (Photo,  by  Moran.) 


E 


FIG.  461. — Scenes  on  the  Campus  of  the  Leland  Stanford  University,  after  the  earth- 
quake of  April,  1906. 

A.  The  Agassiz  statue.     (Branner.)  C.  The  University  Chapel.    (Branner.) 

B.  The  great  arch.     (Branner.)  D=  The  Library.     (Branner.) 


CRUST AL  MOVEMENTS.     D1ASTROPH1SM 


427 


In  general,  the  earthquake  waves  diminish  in  violence  with 
increasing  distance  from  the  centers  of  disturbance  (Fig.  463). 

Frequency.  Earthquakes  are  of  very  common  occurrence, 
though  fortunately  those  which  are  violent  enough  to  be  destruc- 


FIG.  462. — Coseismic  lines  for  each  minute,  Herzogenrath  (Germany),  earth- 
quake of  October  22,  1873.     (Lasaulx.) 

tive  are  rare.  From  1889  to  1899  an  average  of  36  per  year  were 
recorded  in  California  alone,  but  most  of  them  were  so  slight  as  to 
cause  no  destruction.  In  Japan,  earthquakes  have  been  recorded 
at  the  rate  of  several  per  day  for  many  years,  but  this  includes 


FIG.  463. — Diagram  illustrating  the  dimensions  and  intensity  of  vibration 
with  increasing  distance  from  the  epicentrum.      (Belax.) 

many  very  trivial  quakings,  and  only  a  few  of  sufficient  violence 
to  be  destructive. 

The  Isthmus  of  Panama  and  its  surroundings  have  been  under 
careful  observation  with  reference  to  earthquakes  for  a  few  years, 


428 


PHYSIOGRAPHY 


because  the  frequency  and  violence  of  earthquakes  had  a  bearing 
on  the  site  which  was  to  be  selected  for  the  canal  which  was  to 
join  the  Atlantic  and  Pacific.  In  40  months,  between  January, 
1901,  and  April,  1904,  169  earthquakes  were  recorded  at  San  Jose, 
near  the  eastern  end  of  the  proposed  Nicarauguan  route.  Of 


FIG.  464. — Map  showing  in  black  the  principal  earthquake  regions  of  the  Old 
World.     (Montessus  de  Ballore.) 

these,  43  were  mere  tremors,  91  slight  shocks,  and  35  strong  shocks. 
During  the  same  period,  6  tremors  and  4  slight  shocks  were 
recorded  at  Panama. 

The  slight  tremors  would  probably  not  have  been  known  but 
for  the  observatories,  many  of  which  have  been  established  in 
recent  times,  where  all  earth-tremors,  however  slight,  are  recorded 
by  delicate  instruments  (seismographs)  devised  for  this  purpose. 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


429 


In  view  of  the  records  at  the  stations  in  various  parts  of  the 
civilized  world  where  such  apparatus  has  been  set  up,  it  has  been 
said  that  some  part  of  the  earth's  surface  is  probably  shaking  all  the 
time. 

Distribution.  Earthquakes  are  perhaps  most  common  in 
volcanic  regions,  though  not  confined  to  them.  It  can  hardly  be 


FIG.  465. — Map  showing  the  principal  earthquake  regions  of  the  New  World. 

(Montessus  de  Ballore.) 

said  that  all  such  earthquakes  are  caused  by  volcanoes,  since  many 
of  them  do  not  occur  at  the  time  of  volcanic  eruptions.  It  is 
perhaps  better  to  regard  earthquakes  and  volcanoes  as  the  result 
of  a  common  cause,  rather  than  to  regard  one  of  them  as  the  gen- 
eral cause  of  the  other. 

Many  great  earthquakes  have  occurred  near  the  edges  of  the 
continental  platforms.    Mountain  regions  in  general  seem  to  be  more 


430  PHYSIOGRAPHY 

subject  to  earthquakes  than  plains,  though  earthquakes  originat- 
ing in  mountain  regions  sometimes  spread  to  plains.  Earthquakes, 
on  the  other  hand,  do  not  always  start  in  mountain  regions.  As  in 
the  case  of  the  Charleston  earthquake,  they  sometimes  originate 
beneath  plains. 

Causes  of  earthquakes.  Earthquakes  are  probably  due  to 
various  causes.  Small  ones  are  perhaps  sometimes  due  to  the 
falling  in  of  the  roofs  of  underground  caves.  If  the  roof  of  Mam- 
moth Cave,  for  example,  were  to  fall  in,  the  disturbance  would 
cause  an  earthquake  of  small  extent.  Earthquakes  accompany 
violent  volcanic  eruptions,  and  in  these  cases  the  explosions 
which  cause  the  eruption  are  doubtless  also  the  cause  of  the  earth- 
quakes. Great  landslides  and  avalanches  are  the  causes  of  some 
minor  earthquakes,  and  it  is  probable  that  slumping  on  the  slopes 
of  deltas  and  on  the  outer  faces  of  the  continental  shelves  pro- 
duces similar  results. 

Many  great  earthquakes  appear  to  be  connected  with  other 
forms  of  crustal  movement.  As  already  noted,  fissures  are  some- 
times opened  in  the  surface  of  the  land  during  an  earthquake. 
This  is  best  seen  where  there  is  little  or  no  soil,  and  where  the  solid 
rock  lies  close  to  the  surface.  There  is  a  great  crack  of  this  sort 
in  Arizona  (Fig.  467),  and  similar  fissures  have  been  formed  in 
New  Zealand,  Japan,  and  elsewhere  during  earthquakes.  It  is  not 
always  clear  whether  the  fissure  should  be  looked  on  as  the  cause 
or  the  result  of  the  earthquake.  In  some  cases  it  is  found  that  one 
side  of  such  a  fissure  is  higher  than  the  other  after  the  earthquake, 
indicating  that  the  rock  on  one  side  was  raised  or  that  on  the  other 
sunk,  or  both — in  other  words,  that  the  strata  have  been  faulted. 
Faulting  is  probably  the  principal  cause  of  earthquakes,  for  the 
slipping  of  one  great  body  of  rock  past  another  would  cause 
vibrations  which  would  spread  far  from  the  center  of  disturbance. 

There  is  sometimes  horizontal  as  well  as  vertical  displacement 
along  the  cracks,  as  already  noted,  and  the  horizontal  thrust  or 
fault  is  sometimes  the  principal  one,  as  in  the  recent  earthquake 
of  California.  Horizontal  displacement  shows  itself  in  the  distor- 
tion or  breaking  of  lines  which  were  straight  or  continuous  before 
the  faulting.  Thus  fences  or  rows  of  trees  which  were  straight 
before  an  earthquake  may  be  bent  or  broken  and  offset  at  the 
fissure.  The  force  which  causes  this  displacement  is  the  real  cause 
of  the  earthquake. 


CRUSTAL  MOVEMENTS.    DIASTROPHISM 


431 


Again,  great  thicknesses  of  rock  strata  are  sometimes  found 
folded  and  crumpled.  The  process  of  mountain  folding  has  never 
been  seen,  and  it  is  probably  much  too  slow  to  be  seen  from  day 
to  day  or  from  year  to  year.  But  there  can  be  no  doubt  that 


FIG.  466. — Faulting  accompanying  the  Sinjan  earthquake  of  1898. 
(Faidiga.) 

beds  now  folded  so  as  to  stand  on  edge  were  once  horizontal  or 
nearly  so.  No  series  of  horizontal  beds  can  be  folded,  as  many 
beds  have  been,  without  more  or  less  slipping  of  layer  on  layer. 
The  amount  of  slipping  at  any  one  time  may  be  slight,  but  it 


FIG.  467. — Fissure  produced  by  earthquake.     Arizona. 

must  be  real.    This,  too,  is  probably  a  cause  of  earthquakes,  and  of 
earth-tremors  which  are  not  sensible. 

It  is  probable  that  most  earthquakes  are  to  be  looked  upon 
as  but  one  expression  of  the  wide-spread  movements  to  which 


432  PHYSIOGRAPHY 

the  crust  of  the  earth  is  subject,  movements  which  are  due  pri- 
marily to  the  continued  adjustment  of  the  outside  of  the  earth  to 
a  shrinking  interior.  In  general,  these  movements  are  too  slow 
to  produce  sensible  vibrations;  but  locally  and  periodically  they 
are  sufficient  to  cause  distinct  quakings. 

Surface  changes  caused  by  earthquakes.  The  changes  in 
the  surface  of  the  land  made  by  earthquakes  are  numerous  if  not 
important.  In  addition  to  the  cracks  and  fissures,  and  the  risings 
and  sinkings  of  surface  which  have  been  noted,  drainage  is  often 
disturbed.  This  is  partly  because  of  the  cracks  and  fissures  which 
are  opened,  and  partly  for  other  reasons.  If  an  open  fissure  is 
developed  athwart  the  course  of  a  stream,  the  stream  will  plunge 
into  it.  Springs  are  often  disturbed,  old  ones  ceasing  to  flow  and 
new  ones  appearing.  This  is  probably  because  the  earthquake 
movement  has  ruptured  the  rock  beneath  the  surface,  and  so 
changed  the  course  of  the  ground-water  circulation.  Temporary 
spouting  springs  are  sometimes  formed,  water  being  forced  up 
violently  through  them  (p.  414).  Earthquakes  sometimes  cause 
landslides,  and  if  the  material  from  a  mountain-side  slides  down, 
it  may  dam  the  valley  below  so  as  to  disturb  its  drainage  (p.  313). 

From  fissures  and  from  lesser  vents  noxious  gases  sometimes 
issue.  In  some  cases,  too,  loose  sand  is  thrust  up  into  cracks  from 
beneath  during  earthquakes. 

Earthquake  waves  have  a  singularly  destructive  effect  upon 
aquatic  life.  It  has  been  recorded  in  many  cases  that  the  animals 
of  rivers,  bays,  and  even  of  the  ocean  are  killed  in  extraordinary 
numbers  during  an  earthquake. 

MAP  EXERCISES 

Study  the  following  maps  in  preparation  for  the  conference: 

A.  Choptank,  Md. 
Tolchester,  Md. 
Boothbay,  Me. 
Coos  Bay,  Ore. 
Oceanside,  Cal. 
Honey  Lake,  Cal. 
Erie,  Pa. 
Fairview,  Pa. 

B.  U.  S.  Coast  and  Geodetic  Survey  charts,   10,  124,   125,  188,  210, 

5100,  5500. 


CRUSTAL  MOVEMENTS.     DIASTROPHISM  433 

C.  Mt.  Trumbull,  Ariz. 

Echo  Cliffs,  Ariz. 

Diamond  Creek,  Ariz. 

Coast  Survey  Chart,  5100. 

It  is  to  be  borne  in  mind  that  the  relative  change  of  water  and 
land  levels  along  the  shores  of  lakes  may  be  due  to  the  lowering  of 
the  lake  by  the  cutting  down  of  its  outlet,  and  not  to  diastrophism 
at  all.  The  topographic  features  which  give  evidence  of  the  lowering 
of  a  lake-level  are,  however,  the  same  in  kind  as  those  which  arise  from 
the  uplift  of  the  shore-land  or  from  the  sinking  of  the  sea-level. 

D.  Study  the  maps  of  group  A  for  evidences  of  change  of  relative 
level  of  land  and  water.      Answer  in  writing  the  questions  marked  *. 
In  the  case  of  each  map  and  chart, 

1.  Is  submergence,  emergence,  or  warping  (part  up  and  part  down) 
of  the  land  in  recent  times  indicated  by  the  map?  Reasons. 

2.*  Specify  three  well-defined  cases  of  coast-land  (or  shore-land) 
emergence  suggested  by  the  topographic  maps,  with  reasons  therefor. 

3.*  Specify  three  cases  of  coast-land  (or  shore-land)  submergence 
suggested  by  the  topographic  maps,  with  reasons  therefor. 

4.  What  factors  besides  diastrophism  have  probably  been  operative 
in  shaping  the  coast  in  the  Boothbay  region?  In  the  Alaskan  region? 
Distinguish,  if  possible,  between  the  features  due  to  diastrophism 
and  those  due  to  other  causes. 

5.*  Make  an  interpretation  of  the  topography  of  the  coastal  part 
of  the  Oceanside  Sheet,  indicating  the  degree  of  confidence  with  which 
your  conclusions  are  held. 

E.  Study  the  charts  of  group  B  for  evidences  of  submergence   and 
emergence  of  coastal  lands.     Evidences  of  submergence  are  to  be  found 
largely  in  the  configuration  of  the  submerged  surface.     Evidences  of 
emergence   are   much   the  same   as   on    the    contour   maps.     Note   the 
unit  (feet,  fathoms)  in  which  soundings  are  given,  in  the  case  of  each 
chart. 

1.  Note  cases  of  well-defined  apparent  submergence  of  former  lands, 
as  indicated  by  the   configuration  of  the   bottom   (see  p.   397). 

2.  Note  cases    of  well-defined  apparent  emergence  shown  on  these 
charts  (see  p.  394). 

3.  What  are  the  possible  explanations  of  the  larger  features  of  the 
coast-line  shown  on  Chart  5500? 

F.  In  group  C,  the  Hurricane  Ledge  of  the  Mt.  Trumbull  Sheet,  the 
Echo  Cliffs  of  the  Sheet  of  the  same  name,  the  Grand  Wash  Cliffs  of 
the  Diamond  Creek  Sheet,  and  the  steep  cliff  southwest  of  Honey  Lake, 
on  the  Honey  Lake  Sheet,  are  fault-scarps  now  somewhat  eroded.     This 
could,  however,   not  be   certainly  known  from   the   topographic  map. 


434  PHYSIOGRAPHY 

The  steep  cliffs  on  the  northeastern  coast  of  San  Clemen te  Island,  Chart 
5100,  also  represent  an  old  fault-scarp. 

REFERENCES 

1.  Standard   text-books  on   Geology,   under  Changes   of  Level,  Secular 
Changes  of  Level,  Crustal  Movements,  Earthquakes,  etc. 

2.  MILNE,  Earthquakes:    Appleton,  and  Movements  of  the  Earth's  Crust: 
Geog.  Jour.,  Vol.  VII,  1896,  pp.  229-250. 

3.  DUTTON,  Earthquakes:   Putnam. 

4.  HOBBS,  Earthquakes:  Appleton. 

5.  BUTTON,  The  Charleston  Earthquake.    9th  Ann.  Rept.  U.  S.  Geol.  Surv. 

6.  GILBERT,  A  Theory  of  Earthquakes  of  the  Great  Basin,  with  a  Practical 
Application:    Am.  Jour.  Sci.,  Vol.  XXVII,  1884,  pp.  49-53. 

7.  GILBERT:    Pop.  Sci.  Mo.,  Vol.  LXIX,  1906,  p.  97. 

8.  JORDAN:    Pop.  Sci.  Mo.,  Vol.  LXIX,  1906,  p.  289. 

9.  SHALER:    Chapter  in  Aspects  of  the  Earth  (Scribners). 

10.  McGfiE,  The  Gulf  of  Mexico  as  a  Measure  of  Isostasy.     Am.  Jour.  Sci., 
Vol.  XLIV,  1892,  pp.  177-192. 

11.  SHEPARD,  New  Madrid  Earthquake:    Jour.  Geol.,  Vol.  XIII,  pp.  45-56. 

12.  LE  CONTE,  Earth  Crust  Movements  and  their  Causes:    Science,  Vol.  V, 
1897,  pp.  321-330". 

Many  of  the  references  under  Volcanoes  (p.  390)  also  touch  upon  earth- 
quakes. 


CHAPTER  IX 
ORIGIN   AND   HISTORY   OF   PHYSIOGRAPHIC   FEATURES 

WE  may  now  review  the  principal  physiographic  types  in  the 
light  of  the  knowledge  afforded  by  the  preceding  study  of  physio- 
graphic processes. 

Plains 

Plains,  considered  as  one  of  the  three  great  divisions  of  land 
surfaces,  have  arisen  in  various  ways,  as  already  noted.  In  many 
cases  their  materials  show  that  they  were  once  below  sea-level. 
From  this  position  they  were  (1)  bowed  up,  (2)  faulted  up,  (3)  or 
built  up  so  as  to  emerge  from  the  water;  or  (4)  the  sea-level  may 
have  been  drawn  down  so  as  to  leave  them  dry.  Plains  have  arisen 
also  (5)  by  the  degradation  of  plateaus  or  mountains.  In  many 
cases,  two  or  more  of  these  processes  have  operated  jointly  in  the 
development  of  plains. 

After  plains  have  come  into  existence  they  are  modified  by 
gradation,  generally  by  stream  erosion  and  often  by  glacier  erosion; 
by  diastrophism,  which  may  deform  them;  or  by  vulcanism, 
which  may  build  them  up  by  lava-flows  or  diversify  them  by 
the  development  of  volcanic  cones. 

All  existing  plains  of  great  extent  have  been  modified  in  some 
or  all  these  ways.  Thus  the  Coastal  Plain  of  the  Eastern  United 
States,  developed  by  aggradation  and  diastrophism,  has  been  much 
changed  by  erosion,  and  perhaps  somewhat  by  warping,  since  its 
origin.  The  great  Interior  plain  of  the  United  States  has  been 
much  modified  by  rain  and  river  erosion,  and  at  the  North  by 
glaciation.  These  processes  have  led  to  the  development  of  many 
minor  features,  as  already  indicated.  Valleys  have  been  made 
by  running  water,  and  ridges  and  hills  left  in  the  process.  Mounds, 
hills,  and  ridges,  with  associated  kettle-like,  saucer-like,  trough- 

435 


436 


PHYSIOGRAPHY 


like,  and  irregular  depressions,  have  been  made  by  the  continental 
glaciers.     Many  of  these  depressions  have  become  the  sites  ot  lakes, 


fiu.  468. — A  semi-arid  plain,  with  sinks  and  one  water-hole,  in  the  western 
part  of  the  United  States.     (U.  S.  Geol.  Surv.) 

ponds,  and  marshes.     Lands  thus  modified  are  sometimes  called 
glacial  plains.     In  the  bottoms  of  the  larger  valleys,  river  plains 


FIG.  469. — Arid  plain,  western  United  States.     A  mesa  or  plateau  at  the 
right.     (U.  S.  Geol.  Surv.) 

have  been  developed,  and  about  many  lakes  whose  basins  have 
been  partly  filled,  or  whose  levels  have  been  drawn  down  by  the 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    437 


lowering  of  their  outlets,  lacustrine  plains  have  been  developed. 
Similar  flats  occupy  the  sites  of  many  former  lakes  which  have 
become  extinct. 

In  addition  to  the  changes  produced 
by  gradation,  the  Interior  Plain  has 
probably  been  somewhat  changed  by 
unobtrusive  expressions  of  diastro- 
phism. 

Various  plains  in  the  West  have 
been  modified  by  the  ejection  of  volcanic 
matter,  as  well  as  by  gradation  and 
diastrophism,  and  most  plains  have 
been  affected  to  some  extent  by  the 
wind. 

Plateaus 

Plateaus  may  originate  through  the 
operation  of  some  of  the  processes 
which  give  rise  to  plains  (Chap.  1).  Suffi- 
cient up-warping  or  up-wedging  of  the 
sort  which  gives  rise  to  plains  would 
give  rise  to  plateaus.  So  also  would 
sufficient  up-building,  especially  per- 
haps by  lava-flows.  It  may  be  doubted 
whether  the  sea-level  was  ever  lowered 
enough  at  one  time  to  convert  coastal 
plains  into  plateaus,  and  plateaus  are 
not  made  by  the  degradation  of  higher 
lands. 

After  they  come  into  existence,  ^  jji  II)  ^  |y\ 
plateaus  are  modified  by  all  the  proc- 
esses which  modify  plains.  All  exist- 
ing plateaus  have  felt  the  effect  of  some 
of  these  processes,  and  most  of  them  of 
several. 

MOUNTAINS 

Some  study  has  already  been  made 
of  mountains  as  topographic  features,  but  some  points  concerning 
them  could  not  well  be  considered  until  the  processes  of  vulcanism 


438 


PHYSIOGRAPHY 


and  diastrophism  had  been  outlined.  We  have  now  to  see  what 
various  forms  mountains  assume,  how  they  are  grouped,  what 
their  structure  is,  and  what  purposes  they  serve  in  the  economy  of 
nature. 

Mountains  have  been  defined  as  masses  of  land  high  enough 
to  be  very  conspicuous  in  their  surroundings,  but  without  a  great 
expanse  of  surface  at  the  top.  It  is  to  be  understood,  however, 
that  between  large-topped  mountains  and  small  plateaus  there 
are  all  gradations. 

Those  who  have  never  seen  mountains,  but  who  have  seen  hills 
and  ridges,  may  perhaps  best  get  their  conceptions  of  mountains 


FIG.  471. — Dome-shaped  mountain  in  the  Uinta  Mountains.     (Church.) 

by  thinking  of  them  as  hills  and  ridges  which,  in  their  surround- 
ings, appear  to  be  very  high.  They  may  be  but  a  few  hundred 
feet  above  their  environs,  or  they  may  be  many  thousand  feet,  and, 
as  in  the  cases  of  hills  and  low  ridges,  their  slopes  may  be  steep  or 
gentle. 

A  single  mountain  may  be  but  a  big  hill  (Fig.  471  and  PI.  XXIV). 
But,  as  already  stated,  there  are  all  gradations  between  a  big  hill 
and  a  little  mountain,  and  whether  an  isolated  elevation  is  called 
a  hill  or  a  mountain  depends  on  its  surroundings,  or  on  the  judg- 
ment of  those  who  named  it.  A  single  mountain  may  stand  in  the 
same  relation  to  a  mountain  region  that  a  single  hill  does  to  a  hilly 
region. 

A  mountain  may  be  a  high  ridge  rather  than  a  high  hill  (PI. 
XXV).  A  mountain  ridge  is  often  called  a  mountain  range.  A 


PLATE  XXV 


Dunning  Mountain,  Pennsylvania;  a  good  example  of  a  mountain  ridge  due  to 
the  superior  hardness  (resistance)  of  a  tilted  layer  of  rock,  the  outcrop 
of  which  was  left  as  a  ridge  after  the  less  resistant  surroundings  were 
worn  away.  Scale  1  —  mile  per  inch.  (Everett  Sheet,  U.  S.  Geol.  Surv.) 


PLATE  XXVI 


An  area  southwest  of  Denver,  showing  a  mountain  ridge  dissected  by  erosion. 
The  outcropping  hard  layer  appears  in  the  form  of  a  series  of  short 
ridges,  or  "  hog-backs."  (Compare  PI.  XXV.)  Scale  2-miles  per  inch. 
(Denver,  Colo  ,  Sheet,  U.  S.  Geol.  Surv.) 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    439 

mountain  range  may  have  a  nesrly  even  crest  (PI.  XXV),  or  its  crest 
may  be  a  series  of  high  points  partially  separated  from  one  another 
by  depressions  (PI.  XXVI).  A  mountain  range  or  ridge  may  have 


FIG.  472. — An  asymmetrical  mountain  ridge. 

its  opposite  slopes  much  alike,  or  they  may  be  very  unequal  (Fig. 
472). 

A  mountain  group  is  made  up  of  several  or  many  mountain 
peaks,  or  of  short  mountain  ridges.  The  Catskill  Mountains  (Fig. 
473)  and  the  Black  Hills  may  serve  as  examples. 

A  mountain  chain  or  system  is  an  elongate  mountain  group, 
made  up  of  many  single  mountains  or  of  mountain  ranges,  or  of 
both.  The  individual  ridges  commonly  have  a  pronounced  trend 
in  a  common  direction.  The  Appalachian  Mountain  system  is  an 
example  (Fig.  23,  p.  29).  A  few  mountain  systems,  like  the  Alps, 
are  not  conspicuously  longer  in  one  direction  than  in  another. 
The  more  conspicuous  elevations  of  a  system  have  separate  names, 
and  are  individually  mountains. 

Distribution  of  mountains.  In  some  of  the  continents  the 
more  important  mountains  are  toward  the  borders  of  the  land 
rather  than  in  the  interiors.  It  is  to  be  noted,  however,  that 
even  in  some  of  the  continents  where  this  is  true,  the  moun- 
tains are  not  all  near  the  coast.  In  the  western  part  of  North 
America,  for  example,  some  of  the  highest  ranges  are  nearly  1000 
miles  from  the  Pacific,  while  portions  of  the  eastern  mountains 
are  some  400  miles  from  the  Atlantic.  In  the  narrow  part  of  the 
continent  at  the  south,  nearly  all  the  land  is  mountainous. 

In  South  America  the  high  mountains  (the  Andes)  are  con- 
fined to  a  belt  rarely  exceeding  300  miles  in  width  near  the  coast, 
while  some  of  the  lower  mountains  to  the  east  are  farther  from 
the  sea. 

In  Africa,  the  highest  mountains  are  near  the  southeastern 
border  of  the  continent.  Mountains  also  occur  on  the  northwest 
border  and  at  some  other  points;  but,  on  the  whole,  it  can  hardly 


440  PHYSIOGRAPHY 

be  said  that  mountainous  borders  are  especially  characteristic  of 
this  continent.  In  Australia,  also,  the  more  important  moun- 
tains are  near  the  coast,  though  most  of  the  coasts  are  not  mountain- 
ous. 

The  mountains  of  Europe  and  Asia,  taken  as  a  whole,  can 


FIG.  473. — Photograph  of  a  model  of  the  Catskill  Mountains.     (Ho well.) 

hardly  be  said  to  be  near  the  oceans,  though  some  of  them  have 
such  positions. 

Heights.  Mountains  range  in  height  from  large  hills  or  ridges 
but  a  few  hundred  feet  high,  to  elevations  of  nearly  30,000  feet. 
The  highest  mountains  in  the  United  States,  outside  of  Alaska, 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    441 

are  found  in  the  Sierra  Nevada  range  of  California,  where  the 
highest  peak  (Mt.  Whitney)  reaches  an  elevation  of  nearly  15,000 
feet.  The  highest  mountains  of  the  Rockies  are  but  little  lower, 
many  peaks  exceeding  14,000  feet  in  elevation.  In  Colorado  alone 
there  are  about  40  peaks  reaching  an  elevation  of  between  14,000 


FIG.  474 — A  mountain  valley.  The  narrow  part  of  the  canyon  shown  here 
is  to  be  the  site  of  a  dam  310  feet  high,  for  a  reservoir  for  irrigation  pur- 
poses. (U.  S.  Geol.  Surv.) 

and  14,500  feet.  Mt.  Rainier  in  Washington  also  reaches  an 
elevation  of  a  little  more  than  14,000  feet.  The  highest  moun- 
tain in  Alaska,  Mt.  McKinley,  has  an  altitude  of  20,300  feet. 

The  highest  points  in  the  Andes  Mountains  attain  an  elevation 
of  abcut  23,000  feet,  and  many  peaks  rise  above  20,000  feet. 

The  highest  peaks  of  the  highest  mountains  of  Europe,  the  Alps, 


442  PHYSIOGRAPHY 

attain  an  elevation  of  nearly  16,000  feet,  and  the  highest  peaks  in 
the  Caucasus  are  but  little  less.  In  the  Himalayas,  the  loftiest 
mountains  of  Asia  and  of  the  earth,  the  highest  peak,  Mt.  Everest, 
is  nearly  30,000  feet  above  sea-level. 

The  mountains  of  Africa  and  Australia  are,  for  the  most  part, 
much  lower.     A  few  volcanic  peaks  in  the  former  attain  an  eleva- 


FIG.  475.— King's  River  Valley,  Cal.     (U.  S.  Geol.  Surv.) 

tion  of  nearly  20,000  feet,  while  the  greatest  elevation  of  Australia 
falls  short  of  8000  feet. 

Oceanic  mountains.  Mountains  exist  in  the  ocean  basins  as 
well  as  on  the  continental  platforms.  Many  oceanic  mountains  are 
partly  or  wholly  beneath  the  water,  but  the  crests  of  some  of  them 
are  not. 

If  the  height  of  a  mountain  be  reckoned  in  terms  of  elevation 
above  its  base,  rather  than  in  terms  of  elevation  above  sea-level, 
some  of  the  volcanic  cones  of  the  ocean  would  rank  among  the 
highest  mountains  of  the  earth.  Thus  Mauna  (Mount)  Kea  (Fig. 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    443 

392),  on  the  island  of  Hawaii,  rises  nearly  14,000  feet  above 
the  sea.  Measured  from  the  ocean  floor,  from  which  the  island 
rises,  its  height  is  more  than  30,000  feet.  It  is,  in  one  sense, 
nearly  or  quite  the  highest  mountain  of  the  earth,  though  not  the 
highest  above  sea-level.  Parts  of  the  Antillean  mountain  system 
(including  the  West  Indies  and  the  mountains  of  Central  America, 
etc.)  also  rise  from  a  depth  of  16,000  to  18,000  feet  below  sea-level 
to  a  maximum  height  of  more  than  10,000  feet  above.  They  are 
therefore  among  the  greatest  mountains  of  the  earth,  if  their 
elevation  be  reckoned  from  their  real  base. 

Changes  taking  place  in  mountains.  Most  of  the  processes 
of  degradation  already  studied  are  in  operation  in  mountain 
regions,  but  their  relative  importance  is  not  the  same  as  in  lower 
lands.  The  differences  are  due  partly  to  the  steepness  of  the 
mountain  slopes,  and  partly  to  the  differences  of  climate  incident 
to  altitude. 

Because  of  their  steep  slopes,  erosion  by  mechanical  processes 
is  more  rapid  in  mountains  than  on  plains.  Streams  in  mountains 
are,  as  a  rule,  rapid,  at  least  in  the  early  stages  of  an  erosion  cycle, 
and  make  deep  valleys.  Chiefly  for  this  reason,  mountains  are  the 
roughest  portions  of  the  earth's  surface.  Rapid  erosion  means 
that  weathered  rock  is  promptly  removed.  The  accumulation  of 
mantle  rock  is  therefore  less  in  mountains  than  where  erosion 
is  less  rapid,  and  bare  rock  is,  accordingly,  more  common. 

The  temperature  decreases  on  the  average  about  1°  Fahr.  for 
every  300  feet  of  rise.  If  a  mountain  is  3000  feet  higher  than  its 
surroundings,  the  temperature  at  the  top  is  therefore  some  10° 
colder  than  at  the  bottom.  Because  of  their  low  temperature, 
high  mountains  have  little  vegetation.  The  absence  of  vegetation 
allows  running  water  and  wind  to  remove  weathered  rock  readily. 
When  a  mountain  is  so  cold  as  not  to  allow  the  growth  of  vegetation, 
the  absence  of  the  plants,  together  with  the  steepness  of  slopes 
characteristic  of  mountains,  leaves  the  bare  rock  freely  exposed 
to  all  the  processes  of  weathering.  Daily  changes  of  rock  tempera- 
ture are  great  in  high  altitudes,  especially  on  sunny  days,  and 
rock  breaking,  due  to  this  cause,  is  most  effective.  The  steep 
slopes  allow  the  rock-masses  broken  off  in  this  wray  to  fall  or  to 
be  carried  down  readily  (Fig.  476),  thus  exposing  fresh  surfaces 
of  rock  to  the  same  changes. 

In  general,  there  is  more  precipitation  (rain  and  snow)  in  moun- 


PHYSIOGRAPHY 


tains  than  on  plains,  and  more  of  it  falls  as  snow.  The  snow  accu- 
mulates through  a  considerable  part  of  the  year,  to  be  melted  at  a 
later  time.  When  it  melts,  the  water  runs  off  and  has  much  the 
effect  of  concentrated  rainfall.  If  it  accumulates  in  sufficient 
quantity,  it  will  give  rise  to  glaciers,  which,  except  in  very  high 
latitudes,  do  not  occur  outside  of  mountain,  regions.  On  the 


FIG.  476. — Quartzite  Peak,  Wasatch  Mountains,  with  quantities  of  talus  at 
its  base.     (Chamberlin.) 

whole,  therefore,  erosion  is  more  rapid  in  mountains  than  else- 
where. 

The  deposition  of  sediment,  on  the  other  hand,  is  relatively 
less  in  mountains  than  on  plains,  because  of  the  steep  slopes  and 
the  swift  streams.  Much  of  the  debris  which  falls  or  is  carried 
down  steep  slopes  is  however  temporarily  lodged  at  their  bases 
(p.  182). 

Winds  are  often  strong  in  mountain  regions,  though  they  pro- 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    445 

duce  relatively  little  direct  effect  on  the  land,  (1)  because  it  is  less 
commonly  dry,  and  (2)  because  there  is  little  material  fine  enough 
to  be  blown. 

The  winds  in  the  mountains  have  a  notable  effect  on  the  char- 


FIG.  477. — A  mountain  tree.     Near  Granite,  Colo.     (Capps.) 

acter  of  the  trees  (Fig.  477),  especially  where  they  are  scattered, 
or  near  the  upper  limit  of  their  growth  (the  timber  line). 

Origin  of  Mountains 

Volcanic  mountains. — Mountains  originate  in  very  different 
ways.  Figs.  362  and  364  show  isolated  mountains  of  volcanic  origin. 
Single  mountains  having  this  structure  are  among  the  high  moun- 
tains of  the  earth.  Besides  Shasta,  Rainier,  and  others  already 
mentioned  (pp.  378-84),  the  Spanish  Peaks  of  Colorado  (13,620 
feet)  and  Mt.  Wrangell  of  Alaska  (17,500  feet)  belong  to  this  class. 
So,  also,  do  Orizaba  (18,200  feet)  and  Popocatepetl  (17,523  feet) 
of  Mexico;  Tajamulco  (18,317  feet)  and  others  of  Central  America; 
Aconcagua  (22,860  feet),  Chimborazo  (21,498  feet),  and  numerous 
others  in  the  Andes;  Elbruz  (18,470  feet),  Demavend  (18,000 
feet),  Great  Ararat  (nearly  17,000  feet),  Fuji-yama  (12,365  feet), 
and  others  in  Asia;  and  Kilimanjaro  (19,780  feet)  and  Kenia 
(18,000  feet)  in  Africa.  The  highest  mountains  of  Africa  and 
South  America  are  volcanic. 

The  origin  of  many  other  mountains  is  also  clearly  suggested 
by  their  structure. 

Mountains  produced  by  erosion.  One  type  of  mountain  struc- 
ture is  represented  by  Figs.  478  and  479.  Mountains  of  this  sort 


446 


PHYSIOGRAPHY 


may  occur  singly,  but  they  are  often  in  groups.     They  are  clearly 
the  result  of  erosion,  their  surroundings  having  been  worn  away. 


FIG.  478. — Mountains  of  horizontal  strata,  Timpanagas  Mountain,  Utah. 

(Church.) 

Mountains  of  this  sort  are  developed  from  plateaus  in  the  course 
of  their  degradation.  The  Catskill  Mountains  are  an  illustration. 
Other  illustrations  occur  in  the  arid  regions  of  the  West,  where 
the  isolated  masses  of  rock  are  often  called  buttes  (Fig.  173). 


FIG.  479. — Mountains  shaped  by  the  erosion  of  horizontal  beds  of  stratified 
rock.     Castle  Group,  Colo.     (Holmes,  Hayden  Surv.) 

Other  illustrations  of  mountains,  the  outlines  of  which  were  pro- 
duced by  erosion,  are  shown  in  Figs.  479  and  480. 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    447 

Intrusion  and  uplift.  Fig.  37  (p.  39)  represents  another  common 
type  of  mountain  structure.  Mountains  of  this  type  may  con- 
stitute single  mountains  or  groups  of  mountains.  They  may  be 
large  or  small.  In  most  cases  the  bedded  rocks  which  lie  on  the 


FIG.  480. — Mountains  shaped  by  erosion,  where  the  rock  is  massive.     Elk 
Mountains,  Colo.     (Holmes,  Hayden  Surv.) 

sides  once  extended  over  the  crests,  and  have  been  cut  away  by 
erosion.  The  Black  Hills  and  the  Adirondacks  are  examples  of 
large  groups  of  mountains  of  this  type.  Numerous  small  ones 
occur  about  the  Black  Hills  and  at  many  other  points  in  the 
West.  The  Henry  Mountains  of  Utah  (Figs.  402  and  404),  made 
classical  by  the  exhaustive  study  of  Gilbert,  are  a  well-known 
illustration  of  this  general  type. 

Mountains  of  this  type  of  structure  may  be  linear,  making  a 
mountain  range  or  system  rather  than  a  mountain  group.  The 
Sierra  Nevada  Mountains  of  California  are  an  example. 

Mountains  produced  by  folding.  Fig.  481  represents  still 
another  type  of  mountain  structure  illustrated  by  the  Jura  Moun- 
tains, while  Figs.  482  and  483  represent  variations  of  the  type. 


FIG.  481. — Section  of  the  western  Jura  Mountains. 

The  Jura  Mountains  as  they  now  are,  are  the  result  of  the  upswell- 
ing  (without  renewed  folding)  of  a  worn-down  mountain  system 
produced  originally  by  folding.  In  other  words,  the  folded  struc- 
ture of  the  former  mountains  has  been  lifted  up  bodily  in  recent 
times.  The  details  of  the  present  mountains  are  the  result  of 
erosion  on  this  upwarped  structure. 


448 


PHYSIOGRAPHY 


It  is  to  be  noted  that  the  present  topography  of  mountains 
whose  component  strata  are  folded  was  not  always  produced 
by  folding.  It  was  indeed  rarely  produced  in  this  way.  The 

folding  doubtless  gave  rise 
to  ridges,  sometimes  to  ridges 
of  great  height,  as  in  the  case 
of  the  Appalachians.  The 

FIG.  482.— Section  across  the  Shcrtenkopf,    mountains      thus     produced 
Bavarian  Alps.    (Frass.) 

were  then  brought    low    by 

erosion.  Later,  the  planed-down  region  of  folded  rocks  was  bowed 
up,  relative  to  its  surroundings,  but  bowed  up  as  a  unit,  without 
further  folding.  The  present  mountain  crests  are  the  outcrops 
of  the  harder  layers,  isolated  by  erosion  subsequent  to  this  later 

rfiiS*^-  ^jlSr^-^iiillsii^iiS^  -^w^T^S^Sir"^ 

FIG.  483. — Appalachian  structure.    (Rogers.) 

uplift  (Figs.  484  and  485).  It  is  now  known  that  many  other 
mountains  of  folded  structure  have  had  a  similar  history. 

Most  mountains  produced  by  folding  have  been  extensively 


FIG.  484. — Diagram  suggesting  the  type  of  structure  possessed  by  the  simple 
folding  of  strata.     The  diagram  shows  the  folded  surface  worn  down. 


modified  by  erosion,   as  the  accompanying  figures  suggest,  but 
there  are  occasional  exceptions,  as  shown  by  Fig.  486. 

Mountains  produced  by  faulting.  Figs.  487  and  488  represent 
another  type  of  mountain  structure.  Such  mountains  are  some- 
times called  block  mountains,  because  great  blocks  of  the  earth's 
crust,  bordered  by  distinct  planes  of  fracture,  have  been  tilted  so 
that  one  edge  at  least  is  well  above  its  surroundings.  In  many 
such  cases,  it  is  probable  that  the  surroundings  have  sunk  rather 
than  the  mountains  themselves  that  have  been  elevated.  Many  of 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    449 

the  mountains  between  the  Rockies  and  the  Sierras  belong  to  this 
type,  as  elsewhere  noted.  This  type  of  mountains  is  sometimes 
known  as  the  Basin  Range  type. 


FIG.  485. — The  same  as  Fig.  484,  after  an  upwarp  and  subsequent  erosion. 
No  further  folding  is  shown,  and  erosion  has  isolated  the  hard  layers  as 
mountain  ridges.  This  represents,  in  a  general  way,  the  present  condi- 
tion of  the  Appalachians. 

These  illustrations  show,  that  mountains  have  various  struc- 
tures; they  also  suggest  how  mountains  originated,  though  they 
do  not  in  all  cases  indicate  the  causes  which  brought  them  into 
existence. 

Summary.  It  will  be  seen  from  the  foregoing  that  mountains 
are  developed  (1)  by  the  degradation  of  their  surroundings  (Fig. 


CLEMAN  MOUNTAIN 


UMNUM  RIDGE 


i.  ::-..."...  .i  :-  .-^-^ "  '  '^-  -^ 

FIG.  486. — Open  low  mountain  folds,  not  greatly  modified  by  erosion.     Cle- 
man  Mountain  and  Umnum  Ridge,  Washington.     (U.  S.  Geol.  Surv.) 

471);  (2)  by  the  subsidence  of  their  surroundings,  either  by  down- 
warping  or  down-wedging  (Fig.  487);  (3)  by  elevation,  either  by 
up-warping  (folding)  (Fig.  486)  or  up-wedging;  (4)  by  up-swelling, 
due  to  intrusions  of  igneous  rock  (Fig.  403) ;  and  (5)  by  up-building, 


FIG.  487. — Ranges  of  the  Great  Basin.     Length  of  section,  120  miles. 

(Gilbert.) 

as  in  the  case  of  volcanic  cones.  Mountains  which  originate  by 
diastrophism  or  vulcanism  are  subject  to  erosion,  and  most  existing 
mountains  of  volcanic  or  diastrophic  origin  have  been  so  largely 


450 


PHYSIOGRAPHY 


modified   by  erosion,  that  the  details  of   their  present  surfaces 
are  the  result  of  degradation. 


FIG.  489. — The  divide  between  the  head- 
waters of  the  Lake  Fork  of  San  Miguel 
River  and  Cascade  Creek,  southwestern 
Colorado.  The  lowest  point  in  the  di- 
vide, in -the  center  of  the  photograph,  is 
12,700  to  12,800  feet  above  the  sea. 
(Hole.) 


Jx 


"V 


FIG.  490.— Cascade  Pass,  Washington. 
The  trees  have  an  Alpine  aspect. 
(U.  S.  Geol.  Surv.) 


Effects  of  Mountains  on  Mankind 

Climctic  effects.     Directly  and  indirectly,  mountains  play  an 
important  part  in  the  affairs  of  men.     In  the  first  place,  they  affect 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    451 

climate  greatly.  The  winds  blowing  over  them  are  cooled,  and 
as  they  are  cooled  a  part  of  their  moisture  often  condenses  and 
falls.  Thus  mountains,  and  especially  the  windward  sides  of 
mountains,  are  generally  the  sites  of  heavy  rainfall,  and  there- 
fore become  the  sources  of  important  streams.  On  the  other 
hand,  plains  and  plateaus  on  the  leeward  side  of  mountains  often 
have  light  rainfall,  because  the  air,  after  passing  over  the  moun- 
tains where  it  has  left  much  of  its  moisture,  is  drying  rather  than 
rain-giving.  This  is  the  reason  why  the  tracts  east  of  the  Sierra 
and  Rocky  mountains  are  arid  or  semi-arid  (Fig.  492),  and  therefore 
sparsely  settled.  The  state  of  Nevada,  with  an  area  of  more  than 


FIG.  491. — A  view  in  the  Sierras  from  University  Peak.     Ko-ip  Crest, 
Sierra  Nevada  Mountains. 

100,000  square  miles,  had  a  smaller  population  in  1900  than  the 
city  of  Peoria,  Illinois. 

Though  the  mountains  make  the  country  to  leeward  arid, 
they  sometimes  furnish  water  which  may  be  utilized  in  irrigating 
these  lands,  for  as  the  water  which  falls  in  the  mountains  flows 
out  from  them,  it  may  be  diverted  from  its  natural  courses  and 
carried  out  by  ditches  to  the  fields. 

The  work  of  storing  water  in  preparation  for  irrigation  has 
been  well  started  in  the  western  part  of  the  United  States  (p.  193), 
and  still  more  extensive  work  in  this  direction  is  already  planned. 
But  a  small  part  of  the  arid  lands  of  the  West  will  ever  be  irrigated, 
however,  for  the  amount  of  water  available  is  too  small  to  supply 
more  than  a  fraction  of  all  the  land  which  needs  water. 

The  effect  of  mountains  on  the  temperature,  winds,  cloudiness, 


452 


PHYSIOGRAPHY 


etc.,  of  their  surroundings  is  considerable,   though  perhaps  less 
important  than  their  effect  on  precipitation. 

Mountains  are  barriers  to  transportation.  It  is  true  that 
railroads  now  cross  mountain  systems,  but  the  cost  of  building 
and  operating  them  after  they  are  built  is  much  greater  than 
on  the  plains.  A  railroad  map  of  the  United  States  shows  that 
there  are  few  railroads  in  the  eastern  or  western  mountains,  as 
compared  with  the  number  in  the  interior.  Mountains  are  how- 
ever much  less  effective  barriers  to  mankind  now  than  in  earlier 


FIG.  492.— Rainfall  of  the  United  States.     (U.  S.  Weather  Bureau.) 

times,  before  railway  engineering  had  reached  its  present  develop- 
ment. 

Mountains  are  effective  barriers  to  animals  and  plants. 
Most  of  the  animals  lower  than  man  do  not  possess,  and  are  unable 
to  devise,  means  of  crossing  mountains,  and  to  many  of  them  high 
mountains  are  effective  barriers.  The  climate  of  high  altitudes 
is  often  such  as  to  prevent  the  migration  of  plants  from  one  side 
to  the  other,  except  by  human  help. 

Mountains  often  contain  ores  of  various  metals.  The  fact 
that  mining  is  the  most  distinctive  industry  of  many  mountains 
has  been  referred  to  elsewhere.  It  may  be  added  here  that  most 
of  the  gold  and  silver  and  much  of  the  copper  of  the  United  States 
come  from  the  western  mountains.  From  the  same  sources  also 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    453 


454 


PHYSIOGRAPHY 


IOO 35 9O 85- 60 T5 TO 


GOLD  AND  SILVER 


FIG.  494. — Map  showing  the  areal  distribution  of  gold  and  silver  in  the  United 
States.  The  relative  importance  of  the  productions  of  different  areas  is 
not  indicated.  Circles  =  gold,  crosses  =  silver,  and  the  two  combined  = 
gold  and  silver.  (After  Ransome.) 


FIG.  494a. — Map  showing  the  distribution  of  copper  ores  in  the  United  States. 
The  sizes  of  the  dots  indicate  approximately  the  relative  amounts  pro- 
duced. (U.  S.  Geol.  Surv.) 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES     455 


125  iaO  IIS  IIO  105          100         95  90  65  6O  75  .         7O  65 


FIG.  495. — Map  showing  the  distribution  of  iron  ores  in  the  United  States. 
In  1904  Pennsylvania  produced  about  350,000  tons;  Colorado,  about 
400,000  tons;  Virginia  and  New  Jersey,  about  500.000  tons  each;  New 
York,  about  789,000  tons;  the  southern  Appalachian  region,  chiefly 
Alabama,  about  4,459,000  tons;  while  the  Lake  Superior  region  pro- 
duced 20,198,000  tons.  The  production  of  the  Lake  Superior  region  had 
increased  by  1906  to  more  than  33,000,000  tons.  (Based  on  an  unpub- 
lished map  prepared  by  W.  O.  Hotchkiss,  with  C.  K.  Leith.) 


FiG.  496. — Map  showing  the  distribution  of  coal  in  the  United  States. 
(U.  S.  Geol.  Surv.) 


456 


PHYSIOGRAPHY 


comes  much,  but  not  all,  of  the  lead  and  zinc.  Iron  and  coal, 
on  the  other  hand,  the  two  most  important  products  of  mining, 
are  not  won  chiefly  from  mountain  regions,  though  some  iron 
and  much  coal  is  mined  both  in  the  eastern  and  western  moun- 
tains. 


>LEAO  AND  ZINC 


FIG.  497. — Map  showing  the  distribution  of  lead  and  zinc  ores  in  the  United 
States.  Circles  =  lead,  crosses  =  zinc,  and  circles  and  crosses  combined 
=  lead  and  zinc.  (After  Ransome.) 


Agriculture  in  mountains.  Mountain  valleys  are  often  fertile, 
and  many  of  them  are  under  cultivation.  Colorado,  a  moun- 
tainous state,  produces  more  mineral  wealth  than  any  other  state 
in  the  West;  but  the  value  of  the  products  of  the  soil  is  greater 
than  that  of  the  products  of  its  mines.  A  considerable  portion  of 
the  cultivated  land  is  in  the  mountains. 

Scenic  effects.  Quite  apart  from  economic  considerations, 
mountains  have  a  value  not  to  be  estimated  in  dollars  and  cents, 
in  the  scenery  which  they  afford.  The  man  who  has  not  seen 
mountains,  and  who  has  not  lived  with  them  long  enough  to  really 
make  their  acquaintance,  has  missed  one  of  the  good  things  of  life. 
The  Adirondacks,  the  Catskills,  and  the  other  mountains  within 
a  few  hours'  ride  of  such  great  cities  as  New  York,  Philadelphia,  and 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    457 

Boston,  are  of  inestimable  value.  The  mountains  of  the  West  are 
far  grander.  They  are,  unfortunately,  frequented  by  fewer  people, 
because  much  farther  from  great  centers  of  population. 

The  Outlines  of  the  Continents 

Any  good  map  showing  the  outlines  of  the  continents  makes 
it  clear  that  some  coast-lines  are  regular  while  others  are  most 
irregular.  The  coasts  of  the  northern  part  of  North  America  and 
Eurasia  are  strikingly  irregular,  and  in  strong  contrast  with  the 
outlines  of  South  America,  Africa,  and  Australia.  The  west 
coast  of  the  southern  part  of  South  America  is,  however,  very 
irregular. 

Even  the  more  regular  coast-lines  present  contrasts,  for  some 
of  them  are  nearly  straight,  while  others  are  notably  curved;  and, 
where  the  continental  outlines  present  large  irregularities,  certain 
portions  of  the  coast,  considered  by  themselves,  are  regular,  and 
these  may  be  straight  or  curved.  Illustrations  of  such  regularity 
are  found  on  the  west  coast  of  India  and  the  southeast  coast  of 
Arabia,  though  India  and  Arabia  themselves  constitute  coastal 
irregularities  of  great  size. 

The  irregular  coasts  present  much  greater  variety.  Broadly 
speaking,  it  may  be  said  that  there  are  two  great  types  of  irregu- 
larity, namely  (1)  projections  of  the  water  into  the  land,  and  (2) 
projections  of  the  land  into  the  water.  The  former  are  bays,  gulfs, 
etc.,  and  the  latter  are  peninsulas,  capes,  etc. 

Coastal  irregularities  may  be  further  classified  in  various  ways, 
and  each  classification  brings  out  certain  significant  features. 
They  may  be  classified  on  the  basis  of  (1)  size,  (2)  position  with 
reference  to  the  general  trend  of  the  coast,  (3)  relief,  (4)  origin, 
etc. 

Size.  The  projections  of  water  into  the  land,  like  the  pro- 
jections of  land  into  the  water,  may  be  either  large  or  small.  The 
Gulf  of  Mexico,  Hudson  Bay,  the  Bay  of  Bengal,  and  the  Baltic  Sea 
are  examples  of  large  projections  of  water  into  the  land,  while  Dela- 
ware, Chesapeake,  Narragansett,  and  San  Francisco  bays,  and  Puget 
Sound  are  examples  of  smaller  projections  of  water  into  the  land. 
The  little  bays  on  the  sides  of  Chesapeake  Bay  (Fig.  177)  are 
examples  of  still  smaller  indentations  of  the  same  sort.  On  the 
coasts  of  Alaska  (Fig.  498),  Norway,  Chile  (Fig.  499),  and  some 


458 


PHYSIOGRAPHY 


FIG.  498. — Map  of  part  of  the  coast  of  southern  Alaska,  showing  islands, 
which  were  once  a  part  of  the  mainland,  isolated  by  glacial  and  wave 
erosion,  and  by  sinking. 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    459 


other    places,    there   are  numerous    narrow    but  deep   bays    or 
fiords  (p.  248). 

Examples  of  large  projections  of  land  into  the  sea  are  afforded 
by  the  north  coast  of  Australia,  the  east  coast  of  Africa,  the  south 


SKETCH  MAP  Or 

S  O  UT  HERN 

ARGEN  TI NA 


FIG.  499. — Islands  on  the  west  coast  of  South  America  which  were  once  a 
part  of  the  continent.  They  have  been  isolated  by  erosion  of  glaciers 
and  waves  and  by  subsidence. 

coast  of  Asia,  and  both  the  south  and  west  coasts  of  Europe. 
North  America,  too,  furnishes  illustrations  of  this  sort,  especially 
in  Alaska  and  Labrador,  and,  on  a  somewhat  smaller  scale,  in 


460  PHYSIOGRAPHY 

Florida,  Yucatan,  and  Lower  California.  Small  projections  of 
land  into  the  sea  abound  on  most  coasts.  Cape  Cod,  Cape  May, 
and  Cape  Mendocino  are  examples. 

The  projections  of  water  into  the  land  and  of  land  into  the 
water  are  often  closely  related.  If  two  bodies  of  water  not  far 
from  each  other  project  into  the  land,  they  leave  between  them  a 
projection  of  land  into  the  water.  An  illustration  is  afforded  by 
the  Bay  of  Bengal  and  the  Arabian  Sea  with  India  between.  India 
may  therefore  be  looked  upon  as  a  projection  of  land  into  the 
sea,  or  as  an  area  of  land  left  by  the  projection  of  two  great  arms 
of  the  sea  into  the  land.  Similarly,  if  two  areas  of  land  not  far  apart 
project  into  the  sea,  they  enclose  a  body  of  water.  The  land  east 
and  west  of  the  Gulf  of  Carpentaria  on  the  north  coast  of  Australia 
is  an  example. 

Position.  Arms  of  the  sea  may  project  into  the  land,  or 
areas  of  land  into  the  sea,  so  as  to  be  somewhat  nearly  at  right 
angles  to  the  trend  of  the  coast;  or  they  may  have  positions  essen- 
tially parallel  to  the  general  trend  of  the  coast.  Florida,  India, 
Hudson  Bay,  and  the  Gulf  of  Mexico  are  examples  of  the  former, 
and  the  Gulf  and  Peninsula  of  Lower  California  and  many  small 
irregularites  along  the  Atlantic  coast  of  the  United  States  (Fig. 
500)  are  examples  of  the  latter.  It  will  be  seen  that  both  large 
and  small  irregularities  may  occupy  either  position.  They  also 
occupy  positions  intermediate  between  these  two. 
•;;  Relief.  Some  coasts  are  high  and  some  low,  and  the  differences 
are  so  great  that  the  lands  present  strong  contrasts  when  seen 
from  the  sea.  The  irregularities  of  coast-lines  as  seen  on  maps 
affect  both  the  coasts  which  are  low  and  those  which  are  high. 
Some  of  the  great  peninsulas  which  project  out  into  the  water, 
such  as  Scandinavia,  the  Iberian  Peninsula,  India,  Lower  Cali- 
fornia, and  much  of  Alaska,  are  high,  and  others,  such  as  Florida 
and  Yucatan,  are  low.  Small  projections  of  land  into  the  sea 
present  the  same  contrasts.  Some  of  them,  as  those  on  the  coasts 
of  Maine,  Alaska,  and  Chile,  are  high,  while  others,  such  as  those 
along  the  eastern  coasts  of  the  United  States  south  of  New  York, 
are  low. 

The  water  in  bays,  gulfs,  fiords,  etc.,  is  sometimes  deep  and 
sometimes  shallow,  and  its  depth  is  measurably  independent  of 
area.  It  is  deep,  for  example,  in  the  Gulf  of  Mexico,  the  Gulf  of 
California,  the  Mediterranean  Sea,  the  Arabian  Sea,  in  many 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    461 

fiords,  etc.,  but  shallow  in  the  Baltic  Sea,  Hudson  Bay,  and  the 
Gulf  of  Carpentaria. 

Distribution  of  various  types  of  irregularities.  From  the 
maps  showing  the  outlines  of  the  continents,  it  appears  that  great 
irregularities  are  distributed  with  less  inequality  than  the  small 
ones.  While  the  northern  continents  have  both  large  and  small 


FIG.  500. — Portion  of  the  coast  of  Texas,  showing  the  tendency  of  shore 
deposition  to  simplify  the  coast  line.  The  deposits  (narrow  necks  of 
land  parallel  to  the  coast)  shut  in  bays.  (Coast  and  Geodetic  Surv.) 

irregularities  in  greater  numbers  than  the  southern  continents, 
the  contrast  between  the  small  irregularities  of  the  northern 
and  southern  continents  is  greater  than  that  between  the  large 
ones. 

The  great  irregularities  are  not  notably  greater  in  the  northern 
parts  of  the  northern  continents  than  in  their  southern  parts.  So  far 
as  this  class  of  irregularities  is  concerned,  the  coast-lines  of  southern 
Asia  and  Europe  are  as  irregular  as  those  of  other  parts  of  these 
continents.  The  small  irregularities  of  northern  Europe,  and 
especially  of  northwestern  Europe,  are,  however,  more  conspicuous 


462  PHYSIOGRAPHY 

than  those  of  southern  Europe.  The  same  holds,  in  a  general  way, 
for  North  America.  While  this  continent  has  great  and  small 
irregularities  both  at  the  north  and  south,  small  irregularities 
are  more  numerous  in  high  latitudes  than  in  low. 

Again,  the  small  irregularities  in  the  southern  part  of  North 
America  are  more  commonly  low,  while  those  in  the  northern  part 
often  have  greater  vertical  range.  The  former  are  often  parallel 
to  the  trend  of  the  coast,  while  the  latter  are  more  commonly  at 
right  angles  to  it. 

The  irregularities  of  coasts  stand  in  some  relation  to  the  width 
of  the  continental  shelf.  Large  irregularities  of  outline  are,  in 
general,  more  common  where  the  continental  shelf  is  wide  than 
where  it  is  narrow.  High  shores  are,  on  the  whole,  more  irregular 
in  outline  than  low  ones,  though  to  this  general  rule  there  are  many 
exceptions. 

The  islands  along  many  of  the  coasts  of  continents  are  really 
to  be  looked  upon  as  parts  of  the  coastal  irregularities,  for,  as  we 
shall  see  later,  many  islands  along  coasts  were  once  part  of  the 
mainland.  Here  belong  many  of  the  islands  off  the  coast  of  Alaska 
(Fig.  498),  Chile  (Fig.  499),  Scandinavia,  etc. 

All  these  numerous  and  varied  irregularities  call  for  explana- 
tion, and  our  studies  of  processes  now  in  /Operation  have  furnished 
the  data  necessary  for  understanding  why  some  coast-lines  are 
regular  and  others  irregular,  why  some  coasts  are  high  and  others 
low,  why  the  slopes  of  some  coasts  are  steep  and  those  of  others 
gentle.  They  have  also  given  us  a  basis  for  some  conception  of 
the  origin  of  great  and  small  projections  of  land  into  the  sea,  and 
of  great  and  small  projections  of  the  sea  into  the  land. 

Agents  of  gradation.  In  preceding  chapters  wre  have  seen 
the  results  produced  by  agents  of  gradation  on  the  horizontal  con- 
figuration of  coasts.  We  have  seen  (p.  320)  that  waves  tend  to 
develop  indentations  of  water  where  the  rock  is  weak,  leaving 
projections  of  land  where  the  rock  is  resistant,  and  that  irregu- 
larities thus  developed  are  relatively  small.  The  capes,  etc.,  thus 
formed  will  be  low  or  high,  depending  on  the  relief  of  the  land  from 
which  they  were  developed.  The  reentrants  of  water  developed 
by  wave  erosion  are  always  shallow-. 

We  have  also  seen  that  deposition  along  shores  develops  irregu- 
larities, especially  by  the  formation  of  strips  of  land  roughly  parallel 
to  the  trend  of  the  coast,  across  the  debouchures  of  bays,  etc.,  and 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    463 

that  the  irregularities  thus  developed  are  a  step  in  the  direction 
of  final  simplification  of  the  shore-line  (Fig.  500).  The  lands  de- 
veloped by  shore  deposition  are  always  low,  as  left  by  the  waves, 
and  the  lagoons  shut  in  behind  them  are  shallow. 

We  have  also  seen  (p.  248)  that  glaciers  descending  to  the  sea 
may  gouge  out  deep  valleys,  which  become  fiords  when  the  ice 
melts.  Glacial  erosion  may  otherwise  modify  the  coast-line,  both 
by  erosion  and  deposition.  Glaciation,  once  much  more  exten- 
sive than  now,  affords  the  explanation,  or  at  least  a  part  of  the 
explanation,  of  the  many  fiords  of  high  latitudes.  Subsidence  may 
also  be  a  factor  in  the  development  of  fiords. 

Rivers  make  coast-lines  irregular  by  building  deltas  at  their 
debouchures,  but  through  their  erosive  work  they  do  little  to 
make  coast-lines  irregular  horizontally.  On  the  other  hand,  they 
make  high  coast-lands  irregular  vertically,  by  developing  valleys 
in  them. 

Winds  have  little  effect  on  the  horizontal  configuration  of 
coasts,  but  by  piling  up  dunes  they  affect  the  relief  of  coast-lands 
to  some  extent. 

This  brief  review  makes  it  clear  that  agents  of  gradation  are 
competent  to  produce  many  irregularities  of  coast,  especially  those 
of  small  size. 

Diastrophism.  If  the  bottom  of  the  ocean  were  somewhat 
depressed,  increasing  the  capacity  of  the  basin,  the  water  would  be 
drawn  down  about  the  borders  of  the  continents  and  all  the  coast- 
lines would  be  shifted  seaward.  On  such  a  coast  as  that  of  the 
eastern  part  of  the  United  States,  the  border  of  the  continent  would 
become  notably  more  regular  than  now,  because  the  topography  of 
the  continental  shelf,  now  submerged,  is  nearly  plane.  Some  coasts 
which  are  comparatively  regular  owe  their  regularity  to  recent 
emergence. 

If,  on  the  other  hand,  the  borders  of  continents  were  depressed, 
the  coast-lines  would  in  general  become  somewhat  more  irregular 
than  now,  for  the  depression  of  the  land  would  allow  the  sea-water 
to  extend  up  the  valleys,  developing  bays  where  there  are  none 
now,  and  extending  those  which  now  exist  (p.  174). 

Some  indented  coasts,  like  that  of  the  United  States  between 
New  York  and  the  Carolinas,  owe  their  numerous  bays  to  recent 
subsidence.  Where  the  river  valleys  were  normal  to  the  coast,  as 
is  most  commonly  the  case,  roughly  speaking,  the  bays  are  normal 


464  PHYSIOGRAPHY 

to  the  coast.  If  the  valleys  drowned  in  the  making  of  the  bays 
were  not  normal  to  the  coast,  the  bays  would  not  be. 

Again,  the  sufficient  up-warp  of  a  submerged  tract  along  the 
coasts  of  continents  would  develop  peninsulas.  These  penin- 
sulas might  be  normal  to  the  coast  or  roughly  parallel  to  it,  or  at 
any  angle  between.  A  corresponding  down-warp  would  develop 
a  bay  or  gulf,  and  many  large  bays  and  gulfs  have  probably  arisen 
in  this  way.  The  elevated  or  depressed  area  might  be  faulted 
instead  of  warped,  with  similar  results  so  far  as  the  horizontal 
configuration  of  the  coast  is  concerned. 

Vulcanism.  Volcanoes  affect  coast-lines  locally,  but  their 
influence  is  relatively  slight  as  compared  with  that  of  gradation 
and  diastrophism.  Volcanoes  make  islands  near  coasts  more  com- 
monly than  they  produce  modifications  of  the  coasts  of  mainlands. 
Igneous  rocks  are  often  more  resistant  than  sedimentary  rocks, 
and  so  affect  the  forms  of  coast  lines  developed  by  erosion. 

Application 

By  the  application  of  the  above  principles  to  coast-lines,  the  chief 
features  of  many  of  them  may  be  readily  understood.  Where 
there  are  numerous  bays  along  the  coast-line  projecting  into  the 
land  at  right  angles,  roughly  speaking,  to  its  general  trend,  it  may 
be  inferred  with  some  confidence  either  that  the  region  has  re- 
cently sunk,  drowning  the  lower  ends  of  the  rivers,  or  that  it  has 
been  glaciated  recently,  converting  the  lower  ends  of  the  valleys 
into  fiords,  or  both.  If  the  area  concerned  is  in  low  latitude,  the 
chances  are  in  favor  of  the  first  interpretation;  if  in  high  latitude, 
and  especially  if  the  altitude  be  high,  glaciation  is  a  probable  or 
partial  cause  of  the  indentations. 

Chesapeake  Bay  and  the  numerous  bays  tributary  to  it,  Dela- 
ware Bay  and  others  of  the  same  sort  on  the  eastern  coast  of  the 
United  States,  point  clearly  to  recent  submergence  of  the  land. 
Farther  north,  the  indentations  of  the  coast  of  Maine  find  their 
explanation  partly  in  subsidence  perhaps,  but  largely  in  glacier 
erosion,  for  the  ice  of  the  continental  glacier  passed  out  to  sea 
over  this  coast.  The  fiords  of  such  coasts  as  that  of  Alaska, 
Chile,  Scandinavia  and  Scotland  are  largely  the  result  of  glacial 
erosion,  though  subsidence  may  have  deepened  and  extended  the 
indentations  of  water. 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    465 

Where  there  are  long,  narrow  belts  of  low  land,  roughly  parallel 
to  the  general  trend  of  the  coast,  deposition  by  waves  and  shore 
currents  is  to  be  inferred.  Illustrations  are  afforded  by  many 
parts  of  the  coast  between  New  York  and  Texas. 

Where  there  are  great  irregularities  of  outline,  such  as  the 
Gulf  of  Mexico,  the  Gulf  of  California,  the  Adriatic  Sea,  the  Bay 
of  Bengal,  the  Arabian  Sea,  the  Iberian  Peninsula,  Italy,  India, 
Kamchatka,  the  peninsulas  of  Lower  California,  Yucatan,  Florida, 
etc.,  diastrophism  has  probably  been  the  chief  factor  concerned. 

The  irregularities  of  coast  produced  by  diastrophism  are  not 
all  of  great  size.  Puget  Sound,  though  large,  is  much  smaller 
than  most  of  the  irregularities  mentioned,  and  is  believed  to  have 
had  its  origin  in  a  down-warp. 

Where  coasts  are  high,  diastrophism  or  wave-cutting,  or  both, 
are  suggested.  Steep  slopes,  even  where  not  high,  give  the  same 
suggestions,  while  low  coastal  lands  without  cliffs  are  character- 
istic of  areas  of  shore  deposition. 

It  is  to  be  borne  in  mind  that  coast-lines  are  not  permanent, 
and  that  the  coast-lines  of  to-day  are  not  precisely  the  same  as 
the  coast-lines  of  yesterday,  and  those  of  to-morrow  will  not  be 
precisely  the  same  as  those  of  to-day,  for  gradation  and  diastrophism 
are  constantly  changing  them,  and  vulcanism  occasionally. 

Historical  bearing.  The  character  of  the  coast-lines  has  had 
an  important  influence  upon  the  development  of  many  countries. 
The  irregular  coasts  of  northwestern  Europe  and  the  northeastern 
part  of  the  United  States  abound  in  harbors,  and  favor  the  de- 
velopment of  ocean  commerce.  On  the  other  hand,  a  smooth, 
regular  coast  has  always  rendered  difficult,  and  sometimes  com- 
pletely discouraged,  sea  trade.  The  southeastern  states,  eastern 
Mexico,  Africa,  and  India  have  all  experienced,  in  varying  degrees, 
the  disadvantages  of  such  a  coast.  The  greatest  motive  in  Russian 
expansion  has  been  the  possession  of  ice-free  harbors. 

Wherever  a  people  has  occupied  an  indented  coast,  with  off- 
lying  islands  and  an  infertile  hinterland,  it  has  early  turned  to 
the  sea  for  a  living,  and  has  developed  daring  seamen;  and  this 
regardless  of  race  or  inherent  abilities.  Examples  are  the  North- 
men, the  Indians  of  southern  Alaska,  the  blacks  of  northwestern 
Madagascar,  and  the  Malays  of  the  Tenasserim  coast.  On  the 
other  hand,  a  harborless  coast  has  invariably  prevented  the  de- 
velopment of  the  sea-going  habit. 


466  PHYSIOGRAPHY 


ISLANDS 

As  already  indicated,  many  islands  are  really  shore  features, 
being  developed  by  the  same  agents  and  processes  which  develop 
the  horizontal  configuration  of  coasts. 

Like  other  natural  features,  islands  may  be  classified  in  various 
ways,  and  each  classification  brings  out  certain  significant  facts. 
Thus,  on  the  basis  of  size,  they  are  large  and  small;  on  the  basis 
of  height,  they  are  high  and  low;  on  the  basis  of  position,  they  are 
continental  and  oceanic;  and  on  the  basis  of  fertility,  they  are 
fertile  and  barren.  Between  the  extremes  of  each  of  the  above 
groups  there  are  all  gradations.  Other  comparable  bases  of  group- 
ing may  be  suggested.  The  most  significant  classification,  from  the 
physiographic  point  of  view,  is  based  on  origin.  Islands  arise 
through  the  processes  of  diastrophism,  vulcanism,  and  gradation, 
and,  if  the  action  of  organisms  be  excluded  from  gradation,  by 
organic  action. 

1.  By  diastrophism.     The  rise  of  any  portion  of  sea  bottom 
enough  to  cause  it  to  emerge  from  the  wrater,  gives  rise  to  an  island, 
if  the  new  land  is  not  connected  with  a  continent.     Similarly,  the 
subsidence  of  the  sea  might  cause  the  emergence  of  elevated  por- 
tions of  sea  bottom,  giving  rise  to  islands.     Cuba  and  the  other 
large  islands  of  the  West  Indies  belong  to  this  general  class. 

The  rise  of  the  sea-level  might  transform  the  elevations  of  a 
coastal  plain  into  islands,  by  submerging  the  surrounding  land. 
The  same  result  might  be  brought  about  by  the  sinking  of  coastal 
lands  of  strong  relief.  Great  Britain  was  thus  separated  from  the 
mainland.  Had  it  remained  connected  with  the  continent,  the 
course  of  European  history  would  probably  have  been  very  dif- 
ferent. 

2.  By  vulcanism.     Many  submarine  volcanoes  have  built  up 
their  cones  so  that  their  tops  emerge.     Far  from  coasts,  islands  of 
this  sort  are  more  common  than  any  others.     Volcanic  islands 
are,  however,  not  confined  to  the  deep  sea. 

3.  By  gradation,    (a)  By  erosion.     Islands  arise  both  by  aggra- 
dational  and  degradational  processes,  and  both  aggradation  and 
degradation  are  effected  by  different  agents. 

Waves  often  so  erode  a  coast  as  to  isolate  small  areas  of  resistant 
rock,  converting  them  into  islands  (Fig.  501). 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    467 

Glaciers  descending  from  the  land  to  the  sea  may,  by  erosion, 
isolate  coastal  promontories,  converting  them  into  islands.  It  is 
probable  that  some  of  the  islands  on  glaciated  coasts  arose  in  this 

way. 

Islands  are  sometimes  formed  in  rivers  by  the  erosion  of  the 
stream  (Fig.  502).  The  jointing  of  the  rock  seems  often  to  afford 
the  conditions  for  the  development  of  such  islands.  The  erosive 
action  of  the  James  River  transformed  the  Jamestown  peninsula 
into  an  island  toward  the  end  of  the  17th  century,  a  thing  the 


FIG.  501. — Finn  Rock  and  Cape  Blanco,  Oregon.      (U.  S.  Geol.  Surv.) 

colonists  had  planned  to  do  for  purposes  of  defence.  River  islands 
are  also  sometimes  developed  through  the  meandering  of  the 
streams  (Fig.  197). 

(b)  By  deposition.  Islands  arise  by  the  deposition  of  sediment 
along  sea  and  lake  shores  and  in  rivers.  Such  islands  are  usually 
low  and  sandy,  and  always  near  other  land.  The  processes  which 
give  rise  to  them  have  been  indicated  (p.  324).  They  are  often 
affected  by  dunes.  Glacier  deposits  also  give  rise  to  islands,  as  in 
Boston  harbor.  Islands  which  have  cores  of  solid  rock  are  often 
enlarged  by  various  processes  of  deposition. 

4.  By  combinations  of  diastrophism,  gradation,  and  vulcanism. 
Many  existing  islands  owe  their  origin  and  form  to  the  com- 
bination of  two  or  more  of  the  above  agents.  River  or  glacier 
erosion  often  develops  an  uneven  topography  along  shore,  and  a 


468  PHYSIOGRAPHY 

slight  subsidence  of  the  coast,  or  a  rise  of  the  sea-level  there,  gives 
rise  to  islands,  because  the  land  has  been  properly  prepared  in  ad- 
vance. It  is  to  such  a  combination  of  degradation  and  diastroph- 
ism  that  many  of  the  islands  of  glaciated  coasts,  such  as  those  of 
Maine,  Alaska,  Norway,  etc.,  are  due. 

Other  combinations,  too,  of  the  several  agents  operative  on 
coasts  may  give  rise  to  islands.     Thus,  an  island  which  was  pri 
marily  volcanic  may  be  enlarged  in  area  by  the  deposition  of  sedi- 


FIG.  502. — Lone  Rock.  An  island  in  the  Wisconsin  River,  isolated  as  an 
island  by  the  notable  widening  of  a  series  of  joints  in  the  sandstone. 
(Meyers.) 

ment  about  it,  the  sediment  being  brought  down  from  the  higher 
parts  of  the  island.  Iceland  is  an  example. 

Whatever  their  origin,  most  existing  islands  have  been  more 
or  less  notably  modified  by  erosion. 

Island  coasts  are  subject  to  all  the  changes  which  affect  the 
coasts  of  continents.  Islands  are  subject  to  destruction  by  the 
waves,  on  the  one  hand,  and  they  may  cease  to  be  islands  by 
being  attached  to  continents.  Such  connection  may  be  brought 
about  by  diastrophism  or  gradation  (PI.  XXII).  Thus,  a  former 
island  may  be  tied  to  the  mainland  by  deposition.  After  being 
joined  to  the  mainland  the  former  island  becomes  a  part  of  a 
striking  irregularity  of  the  coast. 

5.  By  organic  processes.  There  are  in  some  parts  of  the  world 
numerous  islands  composed  of  coral.  The  little  animals  (polyps) 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    469 


which  secrete  the  coral  live  (1)  where  the  water  is  120  feet  or  less 
in  depth;  (2)  where  the  temperature  never  falls  below  about  68° 
F.;  (3)  where  the  water  has  the  saltness  of  normal  sea-water;  (4) 
where  the  water  is  nearly  free  from  sediment;  and  (5)  where  it 
is  subject  to  some  movement  by  the  wind.  In  such  situations 
they  thrive,  and  sometimes  make  reefs  and  sometimes  islands. 

Polyps  are  not  free-moving  animals,  except  in  the  early  part  of 
their  lives,  before  they  begin  coral-making.     Through  the  larger 


FIG.  503. — Diagram  of  a  fringing 
reef. 


FIG.  504. — Diagram  of  a  barrier 
reef. 


part  of  their  lives  they  are  attached  to  the  bottom.  They  flourish 
about  many  islands  of  volcanic  origin  and  along  some  continental 
coasts,  as  along  the  east  coast  of  Australia.  They  also  flourish 
in  some  places  far  from  islands  or  continents,  if  there  is  shallow 
water  of  the  right  temperature. 

Figs.  503   and  504  show   coral  reefs.      Those  which  are  far 
enough  from  the  land  to  leave  a  somewhat  wide  and  deep  lagoon 


FIG.  505. — Diagram  suggesting  the  development  of  a  barrier  reef  and  an 
atoll,  successively,  from  a  fringing  reef  by  sinking.  1.  Fringing  reef, 
formed  in  shallow  water;  2,  barrier  reef,  developed  from  fringing  reef 
after  subsidence;  3,  the  atoll  which  succeeds  the  barrier  reef. 

inside  are  barrier  reefs;  those  close  to  the  land  are  fringing  reefs. 
li  seems  probable  that  fringing  reefs  sometimes  become  barrier 
reefs  by  the  sinking  of  the  island  or  coast  where  they  occur,  as 
illustrated  by  Fig.  505.  The  sinking  should  not  proceed  faster 
than  the  polyps  build  up  the  reef.  Barrier  reefs,  the  bottoms  of 
which  are  in  deep  water,  were  formerly  thought  to  prove  sub- 
sidence; but  this  conclusion  is  questioned.  A  reef  in  shallow  water 
may  come  to  have  a  long  outer  slope,  with  its  bottom  in  water  far 
below  120  feet,  if  coral  be  broken  off  from  the  upper  part  of  the 


470 


PHYSIOGRAPHY 


reef  and  caused  to  descend  the  slope  into  deeper  water.  This 
process  is  illustrated  by  Fig.  506.  It  is  probable  that  barrier  reefs 
have  been  developed  in  both  these  ways.  Coral  reefs  are  usually 


FIG.  506. — Diagram  suggesting  the  origin  of  a  barrier  reef  without  subsidence. 
The  reef  starts  in  shallow  water  near  shore.  Material  broken  from  it 
falls  down,  making  a  sort  ol  talus  slope,  the  lower  part  of  the  shaded 
portion,  and  the  polyps  build  out  on  this  slope,  but  always  remain  in 
shallow  water  (1,  2,  3,  4).  The  outer  edge  of  the  reef  thus  comes  to  be 
in  deep  water. 

interrupted  where  fresh  water  descends  from  the  land,  so  that  a 
reef  rarely  surrounds  an  island,  and  is  rarely  continuous  for  great 
stretches  along  any  coast. 

It  is  manifest  that  the  barrier  reef  about  a  small  island  may 


FIG.  507. — An  atoll.     (From  Dana's  Corals  and  Coral  Islands  by  permission 
of  Dodd,  Mead  &  Co.) 

become  an  island  or  atoll  by  subsidence.    This  is  illustrated  by 


FIG.  508.— Coral  island  developed  from  a  submerged  volcano  (or  other  rock). 


Fig.  505.     Coral  islands  might  also  arise  by  the  development  of 
reefs  on  volcanic  cones  which  did  not  rise  into  islands  (Fig.  508). 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    471 

The  polyps  do  not  build  the  reef  or  the  atoll  above  water;  but 
when  they  have  built  up  to  water-level  the  waves  may  build  it 
higher,  as  they  convert  sand  reefs  into  land.  Once  land  appears, 
the  wind  may  make  it  higher  by  piling  up  coral  sand.  The  growth 


FIG.  509. — Coral  growing. 

of  vegetation  may  help  along  the  building,  both  by  its  own  growth 
and  by  helping  the  lodgment  of  wind-blown  sediment. 

Coral  islands  and  reefs  would  always  remain  low  if  it  were  not 
for  diastrophism.  There  are  indeed  no  very  high  coral  islands, 
but  there  are  coral  reefs  2000  or  3000  feet  above  sea-level.  Either 
the  land  where  such  reefs  occur  has  risen  greatly,  or  the  sea-level 
has  been  depressed. 


TOPOGRAPHIC  MAP  STUDIES. 

The  following  maps  illustrate  types  of  plains,  plateaus,  and  mountains. 

See  also  the  list  of  maps  on  pp.  54  and  479. 

The  folios  (see  foot-note,  p.  79)  of  the  areas  shown  on  the  maps 
marked  *  are  published,  and  are  always  serviceable  in  the  interpreta- 
tion of  the  topography. 


472  PHYSIOGRAPHY 

A.  PLAINS 

1.  Coastal 

Deal  Island,  Md.  Asbury  Park,  N.  J. 

Dennisville,  N.  J.  Sandy  Hook,  N.  J. 

Cape  May,  N.  J.  *  Coos  Bay,  Ore. 

Atlantic  City,  N.  J.  *  Norfolk,  Va. 
Great  Egg  Harbor,  N.  J. 

2.  Interior 
a.  Flat: 

Maxwell,  Cal.  Chocowinity,  N.  C. 

*  Casselton,  N.  D.  (Casselton-Fargo    Bowling  Green,  0. 
folio). 

6.  Uneven  and  well-drained' 
Petersburg,  Ind.  Sullivan,  Mo. 

*  Ditney,  Ind.  Lancaster,  Wis. — la. — IJ1. 
Tuscumbia,  Mo. 

c.  Uneven  and  ill-drained,  glaciated: 
Crystal  Falls,  Mich.  Geneva,  Wis. 

White  Bear,  Minn.  Madison,  Wis. 

Oswego,  N.  Y.  (special  map).  Briggsville,  Wis. 

B.  PLATEAUS 

Chino,  Ariz.  *  Canyon,  Wyo.   (Yellowstone  Na- 

Burnsville,  W.  Va.  tional  Park  folio). 

*  Charleston,  W.  Va. 

C.  MOUNTAINS 

*  Marysville,  Cal.  Kaaterskill,  N.  Y. 
San  Mateo,  Cal.  Mt.  Marcy,  N.  Y. 
Mt.  Lyell,  Cal.  Saluda,  N.  C.— S.  C. 
Shasta,  Cal.  *  Mt.  Mitchell,  N.  C.— Tenn. 

*  Tellunde,  Colo.  Millersburg,  Pa. 
Leadville,  Colo.  Lykens,  Pa. 

*  Pikes  Peak,  Colo.  *  Greenville,  Tenn. — N.  C. 
Huerfano  Park,  Colo.  *  Morristown,  Tenn. 

*  Holyoke,  Mass. — Conn.  *  Maynardville,  Tenn. 
Greylock,  Mass. — Vt.  Henry  Mountains,  Utah. 
Saypo,  Mont.  *  Tmtic,  Utah. 
Hamilton,  Mont.— Ida.  Tooele  Valley,  Utah. 
Sumpter,  Ore.  *  Ellensburg,  Wash. 

Ishawoot,  Wyo. 

The  types  of  plains  are  indicated  in  the  above  classification,  and 
their  distinctive  features  should  be  observed.  The  plateaus  show 
various  degrees  of  dissection. 


ORIGIN  AND  HISTORY  OF  PHYSIOGRAPHIC  FEATURES    473 

The  mountain  maps  should  be  studied  with  a  view  to  distinguishing 
topographic  types  (pp  435-437).  An  attempt  should  also  be  made  to 
group  them  according  to  their  origin  (p.  445).  The  results  of  this 
latter  classification  should  be  tested  by  the  Structural  Sheets  of  the  folios 
so  far  as  possible. 

MAPS  FOR  REVIEW 

I.  List  of  Maps. 

1.  Fire  Island,  N.  Y.  7.  Glacier  Peak,  Wash. 

2.  Marsh  Pass,  Ariz.  8.  Frostburg,  Md. 

3.  Princess  Anne,  Md. — Va.  9.  Stoughton,  Wis. 

4.  Abilene,  Tex.  10.  Marseilles,  111. 

5.  Hahnville,  La.  11.  Mt.  Taylor,  N.  M. 

6.  Gay  Head,  Mass.  12.  Savanna,  la. — 111. 

These  maps  touch  most  of  the  topics  studied  in  preceding  pages. 

II.  Questions  to  be  answered  in  writing.     In  the  case  of  each  map — 

1.  State  whether  the   area   is    plain,   plateau,  or  mountain,  or  if 
more  than  one  of  these  great  types  appears,  the  location  of  each. 

2.  Name  the  several  agencies  which  have  shaped  the  surface,  and 
indicate  their  relative  importance. 

3.  State  age  of  the  topography  in  terms  of  erosion.      If  different 
parts  of  the  area  are  in  different  stages,  bring  out  this  point. 

4.  How  many  cycles  of  erosion  are  shown?    The  evidence  for  the 
conclusion  stated.     Is  it  conclusive? 

5.  Is  there  any  indication  as  to  the  position  of  the  strata  under- 
lying the  region? 

6.  What  indications  are  there  of  inequalities  of  hardness  of  rock? 

7.  State  what  inferences  may  be  made  (certain,  probable,  possible) 
concerning  the  climate,  and  the  evidence  on  which  they  are  based. 

8.  Note  any  important  features  not  brought  out  by  the  preceding 
questions. 

REFERENCES 

1.  Standard  text-books  on  Geology. 

2.  Folios  of  the  U.  S.  Geol.  Surv.,  of  areas  in  mountain  regions. 

3.  WILLIS,  Mechanics  of  Appalachian  Structure:    13th  Ann.  Rept.  U.  S. 
Geol.  Surv.,  Pt.  II,  pp.  217-283. 

4.  LE  CONTE,  On  the  Structure  and  Origin  of  Mountains,  etc.:  Am.  Jour. 
Sci.,  Vol.  XXXVIII,  1889,  pp.  257-263;  Theories  of  the  Origin  of  Mountain 
Ranges:  Jour.  Geol.,  Vol.  I,  pp.  543-573. 

5.  DANA,  On  the  Origin  of  Mountains:  Am.  Jour.  Sci.,  Vol.  V,  1873,  pp. 
347,  423,  and  474,  and  Vol.  VI,  pp.  6,  104,  161,  304,  and  381. 


474  PHYSIOGRAPHY 

6.  POWELL,  Types  of  Orographic   Structure:    Am.  Jour.  Sci.,  Vol.  XIIt 
1876,  pp.  414-428. 

7.  TARR,  Mountains  of  New  York,  in  Physical  Geography  of  New  York 
State. 

8.  READE  (T.  MELLARD),  The  Origin  of  Mountain  Ranges. 

9.  GULLIVER,  Shoreline   Topography:    Am.   Acad.    Arts   and    Sci.,   Vol. 
XXXIV. 

10.  DARWIN,  The  Structure  and  Distribution  of  Coral  Islands:  Appleton. 

11.  DANA,  Corals  and  Coral  Islands:  Dodd,  Mead  &  Co. 

12.  AGASSIZ,  Various  Papers  in  Bull.  Mus.  Comp.  Zool.,  Harvard. 

13.  HEILPRIN,  The  Bermuda  Islands:  Appleton. 

14.  Various  Papers  on  coral  islands  in  Nature,  Vols.  22,  35,  37,  39, 
40,  41,  42,  51,  and  55. 


CHAPTER   X 
TERRESTRIAL   MAGNETISM 

THE  earth  is  a  great  magnet  and,  like  the  small  magnet  with 
which  we  are  familiar,  has  two  poles.  One  of  these  poles  is  called 
the  North  Magnetic  Pole  and  the  other  the  South  Magnetic  Pole. 
Generally  speaking,  one  end  of  the  magnetic  or  compass  needle 
points  toward  one  of  these  poles,  and  the  other  toward  the  other. 
If  we  were  to  follow  the  directions  pointed  by  the  compass  needle, 
we  would  be  led  to  the  North  Magnetic  Pole  in  the  one  case,  and  to 
the  South  Magnetic  Pole  in  the  other.  The  lines  connecting  these 
poles  are  magnetic  meridians  (Fig.  510). 

The  North  Magnetic  Pole  is  in  latitude  a  little  above  70°,  and 
in  longitude  about  97°  W.,  as  nearly  as  known.  The  South  Mag- 
netic Pole  is  in  latitude  about  72°,  and  in  longitude  about  152°  E. 
These  positions  have  been  calculated  from  the  directions  in  which 
the  compass  needles  point  in  various  positions  in  the  vicinity  of  the 
magnetic  poles.  The  South  Magnetic  Pole  has  never  been  reached, 
and  Captain  Amundsen  reports  that  "the  North  Magnetic  Pole 
has  no  immediate  situation,"  which  probably  means  that  it  is  not 
a  fixed  point. 

It  will  be  seen  from  the  foregoing  that  the  magnetic  poles  are 
far  from  the  geographic  poles,  and  that  they  are  not  exactly  oppo- 
site each  other.  It  is  believed,  too,  that  they  are  not  quite  constant 
in  position,  though  they  are  not  known  to  wander  widely.  The 
North  Magnetic  Pole  has  been  thought  to  have  shifted  its  position 
some  50  or  60  miles  in  as  many  years,  following  1830,  though  this 
determination  does  not  appear  to  be  conclusive. 

Since  the  north  end  of  the  magnetic  needle  points  to  the  North 
Magnetic  Pole,  it  follows  that  the  compass  does  not  indicate  true 
north  and  south  in  many  places.  At  points  northward  from  the 
North  Magnetic  Pole,  the  "north"  end  of  the  needle  points 
in  a  southerly  direction.  At  points  to  the  south  it  points  to 

475 


476 


PHYSIOGRAPHY 


TERRESTRIAL  MAGNETISM 


477 


the  northward,  at  points  east,  westward,  and  at  points  west, 
eastward.  The  departure  of  the  needle  from  the  true  north  and 
south  is  magnetic  declination.  Lines  connecting  places  of  equal 
declination  are  isogonic  lines.  A  line  connecting  places  of  no 
declination  is  an  agonic  line. 

Fig.  511  shows  an  agonic  line  in  the  United  States  passing  from 
Lake  Superior  to  South  Carolina.  Along  this  line  the  magnetic 
needle  points  due  north  and  south.  All  places  east  of  this  line 


FIG.  511. — Isogonic  lines  for  the  United  States,  1902.     The  heavy  line  is  an 
agonic  line,  or  line  of  no  declination.     (U.  S.  Coast  and  Geodetic  Surv.) 

have  west  declination,  and  all  places  west  of  this  line  have  east 
declination.  In  general,  declination  increases  with  increasing  dis- 
tance from  the  agonic  line.  In  Maine,  for  example,  the  declination 
is  more  than  20°  W.  at  a  maximum,  and  in  Washington  more  than 
20°  E.  (Fig.  511).  At  Chicago  the  declination  is  about  3°  E.; 
at  New  York  nearly  10°  W.;  at  Denver  about  13°  E.;  and  at  San 
Francisco  about  16°  E.  It  will  be  seen  that  it  is  important  to 
know  the  magnetic  declination  of  a  region,  if  the  compass  is  to  be 
used  there  for  determining  directions. 

The  declinations  shown  in  Figs.  511  and  512  are  interfered 
with  locally  by  certain  rock  formations,  especially  magnetic  iron 
ore.  In  the  vicinity  of  such  ore,  especially  if  it  be  in  large  bodies, 
the  needle  may  depart  widely  from  the  declination  indicated  by 
these  lines. 


478 


PHYSIOGRAPHY 


1C 

§ 


s 
1 

I 


TERRESTRIAL  MAGNETISM  479 

Since  the  magnetic  poles  shift  slowly,  the  declination  at  any  place 
also  shifts  in  harmony.  It  is  not  certain,  however,  that  all  varia- 
tions in  magnetic  declination  are  due  to  the  shifting  of  the  mag- 
netic pole.  The  declination  at  Chicago  has  shifted  more  than  2° 
since  1820. 

Dip.  The  magnetic  needle  does  not  usually  take  a  horizontal 
position.  At  the  magnetic  poles  it  should  be  vertical,  and  the 
north  end  would  be  down  at  the  North  Magnetic  Pole.  Its  position 
would  be  reversed  at  the  South  Magnetic  Pole.  Half-way  between 
the  magnetic  poles,  that  is,  at  the  magnetic  equator,  the  needle 
should  be  horizontal.  A  compass  which  is  constructed  so  as  to 
show  the  "dip  or  magnetic  inclination  is  a  dip  compass. 

Intensity.  Magnetic  intensity  varies  greatly  from  place  to 
place,  and  slightly  from  time  to  time  in  the  same  place. 

The  causes  and  the  conditions  of  change  of  terrestrial  magnetism 
are  not  well  understood. 

SUPPLEMENTARY  LIST  OF  MAPS 
I.  Topographic  Maps,  U.  S.  Geological  Survey 

The  following  maps  will  afford  opportunity  for  more  extended  map 
study.  The  use  of  as  many  of  them  as  time  permits  will  be  profitable. 
These,  together  with  those  already  mentioned  in  preceding  pages,  make 
a  fairly  adequate  equipment  so  far  as  topographic  maps  are  concerned. 
The  maps  mentioned  in  this  volume  should  be  supplemented  by  those 
of  the  home  region,  and  the  maps  of  the  home  region  should  be  used 
in  the  field,  as  much  as  possible.  In  no  other  way  will  the  maps  be  so 
well  understood.  For  the  areas  marked  *,  folios  have  been  published 
(see  foot-note,  p.  79),  and  they  are  helpful  in  the  study  of  topography. 
See  especially  the  structure-section  sheets. 

CHAPTER  I 

Batesville,  Ark.  Minneapolis,  Minn. 

Tamalpais,  Cal.  Hamilton,  Mont.-Ida. 

Tipton,  la.  Dennisville,  N.  J. 

*  Cottonwood  Falls,  Kan.  Everett,  Pa. 
Montross,  Md.-Va.  *  Gaines,  Pa.-N.  Y. 
Frostburg,  Md.-W.  Va.-Pa. 

CHAPTER  II 

Lakin,  Kan.  Browns  Creek,  Neb. 

Oceanside,  Md.-Del. 

CHAPTER  III 

*  London,  Ky.  *  Kingston,  Tenn. 


480  PHYSIOGRAPHY 

CHAPTER  IV 

A.  River  erosion,  especially  river  valleys. 

Morrilton,  Ark.  Oberlin,  0. 

Dunlap,  111.  *  Wartburg,  Tenn. 

New  Harmony,  Ind.-Ill.  Anson,  Tex. 

Medicine  Lodge,  Kan.  Abajo,  Utah-Colo. 

Tell  City,  Ky.-Ind.  *  Monterey,  Va.-W.  Va. 

Palmyra,  Mo.  St.  Croix  Dalles,  Wis.-Minn. 

Oak  Orchard,  N.  Y.  *  Gallatin,  Wyo.  (Yellowstone  Na- 

*  Mt.  Mitchell,  N.  C.-Tenn.  tional  Park  folio). 

Parmelee,  N.  C.  *  Shoshone,  Wyo.  (Yellowstone  Na- 

*  Fargo,  N.  Dak.  (Casselton-          tional  Park  folio). 

Fargo  folio). 

B.  Topographic  effects  of  unequal  hardness  after  notable  erosion. 

Denver,  Colo.  High  Bridge,  N.  J. 

*  Rome,  Ga.-Ala.  Passaic,  N.  J. 

*  Holyoke,  Mass.-Conn.  Hollidaysburg,  Pa. 
Saypo,  Mont.  *  Uvalde,  Tex. 

C.  Piracy  and  adjustment. 

*  Stevenson,  Ala.-Ga.-Tenn.  *  Franklin,  W.  Va.-Va. 

*  Piedmont,  Md.-W.  Va.  *  Lake,    Wyo.    (Yellowstone    Na- 

*  Chattanooga,  Tenn.  tional  Park  folio). 

*  Ringgold,  Tenn.-Ga. 

D.  Attuviation. 

Morrilton,  Ark.  *  Three  Forks,  Mont. 

Sierraville,  Cal.  Lexington,  Neb. 

Hartford,  Conn.  Paxton,  Neb. 

Camas  Prairie,  Ida.  Silver  Peak,  Nev.-Cal. 

Mountain  Home,  Ida.  Cohoes,  N.  Y. 

Waukon,  la.-Wis.  Watkins,  N.  Y. 

East  Delta,  La.  Williamston,  N.  C. 

Hahnville,  La.  Portland,  Ore.-Wash. 
Independence,  Mo. 

See  also  River  Charts  (IV,  below). 

E.  Cycles  of  erosion. 

Echo  Cliffs,  Ariz.  Everett,  Pa. 

San  Francisco  Mt.,  Ariz.  Harrisburg,  Pa. 

Tusayan,  Ariz.  Huntingdon,  Pa. 

Batesville,  Ark.  Delaware  Water  Gap,  Pa.-N.  J. 

Marshall,  Ark.  Pala  Pinto,  Tex. 

Mountain  View,  Ark.  Wausau,  Wis. 

*  Ditney,  Ind.  Winchester,  W.  Va.-Va. 
Watrous,  N.  M. 

CHAPTER  V 

*  Colfax,  Cal.  Deer  Isle,  Me. 

Durant,  la.  Chief  Mountain,  Mont. 

Clinton,  la.-Ill.  Hamburg,  N.  J. 

Boothbay,  Me.  Plainfield,  N.  J.-N.  Y. 


SUPPLEMENTARY  MAPS  481 

CHAPTER  V — Continued 

Greenwood  Lake,  N.  Y.-N.  J.  Weedsport,  N.  Y. 

Elmira,  N.  Y.-Pa.  Tower,  N.  Dak. 

Little  Falls,  N.  Y.  Pingree,  N.  Dak. 

Hammondsport,  N.  Y.  Masontown,    Pa.    (Masontown- 

Harlem,  N.  Y.-N.  J.  Uniontown  folio). 

Niagara  Falls,  N.  Y.  Methow,  Wash. 

Tonawanda,  N.  Y.  Chelan,  Wash. 

Skaneateles,  N.  Y.  Snoqualmie,  Wash. 

Penn  Yan,  N.  Y.  Delavan,  Wis. 

Tully,  N.  Y.  Briggsville,  Wis. 

Rosendale,  N.  Y.  Baraboo,  Wis. 

Rochester,  N.  Y.  Denzer,  Wis. 

Syracuse,  N.  Y.  The  Dells,  Wis. 

CHAPTER  VI 
Group  K. 

Cayucos,  Cal.  Northport,  N.  Y. 

Haywards,  Cal.  Babylon,  N.  Y. 

Hueneme,  Cal.  Fire  Island,  N.  Y. 

San  Francisco,  Cal.  Hamlin,  N.  Y. 

Oceanside,  Cal.  Euclid,  0. 

Biddeford,  Me.  *  Port  Orford,  Ore. 

Tolchester,  Md.  Erie,  Pa 

Muskegat,  Mass. 

CHAPTER  VII 

San  Francisco  Mt.,  Ariz.  Crater  Lake  (special),  Ore. 

*  Lassen  Peak,  Cal.  Terlingua  (special)  Tex 

Mt.  Lyell,  Cal.  Abajo,  Utah-Colo 

Mt.  Taylor,  N.  M.  Henry  Mts.,  Utah. 

II.  Coast  and  Geodetic  Survey  Charts  l 

Charts  bearing  the  numbers  8,  10,  19,  21,  103,  105,  109,  110,  120, 
122,  123,  124-126,  131-136,  146,  156,  157,  161,  167-169,  177,  184,  204, 
1000,  1001,  1002,  1007,  5100,  5106,  5143,  5200,  5500,  5581,  6300,  6450, 
6460,  8100,  8300,  9302,  S,  and  T. 

III.  Lake  Survey  Charts  l 

The  general  charts  of  Lakes  Superior,  Michigan,  Huron,  St.  Clair, 
Erie,  and  Ontario.  Charts  of  most  parts  of  the  shores  of  these  lakes, 
on  a  much  larger  scale,  are  also  published. 

IV.  River  Charts 

Charts  9,  13,  14,  18,  19,  20,  and  27,  and  Index  charts  I,  II,  and  III 
of  the  Mississippi  River,  issued  by  the  Mississippi  River  Commission.3 
Students  interested  in  any  special  portion  of  the  Mississippi  River  will 
do  well  to  get  the  charts  for  those  regions.  Similar  charts  are  published 
for  certain  other  large  rivers,  such  as  the  Missouri,  the  Tennessee,  etc. 

1  See  foot-note,  p.  203. 

1  Issued  by  the  War  Department,  Washington,  D.  C. 
8  These  maps  may  be  purchased  of  the  Mississippi  River  Commission, 
St.  Louis,  Mo. 


PART  II 

CHAPTER  XI 
EARTH   RELATIONS 

Form.  The  form  of  the  earth  is  very  much  like  that  of  a 
sphere,  but,  since  it  is  not  exactly  a  sphere,  it  is  generally  said 
to  be  a  spheroid.  The  form  has  been  determined  in  various  ways: 
(1)  Ships  have  sailed  quite  around  it.  This  proves  that  it  is 
everywhere  bounded  by  curved  surfaces,  though  it  does  not  prove 
that  it  is  a  sphere  or  even  a  spheroid,  for,  if  it  had  the  shape  of 
an  egg,  it  would  be  possible  to  sail  around  it.  (2)  It  has  been 
found  that  when  vessels  go  to  sea  their  lower  parts  disappear 
first.  When  a  vessel  has  gone  four  miles,  the  lower  five  feet  of 
its  hull  is  out  of  sight  to  an  observer  on  the  shore,  if  his  eye  is 
five  feet  above  the  level  of  the  sea.  Similarly,  when  a  vessel 
approaches  land,  its  highest  parts  are  seen  first  by  observers  on 
the  land,  while  to  observers  on  the  vessel  the  high  lands  are  seen 
first  and  the  low  ones  later.  From  the  vessel  the  spires  and 
chimneys  of  houses  appear  before  the  roofs,  and  the  roofs  before 
the  lower  parts.  These  phenomena  show  only  that  the  earth 
has  a  curved  surface;  but  it  is  found  that  in  whatever  direction 
vessels  sail,  and  from  whatever  port  they  start,  objects  on  land 
disappear  at  about  the  same  rate.  This  means  that  the  curva- 
ture is  nearly  the  same  in  all  directions.  A  body  whose  curva- 
ture is  the  same  in  all  directions  is  a  sphere,  and  a  body  whose 
curvature  is  nearly  the  same  in  all  directions  is  nearly  a  sphere. 
This  is  the  condition  of  the  earth.  (3)  Again,  the  earth  some- 
times gets  directly  between  the  sun  and  the  moon.  It  then  casts 
a  shadow  on  the  moon,  and  this  shadow  always  appears  to  be 
circular,  though  its  edges  are  not  very  clearly  defined.  (4)  The 
direction  of  the  plumb-line  (the  perpendicular  to  a  horizontal  surface 
on  the  earth)  changes  from  point  to  point  on  the  earth's  surface, 
and  it  changes  by  an  angle  which  is  almost  exactly  proportional 

482 


EARTH  RELATIONS 


483 


to  the  distance  between  the  points,  wherever  they  are.  If  the 
change  of  direction  were  exactly  proportional  to  the  distance 
between  two  points,  wherever  taken,  the  earth  would  be  a  sphere 
(Fig.  513).  Since  it  is  only  approximately  true,  the  earth  is  only 
approximately  spherical. 

This  point  may  be  put  in  another  way.    The  stars  are  very  far 
from  the  earth.    As  one  travels  along  the  earth's  surface,  the 


Fio.  513. — The  circle  represents  the  earth's  circumference.  The  extensions 
of  the  radii  represent  the  directions  of  the  plumb-lines  at  various  posi- 
tions. The  distance  from  a  to  b  is  the  same  as  that  from  b  to  c  and 
c  to  d,  and  the  change  in  the  direction  of  the  plumb-line,  that  is,  the 
angle  between  aa'  and  66',  is  essentially  the  same  as  that  between  66'  and 
cc',  cc'  and  dd',  etc.  This  is  true  for  all  parts  of  the  earth. 

apparent  directions  of  the  stars  change,  and  the  angle  of  change 
is  almost  exactly  proportional  to  the  distance  traveled,  wherever 
the  starting-point,  and  whatever  the  direction  of  travel. 

The  significance  of  this  change  in  the  position  of  the  stars 
appears  to  have  been  correctly  interpreted,  in  general  terms  at 
least,  by  certain  Greek  students  (e.g.,  Thales  of  Miletus)  as  early 
as  640  B.C.  The  same  idea  appears  to  have  been  entertained 
at  various  subsequent  times  by  individual  students.  Columbus 
recognized  it  in  the  statement:  "I  have  always  read  that  the 


484  PHYSIOGRAPHY 

world,  comprising  the  land  and  the  water,  is  spherical,  as  testified 
by  the  investigations  of  Ptolemy  and  others,  who  have  proved 
it  by  the  eclipses  of  the  moon  and  other  observations  made  from 
east  to  west,  as  well  as  by  the  elevation  of  the  pole  [pole  star]  from 
north  to  south."  1 

In  these  and  other  ways2  it  is  known  that  the  form  of  the 
earth  does  not  depart  greatly  from  that  of  a  sphere. 

Size.  The  circumference  of  the  earth  is  nearly  25,000  miles, 
and  its  diameter  nearly  8000  miles.  Since  the  earth  is  not  a 
perfect  sphere,  its  various  diameters  and  circumferences  are  not 
exactly  equal.  Its  longest  diameter  is  7926.5  miles,  and  its 
shortest  nearly  27  miles  less  (7899.7  miles).  The  shortest  cir- 
cumference is  about  42  miles  shorter  than  the  longest. 

The  surface  area  of  the  earth  is  nearly  197,000,000  square  miles, 
and  its  volume,  exclusive  of  the  atmosphere,  about  260,000,000,000 
cubic  miles.  The  earth  is  between  five  and  six  times  as  heavy  as 
an  equal  volume  of  water  would  be. 

Motions 

The  earth  has  two  principal  motions.  These  are  (1)  rotation, 
and  (2)  revolution  around  the  sun.  The  earth  rotates  on  its 
shortest  diameter,  which  is  called  its  axis.  The  ends  of  the  axis 
of  rotation  are  the  poles;  and  the  circumference  midway  between 
the  poles  is  the  equator.  The  equator  is  the  longest  circumfer- 
ence of  the  earth.  The  lines  that  pass  from  pole  to  pole  on  the 
earth's  surface  are  meridians.  All  meridians  converge  at  each 
pole.  Meridians  are  parallel  with  one  another  at  the  equator, 
but  nowhere  else. 

Rotation. — The  rotation  of  the  earth  may  be  demonstrated  by 
simple  experiments.  1.  If  a  body  be  dropped  from  a  high  tower, 
it  does  not  fall  so  as  to  reach  a  point  immediately  beneath  that 
from  which  it  fell.  Instead,  it  always  falls  a  little  to  the  east  of 
the  point  from  which  it  started.  This  is  explained  as  follows: 
If  the  earth  rotates,  any  point  must  move  faster  than  any  other 
point  which  is  nearer  its  center,  for  the  same  reason  that  a  point 
on  the  rim  of  a  wheel  moves  faster  than  a  point  between  the  rim 
and  the  hub.  If  the  earth  be  rotating,  the  top  of  a  tower  must 

1  Hakluyt  Soc.  Pub.,  History  of  Columbus's  Third  Voyage,  Vol.  II.,  p.  129. 
'See  Moulton's  Introduction  to  Astronomy,  pp.  114-124. 


EARTH  RELATIONS 


485 


be  moving  forward  faster  than  the  bottom.  In  this  case,  the 
falling  body,  starting  from  the  top  of  the  tower,  has  a  forward 
velocity  greater  than  that  possessed  by  the  base  of  the  tower. 
Under  these  circumstances,  the  falling  body  must  gain  on  the 
base  of  the  tower  in  the  direction  of  rotation;  that  is,  if  the  earth 
rotates  to  the  east,  the  falling  body  would  be  farther  to  the  east, 
relative  to  the  tower,  when  it  reached  the  ground  than  when 
it  started.  If  the  earth  rotated  to  the  west,  the  body  would 
fall  the  other  way.  Since  the  body  always  falls  to  the  east,  and 
since  nothing  but  the  rotation  of  the  earth  to  the  east  seems  to 
explain  this  fact,  it  is  taken  to  be  a  proof  that  the  earth  rotates 


FIG.  514. — The  leaning  tower  of  Pisa, 
where  some  of  Galileo's  famous 
experiments  on  falling  bodies  were 
performed. 


FIG.  515. — Figure  to  illustrate  the 
effect  of  rotation  on  a  falling 
body  as  explained  in  text. 


in  that  direction.     The  actual  deviation  in  our  latitude  is  about 
one  inch  for  500  feet  of  fall. 

Fig.  515  illustrates  the  principle  involved  in  the  falling  body. 
Let  AB  =  the  earth's  radius,  and  ra  a  point  on  a  tower  (height 
greatly  exaggerated)  above  the  earth's  surface.  Suppose  the 
mass  m  is  dropped  from  the  top.  If  the  earth  were  not  rotat- 
ing, it  would  fall  in  the  direction  of  the  plumb-line,  and  would 
strike  the  surface  at  B.  Suppose,  however,  the  earth  is  rotating 
at  such  a  rate  that  BA  turns  to  B'A  while  m  is  falling  to  the 
surface.  If  it  were  not  for  the  attraction  of  the  earth,  m  would 
go  in  a  straight  line  to  m'.  Gravitative  attraction  is  at  right 
angles  to  this  line  mm',  and,  though  it  does  not  change  the 
amount  of  motion  of  m  in  this  direction,  it  impresses  upon  it  a 


486 


PHYSIOGRAPHY 


new  motion  toward  the  earth.  The  result  is  that  it  describes  the 
curved  line  mR,  and  strikes  the  earth  at  R,  a  little  beyond  the 
foot  of  the  perpendicular  m'B'.1 

2.  Another  experiment,  which  shows  the  same  thing,  may  be 
performed  with  a  pendulum  (known  as  Foucaidt's  pendulum). 
If  a  pendulum  attached  to  a  ceiling  is  set  swinging  parallel  to 
a  given  line  on  the  earth's  surface,  as,  for  example,  parallel  to  a 
line  on  the  floor,  it  will  be  found  a  little  later  to  be  swinging  in 
a  plane  which  is  not  parallel  to  the  original  line.  The  pendulum 
changes  its  direction,  with  reference  to  the  line  along  which  it 


FIG.  516. — Diagram  to  illustrate  the  fact  that  the  direction  of  the  swing  of 
the  pendulum  changes  more  rapidly  in  high  latitudes  than  in  low  lati- 
tudes. A  pendulum  set  swinging  with  the  central  meridian  of  the  dia- 
gram, in  different  latitudes,  will  depart  from  the  meridians,  as  shown  at 
the  right,  in  six  hours.  There  is  no  departure  at  the  equator,  much  in 
middle  latitudes,  and  still  more  in  high  latitudes. 

was  started,  more  rapidly  near  the  poles  and  less  rapidly  near 
the  equator.  If  it  could  be  set  swinging  along  a  meridian  so  that 
one  end  of  the  swing  barely  reached  the  pole,  it  would  be  found 
that  the  pendulum  was  swinging  at  right  angles  to  that  meridian 
after  the  earth  had  turned  a  quarter  of  the  way  around.  This  is 
illustrated  by  Fig.  516.  If  the  pendulum  were  set  swinging  half- 
way between  one  of  the  poles  and  the  equator,  it  would  have 
departed  from  the  plane  in  which  it  was  started  much  less  when 
the  earth  had  turned  a  quarter  of  the  way  around.  This  is  also 

1  Moulton's  Introduction  to  Astronomy,  pp.  148-149. 


EARTH  RELATIONS  487 

illustrated  by  Fig.  516.  If  the  pendulum  were  set  swinging  at 
the  equator  so  that  half  the  swing  was  on  either  side  of  it,  the 
swing  would  remain  parallel  with  its  original  position  (Fig.  516). 

The  departure  of  the  pendulum,  except  at  the  equator,  from 
the  plane  of  the  meridian  in  which  it  was  set  swinging  is  often 
said  to  mean  that  the  meridian  in  the  plane  of  which  it  first  swung 
has  changed  its  position,  and  in  its  new  position  it  is  not  parallel 
to  the  position  in  which  the  pendulum  started  to  swing.  The 
pendulum  itself  continues  to  swing  in  its  original  plane,  but  this 
plane  is  no  longer  parallel  to  the  meridian  in  its  changed  position. 
According  to  this  statement,  the  pendulum  seems  to  have  changed 
its  direction,  because  we  determine  direction  by  meridians,  and 
the  successive  positions  of  a  meridian  on  a  spherical  rotating 
body  do  not  remain  parallel  with  one  another,  except  at  the 
equator  of  the  rotating  body. 

This  change  in  the  direction  of  the  pendulum,  which  is  uni- 
versal except  at  the  equator,  is  always  in  the  same  direction  in  the 
northern  hemisphere,  and  always  in  the  same  direction  in  the 
southern  hemisphere,  and  proves  that  the  earth  rotates.  The 
change  in  the  direction  of  the  pendulum  does  not  take  place  at 
the  equator,  so  the  explanation  runs,  because  the  meridians  there 
are  all  parallel  with  one  another  and  the  successive  positions  of  a 
given  meridian  therefore  are  all  parallel.  According  to  this  state- 
ment of  the  case,  the  apparent  change  in  the  direction  of  swing  of 
the  pendulum  takes  place  less  rapidly  midway  between  the  equator 
and  the  poles  than  near  the  poles  (Fig.  516),  because  the  meridians 
are  more  nearly  parallel  with  one  another  in  the  former  position 
than  in  the  latter. 

If  this  were  the  full  explanation  of  the  matter,  the  swing  of  the 
pendulum  should  always  be  parallel  to  its  original  position  at 
the  end  of  24  hours,  whether  the  pendulum  was  near  the  equator 
or  near  the  pole.  This  is  not  the  case,  and  the  above  statement 
is  therefore  not  an  adequate  explanation  of  the  phenomenon. 
Though  the  dependence  of  the  rate  of  variation  of  the  direction 
of  the  pendulum's  swing  on  latitude  cannot  be  given  here,  it  is 
well  understood. 

The  form  of  the  earth  is  consistent  with  its  rotation,  but  can 
hardly  be  said  to  prove  it.  Any  body  which  is  not  perfectly 
rigid  (and  no  body  is)  would  be  somewhat  flattened  at  its  poles, 
and  somewhat  bulged  at  its  equator,  by  rotating.  This  is  the 


488  PHYSIOGRAPHY 

condition  of  the  earth,  for  the  diameter  between  the  poles  is  the 
shortest  diameter,  and  the  diameters  in  the  plane  of  the  equator 
are  the  longest.  The  amount  of  flattening  which  would  result 
from  rotation  depends  on  (1)  the  rate  of  rotation,  and  (2)  the  rigid- 
ity of  the  body.  The  faster  the  rotation  and  the  less  rigid  the 
body,  the  greater  the  polar  flattening.  There  are  other  ways  of 
proving  that  the  earth  rotates,  but  they  need  not  be  cited  here. 

The  rate  at  which  a  point  on  the  surface  of  the  earth  moves,  as 
a  result  of  rotation,  varies  greatly.  Points  on  the  equator  move 
fastest,  because  they  have  farthest  to  go  in  the  time  of  one  com- 
plete rotation.  At  the  equator,  where  the  circumference  is  nearly 
25,000  miles,  a  point  moves  nearly  25,000  miles  a  day,  as  a  re- 
sult of  rotation.  Half-way  between  the  equator  and  either  pole, 
a  point  moves  about  17,600  miles  per  day,  while  at  the  poles  the 
rate  of  motion  resulting  from  rotation  is  zero. 

Effect  of  rotation.  The  most  obvious  effect  of  rotation  is 
the  alternation  of  day  and  night,  for  one  side  of  the  earth  and  then 
the  other  is  turned  toward  the  sun  during  each  rotation.  But  it  is 
to  be  noted  that  the  alternation  of  day  and  night  does  not  of 
itself  prove  rotation.  Day  and  night  might  be  brought  about 
equally  well  by  the  revolution  of  the  sun  around  the  earth  each 
day.  The  period  of  rotation,  24  hours,  determines  the  length  of  a 
day  (day  and  night). 

Revolution.  The  second  principal  motion  of  the  earth  is  its 
revolution  about  the  sun.  No  simple  experiment  can  be  cited  to 
prove  this  motion;  but  the  fact  of  revolution  may  be  illustrated 
in  various  ways. 

If  the  positions  of  individual  stars  be  observed  for  long  periods 
of  time,  they  appear  to  describe  small  circuits  each  year.  Some 
of  the  circuits  are  nearly  circular  and  some  are  nearly  straight  lines. 
Some  of  them  are  larger  and  some  smaller.  This  annual  change  in 
the  apparent  position  of  the  stars  is  their  annual  parallax.  Either 
the  stars  make  this  annual  circuit,  and  all  of  them  in  the  same 
length  of  time,  or  the  earth  makes  a  yearly  circuit  in  space,  which 
causes  the  apparent  annual  movement  of  the  stars.  The  fact  that 
these  apparent  circuits  of  the  stars  are  all  made  in  the  same  length 
of  time  makes  it  more  probable  that  they  are  due  to  the  motion 
of  the  earth,  than  that  they  are  due  to  the  individual  motions  of 
the  stars  themselves.  The  varying  sizes  of  the  apparent  annual 
paths  of  the  stars  is  accounted  for  by  the  fact  that  some  of  them 


EARTH  RELATIONS 


489 


are  nearer  to  the  earth  than  others,  and  the  nearer  they  are,  the 
larger  the  annual  circuits  they  appear  to  describe.  The  varying 
shapes  of  the  annual  paths  would  be  accounted  for  by  the  direc- 
tions of  the  stars,  some  being  in  a  polar  direction  from  the  ob- 
server and  some  in  an  equatorial  direction. 

Various  other  physical  and  astronomical  phenomena,  which  need 
not  be  cited  here,  also  demonstrate  that  the  earth  makes  an  annual 
circuit  around  the  sun. 

The  length  of  time  which  the  earth  requires  to  make  its  revolu- 
tion about  the  sun  determines  the  length  of  the  year.  It  is  a  little 
more  than  365  days. 

The  path  of  the  earth  around  the  sun  is  its  orbit.  The  orbit  of 
the  earth  is  not  a  circle,  but  an  ellipse  (Fig.  517),  and  the  sun  is  in 
one  of  the  foci,  more  than  1,500,000  miles  from  the  center  of  the 
ellipse.  When  the  earth  is  nearest  the  sun,  the  distance  between 

FIG.  517.— The  orbit 
of  the  earth  is  an 
ellipse,  with  the  sun 
in  one  of  the  foci. 
Eccentricity  of  the 
orbit  exaggerated 
10  times;  diameter 
of  the  sun  exagger- 
ated nearly  10 
times;  diameter  of 
the  earth  exagger- 
ated about  50  times. 

the  earth  and  sun  is  more  than  3,000,000  miles  less  than  when  they 
are  farthest  apart.  It  so  happens  that  the  earth  is  nearest  (about 
91,500,000  miles)  the  sun  in  the  early  winter  (early  in  January) 
of  the  northern  hemisphere,  and  farthest  (about  94,500,000  miles) 
from  it  in  early  summer  (early  in  July).  The  perihelion  (nearest 
the  sun)  and  aphelion  (farthest  from  the  sun)  dates  are  subject  to 
slow  periodic  change.  The  perihelion  date  in  4000  B.C.  was  Sep- 
tember 21.  It  will  be  March  21  in  6590  A.D. 

The  motion  of  the  earth  through  space  during  its  revolution 
about  the  sun  is  at  the  rate  of  about  600,000,000  miles  a  year. 
This  means  that  the  earth  travels  about  1,600,000  miles  daily,  or 
about  66,666  miles  hourly. 

The  earth's  axis  is  inclined  toward  the  plane  of  its  orbit  about 
23^°  (Fig.  518).  This  position  of  the  axis,  together  with  the 


Aphelion 


490  PHYSIOGRAPHY 

motions  of  the  earth,  have  much  to  do  with  the  distribution  of  the 
heat  and  light  received  from  the  sun,  and  so  with  the  changes  in 
the  length  of  day  (daylight)  and  night  (darkness),  and  with  the 
succession  of  the  seasons.  But,  before  attempting  to  see  how 


FIG.  518. — Diagram  to  show  the  effect  of  the  inclination  of  the  earth's  axis 
upon  the  distribution  of  light,  heat,  etc.,  on  the  earth.  The  line-shading 
represents  the  plane  of  the  earth's  orbit.  Half  the  earth  is  above  this 
plane,  but  the  plane  does  not  cut  the  earth  symmetrically  with  refer- 
ence to  the  parallels.  In  the  position  Et  more  than  half  the  northern 
hemisphere  is  being  heated  and  lighted.  In  position  E,  less  than  half 
of  the  same  hemisphere  is  heated  and  lighted.  In  positions  E*  and  Z?4 
the  half  of  the  northern  ani  of  the  southern  hemispheres  is  being  lighted 
and  heated. 

these  changes  are  brought  about,  we  must  become  familiar  with 
certain  terms  which  are  to  be  used  in  the  discussion  of  these  changes. 

Latitude,  Longitude,  and  Time 

Latitude.  The  equator  has  been  defined  as  the  circle  about 
the  earth  midway  between  the  poles.  Circles  parallel  to  the 
equator  are  parallels.  The  number  of  parallels  which  might  be 
drawn  is  infinite,  though  but  a  few  are  represented  on  maps.  On 
maps  of  small  scale  parallels  are  drawn  every  5°  or  10°.  On 
maps  of  large  scale  they  are  drawn  for  every  1°  or  2°,  or  sometimes. 
even  for  fractions  of  a  degree.  The  length  of  parallels  varies  greatly, 
those  near  the  equator  being  longer,  and  those  near  the  poles 
shorter. 

The  planes  of  all  parallels  are  perpendicular  to  the  earth's 
axis,  but  no  circle  perpendicular  to  the  axis,  except  the  equator, 
is  a  great  circle,  for  no  other  passes  through  the  ends  of  a  diameter 
of  the  earth.  This  is  shown  in  Fig.  519,  which  represents  the 
earth  in  two  positions.  In  the  left-hand  part,  the  half  of  each 
parallel  and  meridian  represented  is  shown.  In  the  right-hand 
part,  the  relation  of  parallels  to  the  North  Pole  is  shown.  The 


EARTH  RELATIONS  491 

distance  between  the  equator  and  either  pole  is  a  quadrant  (i.e.,  a 
quarter  of  a  circle)  and  is  divisible  into  90  parts  (90°)  called 
degrees.  The  degrees  are  numbered  from  the  equator  to  the  poles. 
Each  degree  is  divided  into  60  parts  (600  called  minutes,  and  the 
minutes,  like  the  degrees,  are  numbered  from  the  equator  toward  the 
poles.  Each  minute  is  divided  into  60  parts  (60")  called  seconds, 
and  the  seconds  are  numbered  in  the  same  direction  as  the  larger 
divisions.  Distance  north  or  south  of  the  equator  may  there- 
fore be  indicated  exactly  by  means  of  parallels.  This  distance 
is  called  latitude,  the  latitude  of  the  equator  being  0°. 

In  reality,  geographic  latitude,  as  distinct  from  astronomic  latitude  and 
geodetic  latitude,  is  the  angle  between  the  plane  of  the  equator,  and  the 
perpendicular  to  the  standard  spheroid  at  the  place  of  observation.  The 
angle  is  measured  by  the  arc  at  the  surface,  and  the  length  of  the  arc  is 
commonly  called  the  latitude. 

If  the  latitude  of  a  place  is  40°  40'  40"  N.,  its  distance  and  its 
direction  from  the  equator  are  accurately  known;  but,  since  the 
parallel  of  40°  40'  40"  runs  quite  around  the  earth,  it  is  clear  that 
the  statement  of  the  latitude  of  a  place  indicates  only  what  paral- 
lel it  is  on,  but  not  its  position  on  that  parallel. 

Longitude.  Position  on  a  parallel  is  indicated  by  means  of 
meridians  (p.  484).  The  number  of  possible  meridians  is  infinite, 
but,  as  in  the  case  of  parallels,  only  a  few  are  commonly  indicated 
on  maps.  One  meridian,  that  passing  through  Greenwich,  Eng- 
land, was  long  ago  arbitrarily  chosen  as  the  meridian  from  which 
distances  east  and  west  are  to  be  reckoned.  This  meridian  is  the 
meridian  of  zero  degrees  (0°) .  Distance  east  or  west  of  this  meridian 
is  known  as  longitude.  Places  east  of  long.  0°  are  in  east  longitude, 
and  those  west  of  it  are  in  west  longitude.  East  and  west  longitude 
respectively  are  regarded  as  extending  180°  from  the  meridian 
0°;  that  is,  half-way  around  the  earth.  The  degrees  of  longitude 
are  divided  into  minutes  and  seconds,  the  same  as  the  degrees  of 
latitude. 

The  position  of  a  place  on  the  earth's  surface  may  be  absolutely 
fixed  by  means  of  meridians  and  parallels.  If  a  place  is  in  longi- 
tude 30°  E.,  its  distance  east  of  the  meridian  0°  is  known.  If,  at 
the  same  time,  it  is  in  latitude  30°  N.,  it  must  be  where  the  parallel 
of  30°  N.  crosses  the  meridian  of  30°  E.  This  gives  its  position 
on  the  earth's  surface  exactly. 


492 


PHYSIOGRAPHY  L 


Every  meridian  reaches  each  pole.  It  might  seem  therefore 
that  each  pole  has  all  longitude.  But  longitude  is  distance  east 
or  west  of  the  meridian  0°,  and  at  the  poles  there  is  neither  east 
nor  west.  At  the  north  pole  the  only  direction  is  south,  and  at 
the  south  pole  the  only  direction  is  north.  The  poles  therefore 
cannot  be  said  to  have  longitude,  since  they  are  not  east  or  west 
of  the  meridian  of  0°. 

Longitude  and  time.  There  is  a  definite  relation  between 
longitude  and  time.  Since  the  earth  turns  through  360°  in  24 


FIG.  519. — Parallels  and  meridians. 

hours,  it  turns  15°  in  one  hour,  or  15'  of  longitude  in  one  minute 
of  time.  The  sun  therefore  rises  one  hour  earlier  at  a  place  in 
longitude  0°  than  at  a  place  in  the  same  latitude  in  longitude 
15°  W.,  and  one  hour  later  than  at  a  place  in  the  same  latitude 
in  longitude  15°  E.  Similarly,  noon  comes  an  hour  earlier  in  longi- 
tude 0°  than  in  longitude  15°  W.  and  an  hour  later  than  in  longi- 
tude 15°  E.  All  places  on  a  given  meridian  have  noon  and  mid- 
right  at  the  same  time,  and  such  places  are  said  to  have  the  same 
time;  but  places  on  different  meridians  have  different  times. 
Thus,  when  places  on  the  meridian  of  Chicago  have  noon,  it  is 
afternoon  on  meridians  farther  east,  and  before  r.oon  on  meridians 
farther  west.  If  the  longitude  of  two  places  is  known  therefore, 
their  difference  of  time  may  be  readily  calculated.  Fig.  520  repre- 
sents three  cities  on  or  near  the  parallel  of  40°,  and  about  15°  apart 
in  longitude.  On  June  21  the  sun  would  be  73^°  above  the  horizon 
at  noon  in  latitude  40°.  Fig.  520  may  be  taken  to  represent  noon 
at  Philadelphia.  At  this  hour  the  sun  is  not  so  high  above  the 
horizon  at  St.  Louis,  which  is  15°  farther  west,  and  it  is  still  lower 
at  Denver,  which  is  15°  farther  west  than  St.  Louis.  After  the 
earth  has  turned  15°,  the  sun  will  be  73£°  above  the  horizon  at 
St.  Louis,  while  it  will  have  become  lower  at  Philadelphia  and 


EARTH  RELATIONS  493 

higher  at  Denver.  When  the'  earth  has  turned  15°  more  (i.e.,  an 
hour  later)  the  sun  will  be  33$°  above  the  horizon  at  Denver.  It 
will  then  be  noon  at  Denver;  ah  hour  past  noon  at  St.  Louis,  and 
two  hours  past  noon  at  Philadelphia. 

Though  all  places  on  a  given  meridian  have  noon  and  midnight 
at  the  same  time,  they  do  not  al  ways  have  sunrise  and  sunset  at 
the  same  hour,  for  reasons  which  will  appear  later. 

The  variations  of  time  with  changes  of  longitude  become  appar- 
ent when  long  journeys  are  made  either  east  or  west.  Thus  a 


FIG.  520. — Diagram  to  illustrate  the  change  in  the  altitude  of  the  sun  from 
hour  to  hour,  in  places  in  the  same  latitude.  The  diagram  represents 
noon  at  Philadelphia  at  the  time  of  the  summer  solstice.  At  this  time 
the  sun  is  there  but  a  few  degrees  from  the  zenith,  as  represented  by  the 
dotted  line.  At  St.  Louis,  in  about  the  same  latitude,  but  farther  west, 
the  sun  is  much  farther  from  the  zenith  at  the  same  hour;  but  when 
the  noon  hour  arrives  at  St.  Louis  the  sun  will  be  as  near  the  zenith 
there  as  it  is  at  Philadelphia  in  the  diagram.  At  Denver,  which  is  still 
farther  west  than  St.  Louis,  the  sun  is  farther  from  the  zenith  than  at 
St.  Louis  at  the  noon  hour  of  Philadelphia..  When  it  is  noon  at  St. 
Louis  the  sun  will  be  as  far  from  the  zenith  at  Denver  as  it  is  in  the 
diagram  at  St.  Louis.  At  this  hour  the  sun  will  be  about  equally  distant 
from  the  zenith  at  Denver  and  at  Philadelphia,  but  at  Philadelphia  it 
will  be  west  of  south  and  at  Denver  east  of  south.  When  it  is  noon  at 
Denver  the  sun  will  be  as  near  the  zenith  there  as  it  is  in  the  diagram 
at  Philadelphia,  and  the  position  of  the  sun  in  St.  Louis  will  be  as  far 
from  the  zenith  as  it  is  in  the  diagram,  but  the  sun  will  be  west  of 
south  instead  of  east  of  south. 

watch  which  has  the  correct  local  time  in  New  York  has  not  the 
correct  local  time  when  it  is  carried  to  Chicago.  To  avoid  the 
difficulties  of  timekeeping  growing  out  of  travel,  railroads  have 
adopted  a  system  of  standard  time.  Under  this  system  the  country 
is  divided  into  north-south  belts,  about  15°  wide,  and  all  places  in 
each  belt  use  the  time  which  is  correct  for  the  central  meridian  of 
that  belt.  The  railway  time  in  adjacent  belts  differs  by  one  hour. 
By  this  system,  the  clocks  and  watches  do  not  show  correct  local 
time  anywhere  except  on  the  central  meridians  of  each  belt. 
Fig.  521  shows  the  standard-time  zones. 


494  PHYSIOGRAPHY 

Lengths  of  degrees.  The  length  of  a  degree  of  longitude,  as 
measured  on  the  surface  of  the  earth,  is  the  ^-$  part  of  a  parallel. 
Since  the  parallels  are  very  much  shorter  near  the  poles  than 
near  the  equator,  the  length  of  a  degree  of  longitude  varies  with  the 
latitude.  At  the  poles,  where  the  length  of  the  parallel  becomes 
zero,  the  length  of  a  degree  of  longitude  also  becomes  zero.  At 
the  equator  the  length  of  a  degree  of  longitude  is  69.652  miles; 
in  latitude  30°,  59.955  miles;  and  in  latitude  60°,  34.914  miles. 

Degrees  of  latitude  are  measured  along  meridians.  They  also 
vary  in  length.  The  length  of  a  degree  of  latitude  has  been  meas- 
ured in  several  places.  In  India  it  is  about  68|  miles,  while  in 
Sweden,  the  most  northerly  point  where  it  has  been  measured, 
it  is  69}  miles.  At  the  poles,  it  is  calculated  that  it  must  be  about 
69-^  miles.  In  the  United  States,  the  average  length  is  about  69 
miles.  The  lengths  of  degrees  of  latitude  and  longitude  in  cer- 
tain selected  latitudes,  are  shown  in  the  following  table: 

Latitude.  Longitude. 

In  latitude  0°,  1°  =  68.704  miles.          69. 1 72  miles. 
«      30o^  1o==68  881      «  59.956      " 

"      45°,  1°  =  69.054      "  48.995      " 

"         "      60°,  1°  =  69. 230      "  34.674      " 

"         "      90°,  1°  =  69.407      "  0.000      " 

All  measurements  which  have  been  made  show  that  the  length 
of  a  degree  of  latitude  increases  as  the  poles  are  approached.  In 
other  words,  the  nearer  the  pole  the  longer  the  degree  of  latitude, 
or,  more  strictly,  the  longer  the  arc  which  subtends  a  degree.  This 
means  that  the  earth  is  flattened  at  the  poles. 

That  this  is  the  meaning  of  the  variation  in  the  length  of  the 
degree  is  shown  by  Figs.  522  and  523.  In  the  study  of  Fig.  522 
it  is  to  be  remembered  that  a  degree  is  -j^  of  the  angular  distance 
about  a  point,  and,  measured  on  a  circumference,  it  is  the  5^-F  part 
of  the  circumference  described  about  the  point  from  which  the 
angle  is  measured.  Since  the  degree  is  longer  in  high  latitudes 
than  in  low,  it  means  that  the  arc  on  which  it  is  measured  is  the 
arc  of  a  larger  circle  than  that  on  which  the  degree  in  low  latitudes 
is  measured.  The  -g^  of  a  larger  circumference  is  longer  than  the 
•y^ff  part  of  a  smaller  circumference.  Thus,  the  distance  on  the 
circumference  between  0°  and  18°  is  much  less  than  that  between 


EARTH  RELATIONS 


495 


72°  and  90°  (=  18°).  In  other  words,  the  center  of  the  circum- 
ference on  which  a  high-latitude  degree  is  measured,  is  not  the 
same  as  the  center  from  which  a  low-latitude  degree  is  measured 
(Fig.  522). 


Fig.  523  shows  the  same  thing  in  another  way.  The  oblate 
curve  S  represents  a  meridional  section  of  the  earth,  with  the  flat- 
tening greatly  exaggerated.  Circle  C  coincides  with  S  at  the  equa- 


496 


PHYSIOGRAPHY 


tor  E,  while  the  circle  M  coincides  with  it  at  one  pole,  P.    A  degree 
of  arc  on  the  curve  S  near  P  is  about  as  long  as  a  degree  on  M, 


M 


FIG.  522. 


FIG.  523. 


FIG.  522. — Figure  to  illustrate  the  fact  that  the  longer  degrees  of  latitude 
toward  the  poles  means  polar  flattening.  The  curve  is  the  half  of  a 
spheroid,  more  oblate  than  the  earth  is.  The  radiating  lines  are  repre- 
sented as  18°  apart;  that  is,  the  distance  from  0°to  1S°  is  18/360  of  the 
circle  of  which  this  arc  is  a  part.  Similarly  the  distance  from  18°  to 
36°  is  18/360  of  the  circumference  of  which  this  curve  is  an  arc,  and  so 
on.  The  curve  between  72°  and  90°  is  much  longer  than  the  curve 
between  0°  and  18°. 

FIG.  523. — The  curve  S  represents  a  meridian  section  of  the  earth  (the  flat- 
tening being  greatly  exaggerated).  The  circle  C  coincides  with  S  near 
the  equator  E,  and  the  larger  circle  M  coincides  with  it  near  the  pole. 
A  degree  of  arc  on  S  near  P  is  of  about  the  same  length  as  one  on  M , 
while  one  on  S  near  E  is  of  about  the  same  length  as  one  on  C.  Since 
the  circle  M  is  larger  than  the  circle  C,  a  degree  on  S  near  P  is  longer 
than  one  near  E. 

while  a  degree  of  arc  on  S  near  E  is  about  as  long  as  a  degree  on 
C.     A  degree  of  arc  on  M  is  clearly  much  longer  than  a  degree  on  C. 

The  actual  measurement  of  the  length  of  a  degree  of  latitude  is  a  diffi- 
cult matter,  but  the  principle  on  which  it  is  measured  is  easily  understood. 
At  any  given  point  in  the  northern  hemisphere  the  north  star  is  a  certain 
number  of  degrees  above  the  horizon.  When  the  observer,  starting  from 
a  given  point,  has  gone  directly  northward  until  the  star  appears  one  degree 
higher  above  the  horizon  at  the  corresponding  hour,  he  has  gone  one  degree 
(Fig.  524).  In  practice,  the  measurement  is  complicated,  because  the  surface 
of  the  land  is  always  somewhat  uneven,  and  allowance  must  be  made  for 
every  irregularity.  A  line  measured  along  the  uneven  land  surface  would 
be  too  long.  Again,  the  degree  is  to  be  measured  at  sea-level.  The  land  is 
above  sea-level,  and  therefore  the  measurement  on  the  land  surface  must 


EARTH  RELATIONS 


497 


be   corrected,  not  only  for  all  unevennesses,  but  for  its  elevation  above  sea- 
level. 

Inclination  of  axis  and  its  effects.     The  sun's  rays  illuminate 
one-half  of  the   earth  all  the  time.     The  border  of  the  illuminated 


FIG.  524. — Diagram  to  illustrate  the  way  in  which  a  degree  of  latitude  is 
measured.  When  the  observer  has  traveled  so  far  along  the  surface 
that  the  position  of  the  pole-star  has  changed  1°,  the  distance  between 
the  two  stations,  A  and  B,  is  a  degree. 

half  is  called  the  circle  of  illumination  (Fig.  525).  All  places  within 
the  circle  of  illumination  have  day,  while  all  places  outside  it 
have  night.  If  the  axis  about  which  the  earth  rotates  were  per- 
pendicular to  the  plane  in  which  the  earth  revolves  about  the 
sun,  the  circle  of  illumination  would  always  pass  through  the 
poles.  Under  these  conditions  the  half  of  each  parallel  would  be 


FIG.  525. — Diagram  to  illustrate  the  fact  that  half  of  the  earth  is  illuminated 
by  the  sun  at  any  one  time.  The  line  between  the  illuminated  half  and 
the  half  which  is  not  illuminated,  is  the  circle  of  illumination. 


illuminated  all  the  time.  If  the  half  of  each  parallel  was  con- 
stantly illuminated,  the  days  and  nights  on  each  parallel  would  be 
equal,  for  it  takes  just  as  long  for  a  place  at  A  (Fig.  525)  to  move  to 
B  (half  of  a  day)  as  for  it  to  move  from  B  to  A'  (half  of  a  night). 


498  PHYSIOGRAPHY 

If,  then,  the  axis  of  the  earth  were  perpendicular  to  the  plane  of 
its  orbit,  days  and  nights  would  always  be  equal  everywhere. 

Since  days  and  nights  are  not  equal  at  all  seasons  on  most 
parts  of  the  earth,  it  follows  that  the  axis  on  which  the  earth  rotates 
is  not  perpendicular  to  the  plane  of  its  orbit. 

Again,  if  the  earth  rotated  on  an  axis  perpendicular  to  the 
plane  of  its  orbit,  the  sun's  rays  would  always  fall  on  a  given  place 
at  the  same  angle  at  the  same  hour  of  the  day.  Thus  at  A,  Fig. 

525,  the  sun's  rays  would  fall   vertically  at  noon;    while  at  the 
same  hour  (noon  at  A)  they  would  fall  at  a  lesser  angle  at  C', 
but   the  angles  of   the  rays   at  A,  C,  and  B  would   always  be 
the  same  at  the  same  hour  of  the  day,  in  whatever  part  of  its 
orbit  the  earth  found  itself.     The  same  relations  would  hold  for 
points  on  all  parallels.     Now,  the  sun's  rays  do  not  fall  at  the  same 
angle  at  the  same  place  at  the  same  hour  at  all  times  of  the  year. 
In  middle  northern  latitudes,  for  example,  the  sun  is  much  higher 
above  the  horizon  at  noon  in  summer  than  in  winter.     This  varia- 
tion of  the  angle  at  which  the  sun's  rays  strike  the  earth  at  a 
given  time  and  place,  as  well  as  the  unequal  lengths  of  days  and 
nights  in  most  places,  is  the  result  of  the  inclination  of  the  axis 
on  which  the  earth  rotates  as  it  revolves  around  the  sun.     The 
position  of  the  axis  is  essentially  constant  throughout  the  year,  and 
though  its  changes  are  more  considerable  in  long  periods  of  time 
they  may  be  disregarded  when  short  periods  are  concerned. 

The  effect  of  the  inclination  of  the  axis  is  illustrated  by  Fig. 

526,  which  represents  the  earth  in  four  positions  in  its  orbit.     In 
the  position  marked  March  21,  the  half  of  each  parallel  is  illu- 
minated.  At  this  time,  therefore,  days  and  nights  are  equal  every- 
where.    In  the  position  marked  June  21,  more  than  half  of  all  the 
parallels  of  the  northern  hemisphere  are  illuminated,  and  there 
the  days  are  more  than  12  hours  long  and  the  nights  correspond- 
ingly shorter.     In  the  southern  hemisphere  the  nights  are  longer 
than  the  days.     In  the  third  position,  September  22,  the  days  and 
nights  are  again  equal  everywhere,  for  the  circle  of  illumination 
bisects  every  parallel.     In  the  fourth  position,  December  22,  more 
than  half  of  each  parallel  in  the  southern  hemisphere  is  within  the 
circle  of  illumination,  and  there  the  days  are  longer  than  the  nights, 
while  in  the  northern  hemisphere  the  nights  are  longer  than  the 
days.     Twice  during  the  year,  therefore,  on  March  21  and  Septem- 
ber 22,  the  days  and  nights  are  equal  everywhere.     These  times 


EARTH  RELATIONS  499 

are  known  as  the  equinoxes.     The  equinox  in  March  is  the  vernal 
equinox,  and  that  in  September  is  the  autumnal  equinox. 

When  the  earth  is  in  the  relation  to  the  sun  shown  in  the  posi- 
tion marked  June  21,  Fig.  526,  the  days  are  longest  in  the  north- 
ern hemisphere,  and  the  rays  of  the  sun  fall  perpendicularly  on  the 
surface  of  the  earth  farther  north  (in  lat.  23°  27^')  than  at  any  other 
time.  This  is  the  summer  solstice.  The  winter  solstice  occurs  six 
months  later,  when  the  sun's  rays  strike  the  earth  vertically  23^° 
(nearly)  south  of  the  equator,  and  when  the  days  of  the  southern 
hemisphere  are  longest  and  those  of  the  northern  shortest.  The 


SEPT.  22 

FIG.  526. — Diagram  showing  the  position  of  the  earth  and  of  its  illumination 
at  the  solstices  and  equinoxes. 

distribution  of  light  and  the  relative  lengths  of  day  and  night  in 
various  latitudes  are  further  shown  for  the  solstitial  dates  by 
Figs.  527  and  528. 

These  figures  also  show  that  the  days  and  nights  are  always  equal 
at  the  equator,  since  the  equator  is  always  bisected  by  the  circle  of 
illumination  (Figs.  527,  528,  and  536).  Days  and  nights  are  not 
always  equal  in  any  other  latitude,  unless  at  the  poles,  where  there 
is  one  day  of  six  months  and  one  night  of  six  months,  each  year. 

Apparent  motion  of  the  sun.  The  effect  of  the  inclination  of 
the  axis  of  the  earth  is  to  make  the  sun  appear  to  move  north  and 
south  once  during  each  revolution  of  the  earth  about  the  sun. 
The  effect  on  the  earth  is  illustrated  by  Fig.  529.  That  is,  the  revo- 
lution of  the  earth  about  the  sun,  while  it  rotates  on  an  axis  in- 


500 


PHYSIOGRAPHY 


clined  toward  the  plane  of  its  orbit,  makes  the  sun  appear  to  move 
from  a  place  where  his  rays  are  vertical  23£°  (nearly)  north  of  the 
equator  (direction  S,  Fig.  529),  to  a  place  where  they  are  vertical 


FIG.  527. — Diagram  to  illustrate  the  effect  of  inclination  of  the  earth's  axis 
on  the  length  of  day  and  night.  In  the  figure,  more  than  half  of  every 
parallel  of  the  northern  hemisphere  is  illuminated.  The  days  are  there- 
fore more  than  twelve  hours  long,  and  the  nights  less,  since  the  half  of 
each  parallel  is  the  measure  of  180°  of  longitude,  and  ISO0  of  longitude 
corresponds  to  twelve  hours  of  time.  Similarly  less  than  half  of  every 
parallel  of  the  southern  hemisphere  is  illuminated,  and  the  nights  are 
therefore  more  than  twelve  hours  long. 


FIG.  528. — The  relation  of  the  earth  to  the  sun's  rays  at  a  time  six  months 
later  than  that  represented  in  Fig.  527.  The  conditions  of  day  and 
night  in  the  hemispheres  are  reversed. 


23£°  (nearly)  south  of  the  equator  (direction  W),  and  back  again, 
in  one  year.1    The  result,  so  far  as  the  earth  is  concerned,  is  as 

1  The  inclination  of  the  earth's  axis  is  not  quite  constant.  Its  present 
inclination  (1908)  is  23°  27'  4.5".  Three  thousand  years  ago  its  inclination 
was  about  23°.  The  extreme  variation  possible  is  2°  37'. 


EARTH  RELATIONS  501 

if  the  sun  moved  from  S,  which  corresponds  to  the  time  of  the  sum- 
mer solstice,  to  A,  which  corresponds  to  the  time  of  the  autumn 
equinox,  to  W,  which  corresponds  to  the  time  of  the  winter  sol- 
stice, then  back  again  to  Sp,  which  corresponds  to  the  spring 
equinox,  and  to  S,  while  the  earth  is  making  one  circuit  about  the 
sun. 

When  the  sun  is  vertical  in  latitudes  north  of  Sp,  the  days  are 
longer  than  the  nights  in  the  northern  hemisphere,  and  the  sun's 
rays  strike  the  surface  in  the  northern  hemisphere  less  obliquely 
than  they  do  in  the  southern  hemisphere.  When  the  sun  is  in  the 
position  Sp,  days  and  nights  are  equal  everywhere,  and  when  the 


Sp&A 


FIG.  529. — The  inclination  of  the  earth's  axis,  as  it  revolves  about  the  sun, 
makes  the  sun  appear  to  travel  north  and  south.  The  sun  is  vertical  at 
the  equatoi  on  the  21st  ol  March  (Sp.),  then  appears  to  move  northward 
until  it  is  vertical  23£°  north  ol  the  equator  (S),  then  appears  to  move 
southward  until  it  is  vertical  again  at  the  equator  (A),  then  south  until 
it  is  vertical  23$°  south  of  the  equator  (W.),  and  then  north  again  until 
it  is  vertical  at  the  equator.  These  chances  are  accomplished  in  the 
course  of  one  year  as  a  result  of  the  revolution. 

sun  is  vertical  south  of  Sp,  days  are  longer  than  nights  in  the 
southern  hemisphere,  and  the  sun's  rays  are  more  nearly  vertical 
than  in  the  northern  hemisphere. 

The  northernmost  parallel  where  the  sun's  rays  are  ever  verti- 
cal is  called  the  tropic  of  Cancer.  The  corresponding  southernmost 
parallel  is  the  tropic  of  Capricorn.  The  tropics  are  nearly  23£° 
(23°  27')  from  the  equator,  because  the  axis  of  the  earth  is  in- 
clined by  that  amount  toward  the  plane  of  its  orbit.  The  sun  is  verti- 
cal at  the  tropic  of  Cancer  at  the  time  of  the  summer  solstice,  and 
at  the  tropic  of  Capricorn  at  the  time  of  the  winter  solstice.  The 
parallels  just  touched  by  the  circle  of  illumination  at  the  time  of  the 


502  PHYSIOGRAPHY 

solstices  are  the  polar  circles.  They  are  as  far  from  the  poles  as 
the  tropics  are  from  the  equator.  They  are,  therefore,  in  latitude 
about  66£°  (66°  33').  The  one  in  north  latitude  is  the  Arctic  circle, 
and  the  one  in  south  latitude  the  Antarctic  circle. 

The  effects  of  inclination  of  the  earth's  axis  on  the  length  of  days  and 
nights  may  well  be  emphasized  by  comparing  the  lengths  of  days  and 
nights  as  they  now  exist,  in  our  own  region,  with  those  which  would  exist 
if  the  axis  of  the  earth  were  inclined  45°  toward  the  plane  of  its  orbit 
instead  of  23J°.  It  is  also  instructive  to  study  the  conditions  which  would 
exist  with  reference  to  day  and  night  (1)  if  the  earth  did  not  rotate  during 
its  revolution  around  the  sun,  and  (2)  if  it  rotated  once  in  the  period  of 
its  revolution.  In  the  latter  case,  the  results  depend  on  the  direction  of 
rotation. 

Latitude  and  sun  altitude.  The  solution  of  certain  problems 
in  the  determination  of  latitude  and  sun  altitude  will  help  to  a 
clearer  understanding  of  the  changes  in  the  relations  of  sun  and 
earth  due  to  the  movements  of  the  latter. 

At  the  time  of  equinox,  the  sun  is  directly  overhead  at  the 
equator  at  noon.  One  degree  north  of  the  equator,  i.e.,  in  latitude 
1°  N.,  the  sun  will  appear  1°  from  the  zenith  (i.e.,  the  point  directly 
overhead)  at  noon,  or  89°  above  the  horizon.  This  is  the  same  as 
saying  that  the  altitude  of  the  sun  is  89°.  Five  degrees  north  of 
the  equator  (lat.  5°  N.)  the  sun  will  appear  5°  from  the  zenith  at 
noon,  and  his  altitude  (above  the  horizon)  is  85°. 

If,  therefore,  the  altitude  of  the  sun  at  a  given  place  at  noon 
at  the  time  of  an  equinox  is  known,  the  latitude  may  be  determined. 
Thus  if  the  altitude  of  the  sun  is  30°  at  the  time  of  an  equinox, 
the  observer  must  be  60°  from  the  place  where  it  is  vertical,  that 
is,  in  latitude  60°  N.  or  60°  S.  Similarly  if  the  latitude  is  known, 
the  altitude  of  the  sun  at  noon  at  the  time  of  equinox  may  be 
determined.  Thus  in  latitude  40°  the  sun  must  be  50°  above  the 
horizon  at  noon,  for  latitude  40°  is  40°  from  the  place  where  the 
sun  is  vertical. 

Any  other  dates  besides  the  equinoxes  may  be  used  if  the  lati- 
tude where  the  sun  is  vertical  is  known.  Thus  at  the  time  of  the 
summer  solstice,  when  the  sun  is  vertical  in  latitude  23i°  N.,  it  is 
23£°  from  the  zenith,  or  has  a  noon  altitude  of  66 J°,  at  the  equator. 
It  has  the  same  altitude  in  latitude  47°  N.,  for  this  place,  like  the 
equator,  is  23J°  from  the  place  where  the  sun  is  vertical. 

Reversing  the  problem,  we  may  determine  the  latitude  of  a 


EARTH  RELATIONS  503 

place  if  we  know  the  noonday  altitude  of  the  sun  there,  and  the 
latitude  where  the  noonday  sun  is  then  in  the  zenith.  Suppose, 
for  example,  the  altitude  of  the  sun  is  40°  at  noon  at  the  time 
of  the  June  solstice,  what  is  the  latitude  of  the  place?  Since 
the  altitude  of  the  sun  is  40°,  the  place  must  be  50°  from  the  place 
where  the  sun  is  vertical,  that  is,  50°  from  latitude  23J°  N.  This 
is  73i°  N.  or  26^°  S. 

Problems. 

Note.  In  the  solution  of  these  problems  the  student  will  find  it 
helpful,  in  many  cases,  to  make  diagrams  representing  the  conditions 
of  the  problem. 

1.  What  is  the  altitude  of  the  sun  at  noon  at  the  time  of  an  equinox, 

(1)  In  latitude  50°  N.? 

(2)  In  latitude  50°  S.? 

(3)  In  latitude  75°? 

2.  What  is  the  altitude  of  the  sun  at  noon  at  the  time  of  the  summer 
solstice, 

(1)  In  latitude  30°  N.? 

(2)  In  latitude  30°  S.? 

(3)  In  the  latitude  of  New  York? 

(4)  In  the  latitude  of  Vancouver? 

(5)  In  latitude  75°  N.? 

(6)  In  latitude  66i°  S.? 

(7)  At  the  north  pole? 

3.  Formulate  a  rule  for  finding  the  altitude  of  the  sun  (a)  at  the 
time  of  an  equinox,  and  (6)  at  the  time  of  a  solstice,  the  latitude  of  the 
place  being  given. 

4.  In  what  latitude  or  latitudes  is  the  sun  30°  above  the  horizon  at 
noon  at  the  time  of  an  equinox? 

5.  In  what  latitude  or  latitudes  is  the  sun  75°  above  the  horizon  at 
noon  at  the  time  of  an  equinox? 

6.  In  what  latitude  or  latitudes  is  the  sun  40°  above  the  horizon  at 
noon  at  the  time  of  the  June  solstice? 

7.  In  what  latitude  or  latitudes  is  the  sun  80°  above  the  horizon  at 
noon  at  the  time  of  the  December  solstice? 

8.  What  is  the  latitude  of  the  place  or  places  where  the  sun  is  10° 
above  the  horizon  at  noon  at  the  time  of  the  June  solstice? 

9.  Formulate  a  rule  for  finding  the  latitude  of  a  place  from  the  noon 
altitude  of  the  sun. 

10.  In  what  direction  and  at  what  altitude  would  the  sun  appear  (a) 
at  midnight,  and  (6)  at  noon,  to  an  observer  in  latitude  75°  N.  at  the 
time  of  the  summer  solstice?    See  2  (5)  above. 


504 


11.  To  an  observer  at  the  equator,  in  what  direction  would  the  sun 
appear  to  rise  on  June  21?    What  would  be  the  noon  altitude  of  the  sun 
at  the  equator  on  the  same  day?     a, 

12.  What  would  be  the  noon  altitude  of  the  sun  at  Chicago  on  June 
21?    On  December  21? 

13.  In  what  latitude  is  the  altitude  of  the  sun  the  same,  at  noon,  at 
the  time  of  a,n  equinox  and  on  June  21? 

The  Solar  System 

The  solar  system  includes  the  sun  and  all  the  bodies  which 
revolve  about  it.  There  are  eight  planets,  of  which  the  earth  is  one. 
Named  in  the  order  of  their  distance  from  the  sun,  commencing 
with  the  nearest,  the  planets  are:  Mercury,  Venus,  Earth,  Mars, 
Jupiter,  Saturn,  Uranus,  and  Neptune.  Most  of  the  planets 
have  satellites  corresponding  to  our  moon.  The  following  table 
shows  some  of  the  more  important  facts  about  the  planets: 


g.sJ 

« 

• 

II 

7 

q 

7 

is* 

IB 

"3 

§3.2 

•sl 

-£  oa 

a>:2 

>>  iJ 

*§     o 

1  —  '  a; 

'•^  *^  ^. 

-  — 

88 

|1 

CQ 

**  "rf 

ES 

2jM 

§15  — 

^3 

Is 

§ 

N 

S  o-^J 

•|.s 

.-   i,    0 

B  os 

§02 

Q 

> 

an 

Q 

S 

n 

« 

i 

Mercury.  .  .  . 

2,765 

0.05 

3.70 

36.0 

0.24 

7°    0' 

0 

9,64  /,  000 

1 

V  ATlllfl 

7,826 

0.89 

4.89 

67.2 

0.62 

3    24 

0 

405,000 

F«.rth 

f  7,926.5* 
{  7,899.7f 

1.00 

1 

5.53 

92.9 

1.00 

0      0 

1 

332,000 

Mcirs 

/  4,352* 
I  4,312f 

0.14 

1 

3.95 

141.5 

1.88 

1    51 

2 

3,020,000 

Jupiter 

1  90,190* 
\  84,570t 

1264.00 

1 

1.33 

483.3 

11.86 

1    19 

7 

1,047 

Saturn  

1  76,470* 
1  69,780f 

759.00 

1 

0.72 

886.0 

29.46 

2    30 

10 

3,502 

1 

T  T  Y>n  iiii 

34,900 

63.40 

1  .22 

1781.9 

84.  C2 

0    46 

4 

22,760 

Neptune.  .  .  . 

32,900 

82.30 

1 

1.11 

2791  .  6 

164.78 

1    47 

1 

19,500 

Equatorial. 


t  Polar. 


EARTH  RELATIONS  505 

Besides  the  planets  and  their  satellites,  the  solar  system  in- 
cludes numerous  (more  than  400)  asteroids,  bodies  much  smaller 
than  the  planets,  intermediate  in  position  between  Mars  and 
Jupiter,  and  those  comets  which  revolve  about  the  sun.  These 
bodies  have  little  influence  on  the  earth,  and  nothing  further  need 
be  said  of  them  in  this  place. 

REFERENCES 

Text-books  on  Astronomy.  Among  the  available  recent  ones  are  Moul- 
ton's  (Macmillan)  and  Comstock's  (Appleton).  Todd's  (Am.  Book  Co.) 
and  Young's  (Ginn  &  Co.)  are  less  recent,  but  serviceable;  also  Johnson's 
Mathematical  Geography  (Am.  Book  Co.). 


PART  III 
THE  ATMOSPHERE 

CHAPTER  XII 
GENERAL    CONCEPTION    OF   THE    ATMOSPHERE 

Substantiality.  When  the  atmosphere  is  still,  we  are  hardly 
conscious  of  its  existence.  We  walk  through  it  without  realizing 
that  we  are  forcing  our  way  through  a  real  substance.  Compared 
with  land  or  even  with  water,  it  seems  most  unsubstantial.  But 
when  the  air  is  in  motion,  that  is,  when  the  wind  blows,  we  are  con- 
scious that  it  is  very  real  and  substantial,  for  the  force  of  the 
wind  may  be  so  great  that  it  is  difficult  to  stand  or  walk  against  it. 
Trees  and  buildings  are  occasionally  blown  down  by  it,  and  quanti- 
ties of  dust  and  sand  are  picked  up  and  sometimes  carried  up  to 
great  heights.  These  familiar  phenomena  show  that  the  air  is  a 
real  substance,  and  that,  when  it  moves  rapidly,  even  strong 
objects  give  way  before  it. 

A  strong  wind  is  not  equally  strong  at  every  instant;  it  comes 
in  gusts.  When  a  strong  gust  of  wind  strikes  a  high  building, 
the  air  is  reflected  from  the  wall,  somewhat  as  a  ball  is  when  thrown 
against  it.  If  a  strong  gust  of  wind  is  followed  the  next  instant  by 
a  weak  wind  or  a  lull,  the  air  rebounding  from  the  wall  may  have 
great  force  in  a  direction  opposed  to  that  of  the  main  wind.  These 
reflected  winds  occasionally  blow  people  down,  for  they  blow  in 
the  direction  opposite  to  that  against  which  the  body  is  braced. 
In  cities  where  there  are  high  buildings,  the  streets  are  sometimes 
protected  from  the  direct  winds;  but  the  currents  of  air,  whether 
direct  or  reflected,  are  often  concentrated  at  the  street  level,  where 
they  sometimes  have  force  enough  to  overturn  cabs. 

The  substantiality  of  the  air  may  be  shown  in  still  another  way. 
If  the  air  be  pumped  out  of  a  cylinder  whose  top  is  covered  by  a 
thin  piece  of  rubber,  the  rubber  covet  is  pressed  down  into  the 

506 


GENERAL  CONCEPTION  OF  THE  ATMOSPHERE        507 

cylinder,  and  may  even  be  broken.  The  force  which  presses  it 
down  is  the  weight  of  the  air  above.  If  the  cylinder  be  of  weak 
material,  such  as  thin  glass,  while  the  cover  is  strong,  the  pressure 
of  the  air  outside  may  burst  the  cylinder  when  the  air  inside  is 
pumped  out.  The  cylinder  does  not  break  when  full  of  air,  because 
the  pressure  on  the  inside  balances  that  on  the  outside.  If  the 
cylinder  be  of  tin  instead  of  glass,  the  pressure  of  the  air  on  the 
outside  may  bend  in  its  walls  when  the  air  is  pumped  out.  These 
experiments,  as  well  as  the  phenomena  of  the  wind,  show  that 
the  air  is  something  real  and  that  it  has  weight. 

The  amount  of  pressure  which  the  air  exerts,  that  is,  its  weight, 
may  be  determined.  At  sea-level,  it  is  found  to  be  nearly  15 
(14.7)  pounds  to  the  square  inch. 

Relation  to  the  rest  of  the  earth.  The  atmosphere  is  com- 
monly called  an  envelope  of  the  earth.  More  properly  it  is  an 
envelope  of  the  rest  of  the  earth,  for  it  is  itself  as  much  a  part  of  the 
earth  as  the  rocks  ^are.  It  goes  with  the  rest  of  the  earth  through 
space,  and  it  is  essential  to  the  life  of  the  earth  and  to  most  of  the 
processes  which  are  in  operation  on  the  earth's  surface.  It  is  the 
medium  through  which  moisture  is  distributed,  and  it  has  much 
to  do  with  the  temperature  of  the  earth,  for  without  an  atmosphere 
the  earth  would  be  very  much  colder  than  now.  Without  the 
air,  therefore,  the  earth  would  be  a  very  different  body.  Some 
conception  of  its  functions  may  be  gained  by  trying  to  conceive 
what  the  earth  would  be  without  it.  This  point  may  be  recalled 
from  time  to  time,  as  our  study  proceeds. 

The  atmosphere  is  in  reality  a  little  more  than  an  envelope  of 
the  rest  of  the  earth,  for  it  penetrates  the  soil  and  rocks  as  far  down 
as  there  are  holes  and  cracks,  and  its  constituents  are  dissolved  in 
the  waters  of  the  sea,  in  all  the  waters  on  the  land,  and  in  all  the 
waters  beneath  its  surface. 

Density  and  altitude.  Many  of  the  laws  which  govern  the 
distribution  of  gaseous  matter  are  known.  From  these  laws  it  is 
known  that  the  air,  which  is  but  a  mixture  of  gases,  must  be  most 
dense  below  and  less  dense  above.  This  is  the  same  as  saying  that 
there  is  more  air  in  a  cubic  foot  of  space  at  sea-level  than  in  a 
cubic  foot  of  space  at  higher  levels.  To  this  general  rule  there  are 
exceptions,  locally  and  temporarily,  but  they  need  not  be  con- 
sidered here.  Similarly  there  is  more  air  in  a  cubic  foot  of  space 
1000  feet  above  sea-level  than  in  the  same  space  2000  feet  above 


508  PHYSIOGRAPHY 

sea-level,  and  so  on.  This  means  that  the  particles  of  which 
gases  are  composed  are  nearer  together  at  low  altitudes  than  at 
high  altitudes.  The  reason  is  readily  understood. 

If  a  cubic  foot  of  air  were  pressed  from  all  sides,  it  could  be 
squeezed  into  a  smaller  space,  and  the  more  the  pressure,  the 
smaller  the  space  into  which  it  could  be  compressed.  Now  at  the 
bottom  of  the  atmosphere  the  air  is  pressed  down  by  all  the  air 
above.  At  the  height  of  1000  feet  above  the  sea,  the  air  is  pressed 
down  by  all  the  air  above  that  level,  and  so  on.  Hence  the 
lowest  air  is  under  most  pressure,  and  is  therefore  (with  certain 
exceptions)  densest. 

It  is  largely  because  the  air  gets  rarer  with  increase  of  altitude 
that  mountain-climbing  is  difficult.  As  the  climber  gets  higher 
and  higher,  it  becomes  more  and  more  difficult  to  breathe.  He 
may  take  in  the  same  number  of  cubic  inches  of  air  each  time  he 
inhales,  but  each  cubic  inch  contains  less  air  the  higher  he  goes. 
Finally  the  air  gets  so  rare  as  to  stop  further  ascent.  It  should  be 
noted,  however,  that  it  is  not  simply  the  decreasing  amount  of  air 
taken  into  the  lungs  which  makes  it  difficult  to  ascend  to  great 
heights.  The  cold,  the  snow  and  ice,  and  often  the  very  steep 
slopes  of  high  altitudes,  are  all  obstacles,  and  the  body  is  not 
adjusted  to  the  lessened  pressures  of  the  higher  altitudes. 

Height.  How  high  above  the  sea  and  land  does  the  air  ex- 
tend? No  positive  answer  can  be  given  to  this  question,  though 
something  is  known  about  it. 

1.  The  greatest  altitude  reached  by  any  mountain-climber  is 
between  four  and  five  miles.     At  this  altitude  there  was  air  enough 
to  make  breathing  possible  to  one  exercising  actively.     A  con- 
siderable quantity  of  air,  therefore,  exists  at  a  height  of  more 
than  four  miles.     Its  upper  limit  must  be  considerably  higher. 

2.  Men  have  ascended  to  heights  of  over  six  miles  in  balloons, 
and  in  one  case  (Coxwell  and  Glaisher,  1802)  to  a  height  of  more 
than  seven  miles.     In  some  cases  the  occupants  of  the  balloons 
have  become  unconscious  at  an  elevation  of  about  29,000  feet, 
but  in  one  case  (Dr.  Berson,  Berlin,  1894)  this  difficulty  was  over- 
come by  carrying  oxygen  for  breathing.    Balloons  without  human 
occupants  have  risen  eighteen  miles.     At  the  upper  limit  of  their 
ascent,  the  air  was  still  dense  enough  so  that  the  amount  displaced 
by  the  balloon  was  at  least  equal  to  the  weight  of  the  balloon. 


GENERAL  CONCEPTION  OF  THE  ATMOSPHERE       503 

3.  From  the  phenomena  of  twilight,  due  to  the  refraction  of  the 
light  as  it  passes  through  the  atmosphere,  it  may  be  demonstrated 
that  the  air  extends  up  to  a  height  of  forty-five  miles. 

4.  On  almost  any  clear  night  "shooting  stars"  may  be  seen. 
These  shooting  stars,  or  meteors,  are  small  solid  bodies  which  come 
into    the   earth's  atmosphere   from  outside  space.      When   they 
enter  the  atmosphere,  they  are  very  cold,  for  the  temperature 
of  space,  outside  the  earth's  atmosphere,  is  believed  to  be  about 
—  459°  F.     As  they  approach  the  earth,  they  are  traveling  (fall- 
ing) very  fast,  say  12  to  45  miles  per  second.     In  passing  through 
the  atmosphere,  their  movement  is  resisted  by  the  air.     The  re- 
sult is  friction,  and  the  friction  with  the  air  heats  them.     When 
they  get  hot  enough  to  glow  (red-hot),  they  may  be  seen.     Now 
the  height  at  which  they  begin  to  glow  has  been  estimated  in 
some  cases,  and  is  found  to  be,  at  a  maximum,  more  than  100  miles 
above  sea-level.     This  shows  that  the  atmosphere  is  much  more 
than  100  miles  high,  for  the  meteors  must  have  come  through  the  rare, 
cold  upper  air  a  very  considerable  distance  before  becoming  red-hot. 

5.  The  aurora  or  "northern  lights"  sometimes  seen  in  high 
latitudes  is  believed  to  be  an  electric  phenomenon  in  very  rare 
air.     The  height  of  the  aurora  is  sometimes  more  than  100  miles. 
The  southern  ends  of  the  streamers  have  even  been  calculated  to 
be  as  much  as  400  miles  high.     This  shows  that  the  air  is  dense 
enough  to  show  electric  phenomena  at  that  height.    It  is  believed, 
however,  that  the  density  of  the  air  100  miles  above  sea-level  is 
not  much  more  than  one  billionth  of  its  density  at  sea-level. 

6.  We  know  the  weight  of  the  atmosphere.     We  know  also 
the  rate  at  which  a  gas,  or  a  mixture  of  gases,  like  the  air  gets 
lighter  with  increase  of  altitude.     The  law  is  that  the  density  is 
proportional  to  the  pressure..    If  we  go  up  till  half  the  air  is  below 
us,  the  air  at  that  height  should  be  half  as  dense  as  it  was  at  the 
bottom.     If  we  rise  again  until  half  of  the  upper  half  of  the  air  is 
below  us,  the  air  at  that  level  is  half  as  dense  as  it  was  at  the  first 
station.     On  this  principle  it  would  appear  that  there  should  be 
no  upper  limit;  the  air  should  simply  get  rarer  and  rarer  without 
having  a  definite  upper  surface. 

Though  the  above  law  holds  in  all  places  where  experiments  can 
be  carried  on,  there  is  some  reason  to  believe  that  it  may  cease  to 
hold  when  the  air  becomes  very  rare. 

7.  All  gases  have  a  tendency  to  fly  away  from  the  earth,  but 


510  PHYSIOGRAPHY 

are  Tield  by  gravity.  Gravity  gets  weaker  and  weaker  with  in- 
creasing distance  from  the  earth's  center,  and,  at  a  sufficiently 
great  distance  from  the  center,  the  earth  would  not  be  able  to  hold 
any  of  the  gases  of  its  atmosphere.  That  distance  would  be  less 
for  lighter  gases,  and  greater  for  heavy  ones.  It  is  calculated  that 
none  of  the  gases  of  the  atmosphere  could  be  held  by  the  earth  at 
a  distance  greater  than  620,000  miles»from  the  center  of  the  earth. 

From  the  above  considerations  it  appears  to  be  certain  that 
the  air  extends  much  more  than  100  miles  above  the  rest  of  the 
earth,  but  how  much  more  is  unknown.  Whatever  its  height,  one- 
half  the  atmosphere  (by  weight)  lies  below  a  plane  about  3.6  miles 
above  sea-level,  three-fourths  of  it  below  a  plane  6.8  miles  above 
the  same  level,  and  seven-eighths  of  it  below  a  plane  10.2  miles 
up.  The  highest  mountain  is  about  6  miles  high,  so  that  nearly 
three-fourths  of  the  atmosphere  lies  below  the  level  of  its  top. 

Volume.  Since  the  height  of  the  air  is  not  known,  its  volume 
cannot  be  determined.  If  it  extends  up  but  200  miles,  its  volume 
is  about  one-sixth  that  of  the  rest  of  the  earth ;  if  it  extends  up  500 
miles,  its  volume  is  nearly  one-half  that  of  the  rest  of  the  earth. 

Mass.  Great  as  is  the  volume  of  the  atmosphere,  its  mass 
(measured  by  its  weight)  is  far  less  than  that  of  the  solid  part  of 
the  earth,  or  even  than  that  of  the  water.  It  has  been  estimated 
at  about  ^TT  that  of  the  water,  and  about  i^org-  that  of  the  solid 
part  of  the  earth.  Its  weight  is  about  equal  to  that  of  a  layer  of 
water  completely  covering  the  earth  to  a  depth  of  33  feet.  Sir 
John  Herschel  estimated  the  weight  of  the  air  resting  on  the  other 
parts  of  the  earth  at  11§  trillion  pounds. 

History.  It  is  probable  that  the  atmosphere  has  undergone 
changes  in  mass  and  volume  in  the  course  of  its  history.  It  was 
formerly  assumed  that  the  atmosphere  is  being  gradually  diminished, 
and  that  it  would  in  time  disappear,  as  the  moon's  atmosphere 
was  assumed  to  have  disappeared.  But  this  assumption  does  not 
appear  to  be  well  founded.  It  is  more  probable  that  the  moon 
never  had  an  atmosphere,  than  that  it  has  lost  one  it  once  had. 
Furthermore,  the  atmosphere  is  now  gaining  various  gases  from 
volcanic  and  other  vents  (p.  368),  and  probably  has  always  done 
so.  It  is  probably  acquiring  gases  from  space  also.  Though 
contributions  from  this  source  are  inconsiderable  now,  they  may 
not  always  have  been  so.  The  atmosphere  is  losing  as  well  as 
gaining.  Some  gases,  especially  light  ones  like  hydrogen,  prob- 


GENERAL  CONCEPTION  OF  THE  ATMOSPHERE       511 

ably  escape  the  attractive  control  of  the  earth  and  pass  off  into 
space.  Other  constituents  of  the  air,  like  oxygen  (p.  72)  and 
carbonic  acid,  are  withdrawn  from  the  air  and  locked  up  for  long 
periods  at  least,  if  not  permanently,  in  the  rocks.  The  rates  both 
of  supply  and  loss  fluctuate.  When  loss  exceeds  supply,  the  mass 
of  the  atmosphere  must  decrease;  when  supply  exceeds  loss,  the 
mass  must  increase.  So  far  as  can  be  judged  from  present  phe- 
nomena, slight  fluctuations  of  mass  must  have  taken  place.  As 
will  be  seen  in  the  next  chapter,  the  fluctuations  in  composition 
may  have  been  more  significant  than  the  fluctuations  of  mass  and 
volume. 

REFERENCES 

The  following  references  apply  to  Chapters  XII  to  XIX.  Numbers 
1  to  5  are  the  most  serviceable  for  the  general  student;  Number  13  is  help- 
iul  for  the  topic  indicated  by  the  title,  and  14  is  constantly  valuable. 

1.  DAVIS,  Elementary  Meteorology:    Ginn  &  Co. 

2.  WALDO,  Elementary  Meteorology:   Am.  Bk.  Co.;    Modern  Meteorology. 
Scribners. 

3.  BARTHOLOMEW,    Physical    Atlas;    Meteorology:    Constable,    London. 

4.  WARD,  Practical  Exercises  in  Meteorology:    Ginn  &  Co.     Also  Bull. 
Am.  Geog.  Soc.,  Vol.  XXXVII,  1905,  p.  385,  and  Vol.  XXXVIII,  1906,  p.  401. 

5.  HANN,  Handbook  of  Climatology:    The  Macmillan  Co. 

6.  GREELT,  American  Weather:   Dodd,  Mead  &  Co. 

7.  FERREL,  Popular  Treatise  on  the  Winds:  Wiley  &  Sons. 

8.  RUSSELL,  Meteorology:  Macmillan. 

9.  Illustrated  Cloud  Forms:   U.  S.  Hydrographic  Office,  Washington. 

10.  DAVIS,  The  Temperature  Zones:  Jour.  Sch.  Geog.,  Vol.  I,  pp.  139-143. 

11.  HAYDEN,  The  Great  Storm  off  the  Atlantic  Coast  of  the  United  States, 
March  11-14,  1888:   Nat.  Geog.  Mag.,  Vol.  I,  pp.  40-58. 

12.  GREELY,  Rainfall  Types  of  the  United  States:  Nat.  Geog.  Mag.,  Vol.  V, 
pp.  45-58. 

13.  MOORE,  Storms  and  Weather  Forecasts:   Nat.  Geog.  Mag.,  Vol.  VIII, 
pp.  65-82,  and  Vol.  XVI,  pp.  255-305. 

14.  The  Monthly  Weather  Review:   The  Weather  Bureau,  Washington. 


CHAPTER  XIII 
CONSTITUTION   OF  THE   ATMOSPHERE 

Principal  constituents.  The  atmosphere  is  remarkably  con- 
stant in  composition,  and  is  made  up  chiefly  of  two  gases,  namely, 
nitrogen,  which  makes  up  nearly  78  per  cent,  of  dry  air,  and  oxygen, 
which  makes  about  21  per  cent. 

Minor  constituents.  Beside  these  two  principal  constituents, 
the  proportions  of  which  do  not  vary  much,  there  are  several  minor 
constituents,  of  which  argon,  about  one  per  cent,  of  the  whole,  is 
most  abundant.  Argon  was  not  separated  from  nitrogen  until 
recent  years.  Another  minor  constituent  of  dry  air,  carbon  dioxide 
or  carbonic-acid  gas,  is  of  great  importance.  It  makes  up  about 
TBT?nr  by  volume  of  the  whole  atmosphere,  and  its  amount  is 
nearly  constant  from  year  to  year. 

There  is  also  a  considerable  amount  of  water  vapor,  that  is, 
water  in  particles  so  small  as  to  be  invisible  in  the  air.  The  total 
amount  in  the  atmosphere  at  any  one  time  varies  within  rela- 
tively narrow  limits;  but  the  amount  varies  greatly  from  place 
to  place  at  the  same  time,  and  from  time  to  time  in  the  same  place. 
Since  this  is  the  case,  and  since  it  is  separated  frequently  from  the 
atmosphere,  in  the  form  of  rain,  snow,  etc.,  it  is  often  regarded  as 
something  in  the  air,  rather  than  as  a  part  of  the  air.  The  weight 
of  the  total  amount  in  the  air  at  one  time  has  been  variously 
estimated  at  from  3-^3-  (1  per  cent.)  to  ?^  (£  per  cent.)  that  of  dry 
air.  The  smaller  figure  is  probably  nearer  the  truth  than  the 
larger.  The  water-vapor  pressure  at  the  bottom  of  the  atmosphere 
is  not  a  measure  of  the  amount  of  water  vapor  above.  It  was 
this  assumption  which  gave  the  larger  of  the  figures  cited  above. 
The  water  vapor  may  make  as  much  as  3  per  cent,  of  the  air  (by 
volume)  in  moist  tropical  regions. 

512 


CONSTITUTION  OF  THE  ATMOSPHERE  513 

Impurities.  The  air  always  contains  some  other  gases  which 
are  commonly  looked  upon  as  impurities,  though  they  are  not  neces- 
sarily injurious  to  life  or  to  natural  processes  in  general.  Gases 
arise  from  combustion  and  decay  of  organic  matter,  from  various 
chemical  processes  used  in  manufacturing,  from  volcanic  and  other 
vents  in  the  earth's  crusts,  etc.  Their  aggregate  amount  is  small, 
but  locally,  as  about  some  vents,  they  are  so  abundant  as  to  be 
injurious  to  life.  This  is  the  case  in  Death  Valley  in  the  Yellow- 
stone Park,  where  animals  straying  into  certain  parts  of  the  valley 
are  often  overcome  and  killed. 

The  air  always  contains  numerous  solid  particles,  which  may, 
collectively,  be  called  dust.  Though  the  dust  in  the  air  serves 
important  functions,  it  is  to  be  looked  upon  as  an  impurity  rather 
than  as  a  constituent. 

Relations  of  constituents  to  one  another.  The  various  gas- 
eous constituents  of  the  air  are  mixed  with  one  another,  and  each 
of  them  retains  its  own  characteristics.  The  oxygen  behaves  essen- 
tially as  if  no  nitrogen  were  present,  and  the  nitrogen  as  if  no 
oxygen  were  present.  That  the  several  constituents  of  the  air 
are  merely  mixed,  and  not  chemically  united,  may  be  shown  in 
various  ways.  One  of  them  is  as  follows:  When  air  is  liquefied 
and  allowed  to  stand,  its  constituents  evaporate  independently. 
Nitrogen  and  carbon  dioxide  evaporate  faster  than  oxygen,  so  that 
as  the  liquid  air  stands,  the  proportion  of  oxygen  increases.  Again, 
heat  is  given  off  whenever  a  chemical  compound  is  formed.  When 
nitrogen  and  oxygen  are  mixed,  no  heat  is  developed. 

>      The  Functions  of  the  Atmospheric  Elements 

.  'i 

The  various  constituents  of  the  air  play  various  roles  in  the 
economy  of  the  earth. 

Nitrogen  is  inactive.  Though  it  is  inhaled  with  the  oxygen 
in  breathing,  it  does  not  appear  to  be  of  direct  use  to  animals. 
Both  animals  and  plants  need  nitrogen,  but  they  cannot  use  the 
nitrogen  of  the  air  directly.  Before  they  can  make  use  of  it, 
it  must  be  combined  with  something  else,  making  what  are 
known  as  nitrogenous  compounds.  From  some  of  these  compounds 
animals  and  plants  derive  the  nitrogen  they  need.  Plant  decay 
sets  some  nitrogen  free,  but  the  aggregate  effect  of  plant  life  and 
plant  decay  on  the  amount  of  nitrogen  in  the  air  is  not  known. 


514  PHYSIOGRAPHY 

Since  nitrogen  makes  up  the  larger  part  of  the  atmosphere,  air- 
pressure  and  wind-strength  are  due  chiefly  to  it. 

Oxygen  from  the  air  is  being  consumed  constantly  by  all 
animals.  Air-breathing  animals  take  it  directly  from  the  air,  and 
water-breathing  animals  -take  it  from  the  water  in  which  it  is  dis- 
solved. Oxygen  is  consumed  by  plants  also,  especially  by  green 
plants  in  the  dark.  Oxygen  is  consumed  wherever  combustion 
is  going  on,  for  combustion  is  primarily  the  union  of  oxygen 
with  other  substances,  especially  carbon.  When  the  oxygen 
enters  into  combination,  it  loses  its  distinctive  character- 
istics. Whenever  organic  matter  decays,  oxygen  is  also  con- 
sumed, for  the  decay  of  such  matter  is  but  slow  combustion.  In 
spite  of  the  constant  and  rapid  consumption  of  atmospheric  oxygen 
by  animals  and  in  all  combustion,  its  amount  does  not  appear  to 
decrease.  We  must  therefore  infer  that  oxygen  is  supplied  to 
the  air  about  as  fast  as  it  is  consumed.  The  sources  of  supply 
are  several.  Plants  break  up  the  CO2  into  its  elements,  C  and  O, 
and  set  some  of  the  oxygen  free.  This  is  perhaps  the  greatest 
source  of  supply  of  free  oxygen.  It  is  to  be  noted  that  oxygen 
received  by  the  air  in  this  way  is  not  (or  may  not  be)  new  to  the 
air.  Much  of  it  at  least  is  only  returned  to  the  air,  after  having 
been  temporarily  withdrawn.  Oxygen  also  reaches  the  atmosphere 
from  volcanic  vents,  by  changes  (deoxidation)  which  take  place 
in  certain  kinds  of  rocks,  and  perhaps  from  other  sources. 

The  carbonic-acid  gas  of  the  atmosphere,  though  a  very  minor 
constituent  so  far  as  quantity  is  concerned,  is  most  important. 
We  have  already  seen  that  it  is  being  constantly  produced  by  the 
burning  of  coal,  wood,  peat,  gas,  etc.,  and  by  the  decay  of  all  or- 
ganic matter.  It  is  also  added  to  the  air  by  all  animal  respiration, 
and  it  is  poured  into  the  air  from  volcanic  vents,  often  in  great 
quantity.  It  is  probable  that  "shooting  stars"  sometimes  con- 
tain carbon,  for  the  corresponding  bodies  (meteorites),  which  are 
so  large  that  they  are  not  reduced  to  dust  in  the  atmosphere,  but 
reach  the  earth  as  masses  of  rock  or  metal,  sometimes  contain 
carbon  (sometimes  in  the  form  of  diamonds).  Any  carbon  which 
meteors  contain  must  be  burned  to  CO2  in  the  upper  air.  It  is 
probable  that  there  are  still  other  minor  sources  of  this  gas. 

Carbonic-acid  gas  is  supplied  to  the  atmosphere  very  rapidly 
from  these  various  sources.  For  example,  about  75  per  cent,  of 
common  bituminous  coal  is  carbon.  When  burned,  a  ton  of  such 


CONSTITUTION  OF  THE  ATMOSPHERE  515 

coal  would  make  about  2|  tons  of  carbonic-acid  gas,  all  of  which 
goes  into  the  atmosphere.  A  ton  of  hard  coal,  which  contains  a 
higher  proportion  of  carbon,  would  produce  still  more  carbonic-acid 
gas.  If  we  knew  the  number  of  tons  of  coal  burned  daily,  we  could 
calculate  the  amount  of  C02  poured  into  the  atmosphere  daily 
as  a  result  of  its  combustion.  Nearly  a  billion  l  tons  of  coal  are 
mined  each  year,  and  if  each  ton  of  coal  makes  2|  tons  of  car- 
bonic-acid gas,  it  will  be  seen  that  the  atmosphere  would  be 
supplied  with  C02  at  the  rate  of  more  than  1\  billion  tons  a 
year  from  this  source  alone.  This  figure  takes  no  account 
of  other  fuels,  such  as  wood,  peat,  natural  gas,  oil,  etc.  Neither 
does  it  take  account  of  the  slow  combustion  (decay)  of  vegetable 
matter,  nor  of  the  CO2  produced  by  respiration.  When  these  and 
all  other  sources  of  carbonic-acid  gas  are  considered,  it  seems  safe 
to  say  that  carbonic-acid  gas  is  being  supplied  to  the  atmosphere 
at  the  rate  of  several  billions  of  tons  per  year;  yet  the  amount  of 
CO-2  in  the  air  does  not  increase  enough  to  be  noted  from  year 
to  year,  or  even  from  generation  to  generation.  It  must  there- 
fore be  taken  out  of  the  atmosphere  about  as  rapidly  as  it 
enters. 

The  losses  of  carbonic-acid  gas  from  the  air  come  chiefly  (1) 
through  green  plants,  of  which  it  is  the  chief  food,  and  (2)  through 
combination  with  mineral  matter;  for  the  CO2  of  the  air  is  con- 
stantly uniting  with  mineral  matter  in  the  solid  part  of  the  earth. 
It  will  be  seen  therefore  that  some  of  the  C02  is  making  a  con- 
tinuous round  of  change.  It  is  taken  out  of  the  air  by  plants,  and 
its  constituents,  or  some  of  them,  become  a  part  of  the  woody 
tissue  of  the  plant.  In  this  process  of  transformation  some  of  the 
oxygen  is  set  free  in  the  air.  The  carbon  of  the  plant  is  then  burned, 
either  in  a  fire  or  by  decay,  and  the  carbonic-acid  gas  produced 
passes  back  into  the  air  to  be  used  by  plants  again.  Much  car- 
bonic-acid gas  goes  through  this  round  each  year,  for  much  vege- 
tation grown  during  one  warm  season  is  burned  or  partially  de- 
cayed before  the  next.  It  will  be  readily  seen,  too,  that  some  of 
this  gas  might  go  through  a  cycle  of  change  involving  its  return 
to  the  atmosphere  more  than  once  in  a  season. 

The  various  sources  of  supply  of  CO.,  are  not  always  equal  at 
the  same  place,  and  are  not  equal  at  different  places.  Thus,  the 

.    'See  Mineral   Resources  of   the    Drilled   States,   an  annual   publication 
ot  the  U.  S.  Geol.  Survey. 


516  PHYSIOGRAPHY 

amount  produced  by  combustion  is  much  greater  in  winter  than 
in  summer,  while  the  amount  produced  by  the  decay  of  plant  and 
animal  matter  is  much  greater  in  summer  than  in  winter.  It  is 
to  be  remembered,  however,  that  the  warm  season  in  one  hemi- 
sphere corresponds  to  the  cold  season  in  the  other;  but  since  there 
are  fewer  people  in  the  southern  hemisphere  than  in  the  northern, 
there  is  less  burning  of  coal  there,  and  since  there  is  much  less 
land  in  the  southern  hemisphere,  there  is  less  decay  of  land  vege- 
tation. Volcanoes  are  more  active  at  some  times  than  others,  and 
probably  give  forth  most  CO2  when  most  active.  The  amount 
produced  by  animal  respiration  is  probably  nearly  the  same 
throughout  the  year. 

The  rate  at  which  CO2  is  taken  from  the  air  also  varies.  Since 
plants  use  it  during  the  growing  season  only,  the  plants  of  middle 
and  high  latitudes  draw  on  the  supply  in  the  atmosphere  chiefly 
during  the  summer.  Though  summer  alternates  in  the  hemi- 
spheres, there  is  much  more  plant  life  on  land  in  the  northern  hemi- 
sphere than  in  the  southern,  and,  so  far  as  land  plants  are  concerned, 
CO2  must  be  consumed  more  rapidly  in  the  northern  summer  than 
in  the  northern  winter.  Carbonic-acid  gas  also  enters  into  com- 
bination with  mineral  matter  more  readily  when  it  is  warm  than 
when  it  is  cold,  so  that  there  must  be  some  variation  from  season 
to  season  in  the  amount  taken  out  in  this  way. 

At  first  thought  it  would  seem  that  carbonic-acid  gas  should 
greatly  increase  in  one  hemisphere  during  the  winter  season,  and 
diminish  in  the  same  hemisphere  during  the  summer;  but  this  is 
not  the  case.  The  reason  is  twofold.  In  the  first  place,  the 
winds  distribute  the  carbonic-acid  gas.  In  the  second  place,  even 
without  winds,  the  carbonic-acid  gas  tends  to  diffuse  equally 
through  the  atmosphere.  It  is,  for  example,  produced  in  great 
quantities  in  a  large  city  in  winter,  for  the  thousands  of  tons  of 
coal  consumed  daily  in  such  a  city  produce  enormous  quantities 
of  carbon  dioxide.  But  instead  of  accumulating  in  great  quanti- 
ties over  the  city,  it  diffuses  through  the  atmosphere,  so  that, 
even  without  winds,  there  would  be  no  great  excess  over  the  region 
where  it  is  produced.  A  slight  excess  in  such  situations  is  often 
noticed,  for  diffusion  does  not  bring  about  equality  of  distribu- 
tion instantaneously. 

At  present  the  supply  and  loss  of  carbon  dioxide  so  nearly 
balance  that  no  change  in  the  amount  of  C02  in  the  air  is  noted; 


CONSTITUTION  OF  THE  ATMOSPHERE  517 

but  it  seems  quite  possible  that  in  the  course  of  long  periods  of 
time  the  supply  may  have  exceeded  the  loss,  or  vice  versa.  While 
therefore  the  amount  of  C02  remains  nearly  constant  from  year 
to  year,  there  is  no  warrant  for  the  inference  that  it  has  remained 
so  from  age  to  age. 

Though  a  very  minor  constituent  of  the  atmosphere,  carbonic- 
acid  gas  has  an  important  function  other  than  in  supplying  food 
to  plants.  It  has  the  power  of  retaining  some  of  the  heat  radiated 
from  the  solid  part  of  the  earth  into  space.  It  therefore  serves  as 
a  blanket  to  hold  in  the  heat  of  the  earth,  and  thin  (tenuous)  as 
the  blanket  now  is,  it  is  more  effective,  in  this  respect,  than  the 
denser  blanket  of  oxygen  and  nitrogen.  If  it  were  thicker,  it  would 
be  still  more  effective,  making  the  earth  warmer.  So  important 
is  its  function  in  this  respect,  that,  if  the  amount  of  this  gas  were 
doubled,  the  temperature  of  the  earth,  and  especially  of  high  lati- 
tudes, would  be  notably  increased.  It  has  been  estimated  that 
if  its  amount  were  doubled  or  trebled,  magnolias  might  grow 
again  in  Greenland,  as  they  once  did.  On  the  other  hand,  it  has 
been  estimated  that  if  the  amount  of  carbonic-acid  gas  in  the 
atmosphere  were  decreased  one-half,  the  climate  would  be  so  much 
colder  than  now,  that  much  of  the  land  in  the  northern  part  of  our 
continent  would  be  covered  by  a  sheet  of  snow  and  ice,  some- 
what as  it  was  in  the  glacial  period  (p.  271).  While  these  con- 
clusions have  been  called  into  question,  so  far  as  the  amount  of 
change  of  temperature  for  a  given  increase  or  decrease  of  carbonic- 
acid  gas  is  concerned,  there  seems  to  be  no  doubt  that  an  increase 
of  carbonic-acid  gas  in  the  atmosphere  would  ameliorate  climate, 
while  a  decrease  would  make  it  more  rigorous. 

It  has  been  noted  that  the  water  vapor  in  the  atmosphere  is  a 
variable  quantity.  It  is  constantly  entering  the  atmosphere,  and  it 
is  constantly  being  condensed  and  precipitated  in  the  form  of  rain, 
snow,  dew,  frost,  etc.,  to  be  again  evaporated,  condensed,  and  pre- 
cipitated. Like  much  of  the  C02,  it  is  making  continuous  rounds. 
The  amount  which  the  atmosphere  may  contain  at  any  time  is  de- 
pendent on  temperature;  but  various  other  factors,  especially  the 
available  local  supply,  help  to  determine  the  amount  which  is 
actually  held.  The  importance  of  water  vapor  in  the  general  econ- 
omy of  the  earth  will  be  referred  to  in  later  chapters,  but  it  may  be 
stated  here  that,  like  the  carbonic-acid  gas,  it  serves  as  a  blanket 
to  keep  the  earth  warm.  Furthermore,  it  is  to  be  remembered 


518  PHYSIOGRAPHY 

that  the  water  vapor  of  the  air,  constantly  renewed,  is  the  source 
of  all  the  rain  and  snow,  the  work  of  which  has  been  described 
on  preceding  pages. 

The  dust  in  the  atmosphere  includes  all  solid  particles  held  in 
it.  We  do  not  ordinarily  see  them,  though  clouds  of  dust  are  some- 
times visible  on  windy  days.  The  settling  of  dust  from  the  air 
on  all  objects  in  doors  or  out  is  sufficient  evidence  of  its  universal 
presence  (p.  55).  It  may  be  readily  seen  in  indoor  air  if  the 
room  be  darkened  and  the  light  allowed  to  enter  only  through  a 
narrow  crack  or  small  hole.  Even  air  which  appears  clear  may 
in  this  way  be  seen  to  contain  innumerable  particles  of  solid  matter. 
The  amount  of  dust  is  sometimes  very  great,  as  over  cities  and  in 
dry  and  windy  regions.  During  the  fogs  of  February,  1891,  it  was 
estimated  'that  the  amount  of  dust  deposited  on  glass  roofs  in  and 
near  London  was  six  tons  per  square  mile.  The  variety  of  matter 
in  the  dust  was  also  great,  carbon  (soot)  being  most  abundant. 

Some  years  since  a  method  was  devised  for  counting  the  dust 
particles  in  a  given  volume  of  air.  The  result  showed  that  in  the 
air  of  great  cities  there  are  hundreds  of  thousands  of  dust  particles 
in  each  cubic  centimetre  (a  centimetre  is  less  than  fo  of  an  inch) 
of  air;  and  that  even  in  the  pure  air  of  the  country,  far  from  towns 
and  factories,  there  are  hundreds  of  motes  per  cubic  centimetre. 
It  has  been  estimated  that  "every  puff  of  smoke  from  a  cigarette 
contains  about  4000  million  separate  granules  of  dust." 

The  amount  of  dust  in  the  air  is  greater  over  the  land  than 
over  the  sea,  and  in  the  lower  atmosphere  than  in  the  upper. 

The  dust  particles  consist  of  inorganic  materials,  such  as  (1)  tiny 
particles  of  mineral  matter  blown  up  from  dry  roads  and  fields,  (2) 
particles  of  smoke  from  chimneys,  (3)  frequently  of  tiny  bits  of  rock 
matter  blown  out  of  volcanoes,  and  (4)  meteoric  dust,  or  the  dust 
which  comes  to  the  .earth  from  outside  space — such  as  the  dust  to 
which  "shooting  stars  "  are  reduced  in  the  atmosphere;  and  organic 
particles.  Among  the  organic  dust  particles  are  bacteria  of  various 
sorts,  and  the  spores  of  many  plants.  The  dust  that  is  thrown 
into  the  air  when  a  dry  puffball  is  broken  may  serve  as  an  illus- 
tration of  the  spores  of  plants  which  are  often  abundant  in  the  air. 
The  fact  that  plant  spores  are  nearly  universal  in  the  air  is  shown 
by  the  promptness  with  which  a  moist  piece  of  bread  or  cake,  or  a 
moist  piece  of  leather,  gets  mouldy,  especially  in  a  dark,  warm  place. 
The  moulds  are  plants,  and  the  spores  from  which  they  grow  were 


CONSTITUTION  OF  THE  ATMOSPHERE  519 

floating  in  the  air,  until  they  found  a  lodging-place  suitable  for  their 
growth.  In  the  blossoming  season  also,  the  winds  get  much  pollen 
dust  from  flowers.  The  scattering  of  pollen  by  the  wind  serves 
an  important  purpose  in  the  plant  world. 

Some  diseases  are  spread  by  means  of  germs  in  the  air,  though 
fortunately  most  of  the  germs  in  the  air  are  not  injurious. 

The  number  of  bacteria  found  in  a  cubic  metre  of  air  at  Mont- 
souris  (France)  Observatory  was  345,  while  in  the  same  amount 
of  air  in  the  heart  of  Paris  the  number  was  4790.  These  figures 
give  some  idea  of  the  relative  purity  of  country  and  city  air. 

The  dust  particles  in  the  atmosphere  play  an  important  role 
in  various  other  ways.  They  "scatter"  the  light  of  the  sun,  so 
as  to  illuminate  the  whole  atmosphere.  Without  the  dust  in  the 
air,  all  shady  places  would  be  in  darkness.  The  sun  would  prob- 
ably appear  in  dazzling  brilliance,  shining  from  a  black  sky  in 
which  the  stars  would  be  visible  even  in  the  daytime.  The  blue 
color  of  the  sky,  and  the  sunset  and  sunrise  tints,  are  determined 
or  affected  by  the  dust  in  the  atmosphere.  Dust  particles  also  serve 
as  nuclei  about  which  water  vapor  condenses.  It  was  formerly 
held  that  they  were  necessary  for  the  condensation  of  water  vapor 
in  the  atmosphere,  but  this  appears  not  to  be  the  case. 


CHAPTER  XIV 
TEMPERATURE   OF  THE   AIR 

THE  temperature  of  the  air  varies  from  season  to  season,  from 
day  to  day,  and  even  from  one  part  of  a  day  to  another.  Because 
of  the  importance  of  temperature  in  all  human  affairs,  it  is  con- 
venient to  have  some  method  of  measuring  and  recording  it. 

The  thermometer.  The  temperature  is  measured  by  means 
of  the  thermometer.  The  principle  of  the  thermometer  is  readily 
understood.  It  consists  of  a  glass  tube  of  uniform  diameter,  except 
for  a  bulb  at  one  end.  The  bulb  and  the  lower  part  of  the  tube 
are  filled  with  some  liquid,  generally  mercury.  The  mercury  is 
then  heated  to  its  boiling  temperature,  so  as  to  expel  all  air. 
When  the  tube  is  full  of  boiling  mercury  from  which  all  air  has 
been  driven  by  the  heat,  it  is  sealed. 

The  mercury  contracts  on  cooling,  so  that  it  but  partly  fills  the 
tube.  Above  it  is  a  vacuum.  When  the  temperature  rises,  the 
mercury  in  the  tube  expands  and  rises.  When  the  temperature 
falls,  the  mercury  contracts  and  sinks.  The  amount  of  rise  or 
fall  of  the  mercury  in  the  tube  indicates  the  amount  of  the  change 
of  temperature. 

That  the  temperature  may  be  read  directly  from  the  ther- 
mometer, it  is  necessary  to  have  a  scale  marked  on  the  tube.  Two 
scales  are  in  common  use — the  Fahrenheit  and  the  Centigrade. 
The  scales  are  made  as  follows:  The  thermometer  tube  is  placed 
in  boiling  water,  or  in  steam  just  over  boiling  water,  at  sea-level 
(760  mm.  pressure),  and  allowed  to  remain  there  until  the  tube  and 
its  contents  acquire  the  temperature  of  the  water.  The  point  to 
which  the  mercury  rises  in  the  tube  under  these  circumstances 
is  then  marked  212°,  if  the  Fahrenheit  scale  is  to  be  used.  The  tube 
of  mercury  is  then  put  into  moist  pounded  ice  or  snow,  where  it 

520 


TEMPERATURE  OF  THE  AIR  521 

remains  until  the  level  of  the  mercury  in  the  tube  becomes  stationary, 
and  the  level  at  which  the  mercury  then  stands  is  marked  32°. 
The  space  between  the  212°  mark  and  the  32°  mark  is  then  divided 
into  180  equal  parts,  each  being  called  a  degree  (1°  Fahr.)  The 
marks  on  the  tube  may  be  made  for  each  degree,  for  every  two 
degrees,  or  for  every  five  degrees,  according  to  the  delicacy  which 
is  required  of  the  thermometer. 

The  space  below  the  freezing  temperature  is  divided  similarly 
into  degrees,  each  degree  below  32°  having  the  same  length  on  the 
tube  as  each  degree  above.  The  0°  of  this  scale  is  32°  below  the 
freezing-point.  The  scale  is  carried  still  lower  on  the  tube,  and 
the  temperature  below  0°  is  called  "below  zero."  Thus  20°  be- 
low zero  means  52°  below  the  freezing-point,  and  is  written  —20° 
Fahr.  or  -20°  F. 

The  Centigrade  scale  is  much  simpler  and  better,  though  un- 
fortunately not  in  common  use  in  English-speaking  countries.  The 
height  of  the  mercury  at  the  freezing  temperature  under  normal 
atmospheric  pressure  is  marked  0°,  and  the  boiling  temperature 
100°.  The  space  between  is  divided  into  100  parts,  each  of  which 
is  a  degree  (1°  €.)„  The  degrees  below  zero  have  the  same  length 
on  the  scale  as  the  degrees  above.  It  will  be  seen  that  1°  C.  is 
equal  to  lf°  Fahr.  If  this  relation  of  degrees  is  remembered,  de- 
grees Fahrenheit  may  be  readily  changed  to  degrees  Centigrade, 
or  vice  versa. 

The  Heating  of  the  Atmosphere 

Sources  of  heat.  The  atmosphere  receives  heat  from  several 
sources,  but  that  received  from  the  sun  so  far  exceeds  that  from 
all  other  sources  that  the  others  may  almost  be  neglected. 

That  much  heat  is  received  from  the  sun  is  shown  by  the  fact 
that  the  temperature  generally  rises  when  the  sun  rises  and  sinks 
when  the  sun  goes  down,  and  by  the  further  fact  that  the  tempera- 
ture is  generally  warmer  on  a  sunny  day  than  on  a  cloudy  one. 
It  is  true  there  are  occasional  exceptions  to  these  general  rules, 
for  now  and  then  a  night  is  warmer  than  a  day,  and  a  cloudy  day 
is  sometimes  warmer  than  a  sunny  one  of  the  same  season.  But 
these  exceptions  do  not  interfere  with  the  truth  of  the  general 
statement. 

The  source  of  atmospheric  heat  which  is  second  in  importance 
is  the  interior  of  the  earth;  but  the  heat  from  this  source  is  not 


522  PHYSIOGRAPHY 

enough  to  affect  the  temperature  of  the  atmosphere  sensibly.  This 
is  indicated,  in  a  general  way,  by  the  fact  that  in  polar  regions, 
during  the  long  night,  the  temperature  is  very  low,  and  all  the  heat 
received  from  the  interior  of  the  earth  has  no  apparent  effect  on 
the  snow  and  ice. 

Sun  heating :  insolation.  The  temperature  of  space  is  supposed 
to  be  about  —273°  C.  (  —  459°  F.).  The  more  genial  temperature 
which  we  enjoy  results  chiefly  from  the  heat  received  from  the  sun; 
yet  the  earth  receives  less  than  5-<rffWo~o<jotf  °f  ^ne  nea^  given  off  by 
that  luminary.  The  amount  received  each  year,  if  equally  dis- 
tributed, is  enough  to  melt  a  layer  of  ice  about  141  feet  thick  over 
the  entire  earth,  or  to  evaporate  a  layer  of  water  18  feet  deep. 

Each  hemisphere  receives  the  same  amount  of  heat  from  the 
sun  each  year  (Fig.  518),  but.  because  of  the  inclination  of  the 
earth's  axis,  the  heat  is  very  differently  distributed  in  different 
latitudes.  Other  things  being  equal,  the  earth  gets  most  heat 
per  unit  area  where  the  sun  shines  the  greatest  number  of  hours  per 
day.  In  summer,  the  days  are  longest  in  the  highest  latitudes. 
So  far  as  length  of  day  is  concerned,  therefore,  the  highest  lati- 
tudes, namely  the  poles,  should  get  more  heat  than  any  other  part 
of  the  earth  in  summer. 

Again,  other  things  being  equal,  the  earth  (land  or  water  sur- 
face) gets  most  heat  per  unit  area  where  the  sun's  rays  fall  most 


FIG.  530. — Diagram  to  illustrate  the  unequal  heating  power  of  the  sun  at 
different  altitudes.  When  its  rays  are  vertical  they  are  concentrated  on 
less  space  on  the  surface  of  the  earth,  and  at  the  same  time  pass  through 
less  atmosphere,  than  when  they  strike  the  surface  of  the  earth  obliquely. 

nearly  vertically,  both  because  they  are  there  most  concen- 
trated, and  because  they  there  pass  through  a  lesser  thickness 
of  the  air,  which  absorbs  some  of  their  heat.  This  is  shown  by 
Fig.  530.  A  given  bundle  of  rays,  1,  falling  vertically  on  the  sur- 
face, is  distributed  over  a  given  space,  while  an  equal  bundle  of 
rays,  2  or  3,  falling  obliquely  on  the  surface,  is  distributed  over  a 
much  greater  area,  and  therefore  heats  each  part  less.  Again, 


TEMPERATURE  OF  THE  AIR  523 

the  oblique  rays,  2  and  3,  pass  through  a  greater  thickness  of  at- 
mosphere, and  more  of  their  heat  has  been  absorbed  before  they 
reach  the  surface  of  the  solid  part  of  the  earth.  The  angle  at  which 
the  sun's  rays  reach  the  earth  varies  from  place  to  place.  It  also 
varies  at  the  same  place  from  time  to  time,  because  the  earth's 
axis  of  rotation  is  inclined  to  the  plane  of  its  orbit  as  the  earth 
revolves  about  the  sun.  This  is  illustrated  by  Fig.  529,  which  has 
already  been  explained. 

Primary  distribution  of  heat.  Remembering  that  it  is  the 
rotation  of  the  earth  on  an  inclined  axis  while  it  revolves  about  the 
sun  (Fig.  526)  which  makes  the  sun  appear  to  move  north  and 
south  during  the  year  (Fig.  529),  we  may  study  the  effects  of  this 
apparent  motion  of  the  sun  on  the  distribution  of  heat  received  by 
insolation.  From  Fig.  528  we  see  that  when  the  sun's  rays  are 
perpendicular  to  the  surface  of  the  earth  23i°  south  of  the  equator, 
they  are  most  oblique  at  all  points  in  the  northern  hemisphere, 
and  least  oblique  at  all  points  in  the  southern  hemisphere.  At  this 
time,  therefore,  the  southern  hemisphere  is  receiving  more  heat 
than  the  northern,  because  of  the  direction  of  the  sun's  rays.  At 
the  same  time,  the  days  are  longer  in  the  southern  hemisphere 
than  in  the  northern,  and  this  is  a  second  reason  why  the  southern 
hemisphere  is  receiving  more  heat  than  the  northern  at  this  time. 
After  the  time  (winter  solstice,  December  22)  when  the  sun's 
rays  are  vertical  at  23^°  S.,  they  become  perpendicular  to  the  sur- 
face at  points  farther  and  farther  north,  and  on  March  21  they 
are  vertical  at  the  equator  (Fig.  529).  Days  and  nights  are  then 
equal  everywhere,  and  the  sun's  rays  are  equally  oblique  in  corre- 
sponding latitudes  north  or  south  of  the  equator.  Any  latitude 
in  one  hemisphere  is  then  receiving  the  same  amount  of  heat  as 
the  corresponding  latitude  in  the  other  hemisphere. 

After  March  21,  the  sun  appears  to  continue  its  journey 
northward  until,  on  June  21,  its  rays  are  vertical  at  the  tropic 
of  Cancer,  23^°  N.  (Fig.  529),  when  the  days  of  the  northern  hemi- 
sphere attain  their  greatest  length,  and  the  nights  of  the  same 
hemisphere  become  shortest  in  all  latitudes  where  there  is  alterna- 
tion of  day  and  night  (Fig.  527).  At  the  same  time,  the  rays  of  the 
sun  are  less  oblique  in  the  northern  hemisphere,  as  a  whole,  than 
at  any  other  time.  In  the  southern  hemisphere  the  conditions  are 
reversed.  At  this  time,  therefore,  the  northern  hemisphere  is  being 
heated  by  the  sun  faster  than  at  any  other  time  of  the  year,  while  the 


524 


PHYSIOGRAPHY 


southern  hemisphere  is  receiving  less  heat  than  at  any  other 
time. 

From  June  21  to  December  22  the  sun  appears  to  move 
so  that  its  rays  become  vertical  farther  and  farther  south,  and  the 
preceding  sequence  of  events  is  reversed. 

The  latitudes  where  the  sun's  rays  fall  vertically  range  from 
the  tropic  of  Cancer  to  the  tropic  of  Capricorn.  For  the  whole 
year,  however,  the  sun's  rays  are,  on  the  average,  least  oblique  in 
the  lowest  latitudes.  This  is  why  the  low  latitudes  are,  on  the 
whole,  warmer  than  the  high  latitudes. 

The  actual  amount  of  sun  heat  received  in  different  latitudes 
is  determined  by  the  length  of  day  (hours  of  sunshine)  and  the 
direction  of  the  sun's  rays ;  but  it  is  to  be  noted  that  the  latitudes 
which  have  the  longest  days  never  have  the  vertical  rays  of  the  sun. 
Calculations  based  on  these  two  factors  have  been  made,  showing 
the  proportion  of  heat  received  in  different  latitudes  during  the 
whole  year  and  during  different  seasons.  For  the  year,  the 
equator  receives  more  heat  than  any  other  part  of  the  earth.  If 
the  average  amount  of  heat  received  there  each  day  be  taken  as 
1,  the  amount  of  heat  received  in  a  year  is  365.2.  The  proportionate 
amount  received  in  various  other  latitudes  is  shown  in  tne  following 
table: 


Latitude 

0° 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

90° 

Thermal  days, 

or     relative 

amount      of 

heat  yearly 

365.2 

360.2 

345.2 

321.0 

288.5 

249.7 

207.8 

173.0 

156.6 

151.6 

From  this  table  it  is  seen  that  latitude  40°  receives  about  three- 
fourths  as  much  heat  as  the  equator,  and  latitude  70°  a  little  less 
than  one-half  as  much. 

During  the  half  of  the  year  when  the  sun's  rays  are  vertical  north 
of  the  equator,  most  heat  is  received  in  latitude  25°  N.  During  this 
half  of  the  year  the  sun's  rays  are  most  nearly  vertical,  on  the 
average,  in  latitude  11|°  (half-way  between  the  equator  and  lati- 
tude 23£°);  but  the  days  are  longer  farther  north.  During  the 
three  months  centering  about  June  21,  the  zone  of  greatest  heat 
is  iri  latitude  41°  N.  The  sun's  rays  are  here  less  nearly  vertical 
than  in  latitudes  about  the  tropic  of  Cancer,  but  the  days  are  much 


TEMPERATURE  OF  THE  AIR 


525 


longer.  Between  May  31  and  July  16  l  the  north  pole  receives 
more  heat  than  any  other  part  of  the  earth,  the  continuous  day 
offsetting  the  great  obliquity  of  the  sun's  rays  at  this  time.  At 
the  time  of  the  summer  solstice,  the  area  immediately  about  the 
north  pole  receives  20%  more  heat  than  an  equal  area  at  the 


FIG.  531. — Diagram  showing  receipt  of  heat  in  different  latitudes  of  the 
northern  hemisphere  for  four  dates  between  the  vernal  equinox  and  the 
summer  solstice.  The  latitudes  are  indicated  at  the  top  of  the  figure, 
and  the  relative  amounts  of  heat  at  the  right.  (After  Wiener.) 

equator  ever  receives,  and  36%  more  than  the  equatorial  region 
receives  at  that  time.  Fig.  531  shows  the  amount  of  heat  re- 
ceived from  the  sun  in  various  latitudes  of  the  northern  hemi- 
sphere from  the  time  of  the  vernal  equinox  to  the  time  of  the  sum- 
mer solstice. 

The  temperature  of  one  place  is  not  necessarily  higher  than 
that  of  another,  because  it  receives  more  heat.  No  amount  of  heat, 
for  example,  would  make  the  temperature  of  Greenland  warm 
until  after  the  snow  and  ice  was  melted.  All  the  heat  received  tend- 
ing to  raise  the  temperature  above  32°  F.  would  be  expended  in 
melting  and  evaporating  the  snow,  without  raising  its  tempera- 
ture above  32°  F.  (0°  C.).  The  region  about  the  north  pole  does 
not  get  very  warm,  even  when  it  receives  more  heat  than  the 
equator,  because  much  of  the  heat  is  expended  in  melting  ice  and 
in  warming  up  ice-cold  water,  which  heats  very  slowly  and  runs 
away  as  soon  as  the  heating  is  well  begun. 

What  the  sun  does  for  the  earth  in  the  matter  of  heat  is  shown 

1  Hann  gives  these  dates  May  10  to  August  3,  a  period  of  56  days. 


526 


PHYSIOGRAPHY 


by  the  following  table,  which  gives  the  estimated  temperatures 
(Centigrade)  which  would  exist  on  the  earth  in  different  latitudes 
if  there  were  no  atmosphere.  The  figures  in  the  upper  part  of  the 
table  are  for  the  warmest  and  coldest  months. 


Equator 

10° 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

Pole. 

67 

67 

70 

74 

75 

75 

73 

76 

80 

82 

56 

50 

36 

16 

-10 

-45 

-103 

-273 

-273 

-273 

ANNUAL    MEANS 


62 

61 

57 

50 

39 

24 

1 

-43 

-81 

-105 

HEATING  AND  COOLING 

There  are  three  ways  in  which  the  air  receives,  loses,  and  trans- 
fers heat.     These  are  radiation,  conduction,  and  convection. 

1.  Radiation.     When  the  sun  shines,  the  surface  which  its  rays 
strike  is  warmed  by  the  absorption  of  heat  radiated  from  the  sun. 
An  object  in  front  of  a  fire  is  warmed  by  heat  radiated  from  it. 
A  body  need  not  be  glowing  hot,  like  the  sun,  to  radiate  heat.     A 
hot  stove  would  continue  to  radiate  heat  if  all  the  fire  were  taken 
out.     The  body  which  radiates  heat  is  itself   cooled.     The  hot 
stove  from  which  the  fire  has  been  taken  presently  ceases  to  radiate 
heat.     The  land  warmed  by  radiation  from  the  sun  during  the 
day  is  cooled  by  the  radiation  of  its  heat  during  the  night.     The 
rate  at  which  a  given  body  loses  heat  by  radiation  depends  upon 
the  difference  of  temperature  between  it  and  its  surroundings. 
Thus  a  hot  stove  will  cool  much  more  quickly  in  a  cold  room  than 
in  a  warm  one. 

2.  Conduction.    If  one  end  of  a  bar  of  iron,  such  as  an  iron 
poker,  be  put  in  the  fire,  the  other  end  soon  becomes  hot.     The 
heat  passes    along    the   iron  rod   from  one   end    to   the  other. 
This  means  that  the  molecular  motion,  known  as  heat  or  heat- 
energy,  set  up  in  one  end  of  the  rod  by  the  fire,  is  passed  along  from 
particle  to  particle  to  the  other  end.     This  method  of  transmitting 
heat  is  conduction.     Any  cold  body  in  contact  with  a  hot  body  is 
warmed  by  conduction.     Thus,  the  bottom  of  the  air  is  warmed 


TEMPERATURE  OF  THE  AIR  527 

by  contact  with  the  land,  that  is,  by  conduction,  wherever  the 
temperature  of  the  land  is  higher  than  that  of  the  air. 

3.  Convection.  When  a  kettle  of  water  is  placed  on  a  hot  stove, 
the  water  in  the  bottom  is  heated  by  conduction,  that  is,  by  con- 
tact with  the  hot  kettle.  The  heating  of  water  causes  it  to  expand, 
and  when  the  water  in  the  bottom  of  the  kettle  expands,  it  becomes 
lighter  than  the  water  above  it.  The  heavier  water  above  then 
sinks  and  pushes  the  lighter  water  below  up  to  the  top.  This  sort 
of  movement  is  convection.  Other  illustrations  of  convection  are 
afforded  by  stoves,  fireplaces,  etc.  A  thin  sheet  of  light  paper 
may  be  momentarily  sustained  in  the  air  over  a  hot  stove,  or  even 
carried  up  by  the  rising  air  of  the  convection  current.  Again, 
as  the  air  in  a  chimney  is  heated,  it  expands  and  becomes  less 
dense  than  the  air  about  it.  The  cooler,  denser  air  about  the  base 


\ 


It 


FIG.  532. — Diagram  to  illustrate  convection  in  a  vessel  of  water  heated  at 
one  point  at  the  bottom. 

of  the  chimney  or  stove  crowds  in  below  the  expanded  air  in  the 
chimney,  and  crowds  it  up  out  of  the  chimney.  Since  the  air  com- 
ing into  the  chimney  is  continually  being  expanded,  the  up-draught 
continues  as  long  as  there  is  fire.  Every  draught  from  a  chimney 
is  therefore  an  example  of  convection. 

It  will  be  seen  that  in  convection  the  molecules  of  the  gas  or 
liquid  change  their  position  relative  to  one  another,  while  in  con- 
duction in  a  solid,  they  do  not. 

Convection  is  of  so  much  importance  in  connection  with  the 
temperature  of  the  air  and  of  water  that  the  process  may  be 
analyzed  a  little  more  fully.  Suppose  a  vessel  of  water  (Fig.  532) 
heated  at  the  central  point  of  its  bottom.  (1)  The  water  heated  at 
a  expands,  lifting  the  overlying  column  of  water,  producing  a  very 
low  dome  on  the  surface  at  1.  (2)  Under  the  influence  of  gravity, 


528 


PHYSIOGRAPHY 


water  flows  off  the  dome.  There  are  now  unequal  pressures  at  the 
bottom  of  the  dish.  It  is  greater  at  c  than  at  a,  because  there  are 
more  molecules  above  c  than  above  a.  (3)  Because  of  the  excess 
of  pressure  at  c,  water  moves  from  c  to  a,  displacing  (lifting)  the 
warmer  water  at  that  point,  and  producing  the  upward  move- 
ment indicated  in  the  center  of  the  figure.  (4)  The  centerward 
movement  from  c  causes  the  water  above  c  to  sink  and  to  occupy 
the  abandoned  space,  while  the  lifting  of  the  water  over  a,  by  the 
inflow  from  c,  renews  the  dome,  and  the  lateral  motion  from 
center  to  side  at  the  surface. 

When  the  surface  of  the  land  is  warmed  by  the  heat  radiated 
from  the  sun,  it  heats  the  air  above,  partly  by  conduction,  but 
chiefly  by  radiation.  Some  parts  of  the  surface  are  heated  more 
than  others.  The  heated  air  expands  and  rises.  The  beginning 
of  the  rise  is  due  to  expansion  (Fig.  533).  If  the  air  in  a  given 
region  were  expanded  as  shown  in  Fig.  533,  the  air  at  the  top  of 
the  expanded  column  would  run  over  (spread),  much  as  water 
would  under  similar  conditions.  After  this  has  taken  place,  the 


FIG.  533. — The  initial  rise  of  air,  as  a  result'  of  heating,  is  due  to  the 
expansion  of  the  part  heated. 

amount  of  air  at  the  base  of  the  column  h  will  be  less  than  the 
amount  at  the  same  level  outside  the  heated  are"a,  and  air  from 
outside  the  heated  column  will  flow  in  to  balance  the  deficiency. 
This  inflow  will  push  up  the  column  of  warmed  and  expanded  air, 
and  further  overflow  above  will  cause  more  inflow  at  the  bottom. 
If  the  heated  area  continues  to  be  heated,  a  permanent  convec- 
tion current  will  be  established  in  the  heated  area  (Fig.  534). 

It  is  not  necessary  that  the  expanding  air  actually  raise  the 
upper  surface  of  the  air  sensibly,  as  shown  in  Fig.  534,  to  establish 
a  convection  current.  As  it  expands  upward  it  compresses  the 
air  above  the  lower  heated  part  (Fig.  535).  Where  this  compres- 
sion takes  place,  the  air  is  denser  than  that  at  the  same  level  about 
it,  and  flows  sideways  to  balance  the  discrepancy.  This  is  what 


TEMPERATURE  OF  THE  AIR 


529 


actually  takes  place  in  the  air.  It  will  be  seen  that  convection 
gives  rise  to  horizontal,  as  well  as  to  vertical,  air  movements, 
and  that  the  horizontal  movements  take  place  at  various  levels. 


^•••^, 

FIG.  534. — The  permanent  heating  of  the  air  over  a  given  region  gives  rise  to 
permanent  convection  currents. 

How  the  sun  heats  the  atmosphere.  The  atmosphere  is 
heated  by  the  sun  in  two  principal  ways:  (1)  It  is  warmed  by 
heat  radiated  from  the  sun  as  the  rays  of  the  sun  come  through  it 
and  (2)  the  land  and  water  below  the  atmosphere  are  warmed  by 
absorbing  the  heat  radiated  from  the  sun,  and  then  radiate  much 


Condensed  Air 
• f f 


FIG.  535. — Flow  of  air  from  above  a  heated  area  would  take  place  even  if 
the  surface  of  the  air  were  not  raised. 

of  the  heat  they  have  received.     Much  of  the  heat  thus  radiated 
is  absorbed  by  the  air,  which  is  thus  warmed. 

The  amount  of  heat  absorbed  by  the  air  from  the  direct  rays 
of  the  sun  is  different  in  different  latitudes,  and  depends  chiefly 
on  the  distance  the  rays  travel  in  the  atmosphere;  that  is,  on  the 
verticality  of  the  sun's  rays  (Fig.  530).  The  amount  for  different 
altitudes  of  the  sun  is  shown  in  the  following  table: l 


Altitude  of  the  sun 

0° 

5° 

10° 

20° 

30° 

50° 

70° 

90° 

Thickness  of  the  atmos- 
phere in  units  

35  5 

10  2 

5  56 

2  90 

1  99 

1  31 

1  06 

1  00 

Proportion  of  solar  radia- 
tion reaching  the  bottom 
of  the  atmosphere  

0  00 

0  05 

0  20 

0  43 

0  56 

0  69 

0  74 

0  75 

1  Copied  from  Waldo's  Elementary  Meteorology,  p.  28. 


530  PHYSIOGRAPHY 

The  heat  radiated  into  the  air  from  land  and  water  is  more 
readily  absorbed  by  the  air  than  the  luminous  heat  radiated  by  the 
sun,  so  that  the  atmosphere  is  heated  more  by  radiation  from 
below  than  by  direct  insolation.  The  lowest  air  is  heated  most 
by  both  earth  radiation  and  insolation,  because  it  is  densest,  and, 
being  warmed,  it  gives  rise  to  convection  currents  which  warm 
the  air  above.  Since  convection  currents  involve  horizontal  as 
well  as  vertical  movements,  regions  which  are  heated  much  give 
of  their  heat  to  regions  which  are  heated  less. 

Whenever  the  land  and  water  are  .warmer  than  the  air  which 
rests  on  them,  they  also  warm  it  by  conduction,  and  convection 
results.  Warmer  air  also  radiates  heat  to  cooler  air. 

The  heating  of  land  and  water.  Land  and  water  are  heated 
unequally  by  the  sun,  the  former  being  heated  four  or  five  times 
as  fast  as  the  latter  by  insolation.  The  reasons  are  several: 

1 .  The  absorption  of  a  given  amount  of  heat  by  a  given  amount 
of  soil  or  rock  raises  the  temperature  of  the  soil  or  rock  more 
(about  four  times  as  much)  than  that  of  the  same  amount  of  water; 
that  is,  the  specific  heat  of  water  is  higher  than  that  of  the  land. 

2.  Water  is  a  good  reflector,  while  the  land  is  not,  and  the 
latter  therefore  absorbs  a  larger  proportion  of  the  heat  of  the  sun's 
rays. 

3.  The  land  cools  more  readily  than  water. 

4.  Convection  currents  or  movements  are  established  in  water 
as  soon  as  its  surface  is  heated  locally.     This  prevents  excessive 
heating  at  any  one  point.     The  land,  on  the  other  hand,  being 
solid,  is  without  movements  of  convection. 

5.  There  is  more  evaporation  from  a  water  surface  than  from 
a  land  surface,  other  conditions  being  the  same,  and  evaporation 
cools  the  surface  from  which  it  takes  place. 

G.  Soil  and  rock  are  essentially  impenetrable  to  light  and  heat 
rays,  while  water  is  not.  The  heat  of  insolation  is,  therefore,  dis- 
tributed, at  first  hand,  through  a  greater  thickness  of  water  than 
of  land.  Being  confined  essentially  to  the  surface  of  the  latter, 
the  temperature  of  the  surface  is  made  higher. 

7.  Rock  is  a  poor  conductor  of  heat,  but  water  is  even  poorer. 

Secondary  distribution  of  heat.  From  what  has  preceded,  it  is 
clear  that  after  the  heat  is  received  by  the  earth  from  the  sun,  it 
is  to  some  extent  re-distributed.  This  re-distribution  is  accom- 
plished in  part  in  ways  just  noted.  It  is  also  effected  by  air  move- 


TEMPERATURE  OF  THE  AIR  531 

merits  (other  than  convection  currents)  and  water  movements 
(especially  ocean  currents)  both  of  which  are  of  great  importance 
in  the  secondary  distribution  of  heat.  It  has  been  estimated  that 
without  them  the  average  temperature  of  the  equator  would  be 
about  131°  F.,  instead  of  about  80°  F.  as  now,  and  that  of  the 
poles  about  —  108°  F.;  instead  of  0°  as  now. 

THE  SEASONS 

We  are  now  prepared  to  understand  the  seasons,  and  the 
reasons  for  their  differences  so  far  as  temperature  is  concerned. 
In  most  latitudes  the  seasons  are  usually  said  to  be  four— spring 
summer,  autumn,  and  winter;  but  they  are  not  sharply  separated 
from  one  another,  each  grading  into  the  one  which  follows. 

The  exact  limits  of  the  four  seasons  are  arbitrarily  defined. 
In  the  United  States,  March,  April,  and  May  are  commonly  called 
the  spring  months ;  June,  July,  and  August,  the  summer  months ; 
September,  October,  and  November,  the  autumn  months;  and 
December,  January,  and  February  the  winter  months.  Some- 
times, however,  spring  is  defined  as  the  time  between  the  vernal 
equinox  and  the  summer  solstice.  On  this  basis,  summer  is  the 
time  between  the  summer  solstice  and  the  autumnal  equinox, 
autumn  the  time  between  the  autumnal  equinox  and  the  winter 
solstice,  and  winter  the  time  between  the  winter  solstice  and  the 
vernal  equinox.  In  the  southern  hemisphere  spring  comes  in 
September,  October,  and  November;  summer  in  December,  Janu- 
ary, and  February;  and  so  on.  The  vernal  equinox  of  the  north- 
ern hemisphere  is  the  autumnal  equinox  of  the  southern,  and  the 
summer  solstice  of  the  northern  is  the  winter  solstice  of  the  southern. 

The  first  of  these  subdivisions  is  based  primarily  on  tempera- 
ture. The  summer  is  made  up  of  the  three  warmest  months,  so 
far  as  intermediate  (temperate)  latitudes  are  concerned,  and  the 
winter  of  the  three  coldest.  The  second  has  an  astronomical  basis. 
The  limits  of  the  seasons  are  defined  in  still  other  ways  in  some 
countries  even  in  middle  latitudes,  and  in  some  cases  the  lengths 
of  the  seasons,  according  to  popular  use  of  the  terms,  are  not  equal. 

In  middle  latitudes  the  distinction  between  the  seasons  is 
primarily  one  of  temperature;  but  in  some  parts  of  the  earth 
the  distinction  between  the  seasons  is  based  partly,  or  even  largely, 
on  elements  other  than  temperature.  Thus,  in  some  regions  the 


532  PHYSIOGRAPHY 

wet  and  dry  seasons  are  more  distinct  than  the  warm  and  cold 
ones.  This  is  true,  for  example,  in  some  low  latitudes  where  the 
temperature  is  always  high.  In  the  polar  regions,  on  the  other 
hand,  while  the  temperature  of  the  cold  seasons  is  very  much 
lower  than  that  of  the  warm  ones,  there  is  also  a  striking  difference 
in  the  matter  of  light.  At  the  poles  the  wrarm  season  is  the  light 
season,  and  the  cold  season  is  the  dark  one. 

Differences  between  summer  and  winter.  N  Aside  from  the 
higher  temperature  of  summer  in  our  latitude  (middle  latitude  of 
the  northern  hemisphere),  there  are  certain  other  obvious  dif- 
ferences between  summer  and  winter.  (1)  In  summer  the  days 
are  more  than  12  hours  long,  and  the  nights  less.  (2)  The  sun  is 
much  higher  above  the  horizon  at  noon  in  summer  than  at  the  cor- 
responding hour  in  winter.  This  is  the  same  as  saying  that  the  sun's 
rays  are  less  oblique,  at  any  given  hour  of  the  day,  in  summer  than 
in  winter  (Fig.  526).  (3)  A  third  difference  between  summer  and 
winter  in  our  latitude  is  the  direction  in  which  the  sun  rises  and 
sets.  In  summer  the  sun  rises  to  the  north  of  east,  and  sets  to 
the  north  of  west.  At  the  equinoxes  it  rises  in  the  east  and  sets 
in  the  west.  In  the  winter  it  rises  to  the  south  of  east  and  sets 
to  the  south  of  west.  (4)  The  amount  of  moisture  in  the  air  often 
varies  with  the  season ;  but  in  some  regions  it  is  the  warm  season 
which  is  wet,  while  in  others  it  is  the  cool  season.  (5)  In  some 
regions  the  winds  change  their  direction  and  force  with  the  change 
of  seasons,  as  will  be  noted  later.  The  first  and  second  of  these 
differences  are  the  most  important,  so  far  as  concerns  the  seasons 
of  middle  latitudes. 

The  differences  between  summer  and  winter,  other  than  the 
differences  of  temperature,  are  dependent  primarily  upon  the 
differences  in  temperature. 

Why  we  have  summer  when  we  do.  Since  the  earth  receives 
most  of  its  surface  heat  from  the  sun,  it  follows  that  the  period  of 
the  year  when  the  days  are  long  and  the  nights  short  must  be 
warmer  than  the  period  when  the  days  are  short  and  the  nights 
long;  for  long  days  and  short  nights  mean  long  periods  of  heat- 
ing and  short  periods  of  cooling  daily,  while  short  days  and  long 
nights  mean  short  periods  of  heating  and  long  periods  of  cooling 
daily.  Not  only  this,  but  the  sun's  rays  are  more  nearly  vertical 
when  the  days  are  long,  as  shown  by  Fig.  536,  and  so  have  greater 
heating  power.  It  follows,  therefore,  that  during  the  summer  the 


TEMPERATURE  OF  THE  AIR  533 

surface  is  not  only  heated  more  hours  a  day  than  during  the  winter, 
but  the  heat  per  hour  is  greater  while  the  sun  shines.  These  are 
the  immediate  reasons  why  summer  is  warmer  than  winter. 

The  reasons  why  the  days  are  longer  at  one  time  of  the  year 
than  another  have  already  been  given  (p.  498). 

Change  of  seasons.  The  change  of  seasons  may  be  understood 
from  a  study  of  Figs.  526  and  536.  We  have  already  seen  (1)  that 
the  sun's  rays  are  vertical  at  the  equator  at  the  equinoxes,  and 
that  the  days  and  nights  are  then  equal  everywhere;  (2)  that  the 
northern  hemisphere  is  being  heated  most  by  the  sun  at  the  time 
of  the  summer  solstice,  and  least  at  the  time  of  the  winter  solstice; 
(3)  that  the  days  are  longer  than  the  nights  in  the  northern  hemi- 
sphere (except  where  there  is  continuous  day)  from  March  21 
to  September  22;  (4)  that  the  sun's  rays  are  less  oblique  in  either 
hemisphere  during  the  half  of  the  year  when  the  days  are  longer 
than  the  nights;  and  (5)  that  the  relative  lengths  of  day  and  night, 
and  the  angle  of  the  sun's  rays,  are  reversed  in  each  hemisphere 
every  half-year. 

Since  the  northern  hemisphere  is  being  heated  most  at  the 
time  of  the  summer  solstice  and  least  at  the  time  of  the  winter  sol- 
stice, it  would  seem,  at  first  thought,  that  these  dates,  respectively, 
should  be  the  middle  points  of  the  hot  and  cold  seasons.  This  is 
not  the  case.  It  therefore  follows  that  the  temperature  of  any 
given  latitude  is  not  altogether  dependent  on  the  amount  of  heat 
it  is  receiving  from  the  sun  (p.  524).  Again,  since  the  value  of 
insolation  in  corresponding  latitudes  in  the  two  hemispheres  is 
equal  at  the  equinoxes,  it  would  seem,  at  first  thought,  that 
corresponding  latitudes  in  the  two  hemispheres  should  have  the 
same  temperature  at  these  times;  but  this,  again,  is  not  the  case. 
In  our  own  latitude,  for  example,  March  21  is  much  colder  than 
September  22.  There  is  some  discrepancy,  too,  between  the  tem- 
peratures of  the  northern  and  southern  hemispheres  on  corre- 
sponding dates  in  corresponding  latitudes,  because  of  the  greater 
preponderance  of  water  in  the  latter. 

The  reason  why  a  place  in  our  latitude  is  warmer  at  the  time 
of  the  autumnal  than  at  the  time  of  the  vernal  equinox  is  because 
the  warmth  of  the  summer  just  passed  has  not  all  been  lost.  At 
this  time,  ^theref ore,  the  northern  hemisphere  has  a  temperature 
higher  than  that  which  it  would  have  if  it  depended  solely  on 
daily  insolation.  On  the  other  hand,  the  temperature  at  the  time 


534  PHYSIOGRAPHY 

of  the  vernal  equinox  is  lower  than  that  which  would  seem  appro- 
priate from  the  insolation  then  taking  place,  because  the  cold  of 
the  winter  just  passed  has  not  been  altogether  overcome.  The 
cold  of  the  spring  is  rather  more  enduring  than  the  heat  of  the 
autumn,  for  it  is  in  some  sense  "stored  up"  in  the  snow,  the  ice, 
and  the  frozen  ground. 

Similarly,  our  summer  solstice  is  not  the  hottest  part  of  the 
year  in  the  northern  hemisphere,  or  the  coldest  in  the  southern, 
for  the  summer's  heat  has  not  altogether  overcome  the  effect  of 
the  preceding  winter  in  the  northern  hemisphere,  or  the  effect  of 
the  preceding  summer  in  the  southern  hemisphere.  The  time  of 
maximum  heat  therefore  lags  behind  the  season  of  maximum 
heating.  Similarly,  the  time  of  maximum  cold  does  not  come 
till  after  the  season  of  minimum  heating.  In  middle  latitudes 
the  lag  is  about  a  month,  but  it  is  greater  over  the  ocean  than  over 
the  lands,  because  the  latter  are  heated  and  cooled  the  more  readily. 

Seasons  in  other  latitudes.  Attention  to  the  subdivisions  of 
the  year  in  latitudes  other  than  our  own  will  help  to  an  under- 
standing of  the  fundamental  principles  involved.  At  the  equator, 
for  example,  the  sun's  rays  are  vertical  twice  each  year,  that  is, 
at  the  time  of  the  equinoxes.  Twice  a  year,  too,  the  sun's  rays 
are  vertical  23^°  from  the  equator,  once  to  the  north  and  once  to 
the  south.  The  equator,  therefore,  has  two  seasons,  occurring  at 
the  time  of  our  spring  and  autumn,  which  are  somewhat  warmer 
than  two  other  seasons  occurring  at  the  time  of  our  summer  and 
winter.  The  variations  in  temperature  are  much  less  than  in  our 
own  latitude,  for  the  length  of  day  and  night  never  varies,  and  the 
angle  of  the  sun's  rays  varies  but  23^°,  while  with  us,  in  middle 
temperate  latitudes,  it  varies  47°.  At  the  equator,  therefore,  there 
is  a  fourfold  division  of  the  year,  but  the  divisions  do  not  corre- 
spond very  closely  with  those  of  middle  latitudes. 

In  high  latitudes  the  conditions  are  still  different.  The  suc- 
cession of  seasons  in  latitude  75°  N.  may  be  taken  to  illustrate 
the  conditions  in  latitudes  above  the  polar  circles  generally.  When 
the  sun's  rays  are  vertical  15°  south  of  the  equator,  the  sun  would 
appear  on  the  horizon  at  noon  in  latitude  75°  N.  (Fig.  536),  for  this 
latitude  is  90°  from  the  place  where  the  sun's  rays  are  vertical. 
When  they  are  vertical  farther  south  than  15°  S.,  points  on  the  par- 
allel of  75°  N.  will  not  see  the  sun.  When  the  sun's  rays  are  vertical 
in  latitude  15°  N.,  or  in  any  latitude  farther  north,  no  point  on  the 


TEMPERATURE  OF  THE  AIR 


535 


B 


FIG.  536. — Diagram  to  illustrate  seasons  in  latitude  75°.  When  the  sun's 
rays  are  vertical  at  C,  the  circle  of  illumination  is  represented  by  the 
line  90°-90°.  The  half  of  each  parallel  of  75°  is  then  illuminated,  and 
days  and  nights  on  that  parallel  are  therefore  equal.  The  same  is  true 
of  all  other  latitudes.  When  the  sun's  rays  are  vertical  at  B,  in  latitude 
15°  N.,  the  circle  of  illumination  is  represented  by  b-b,  the  whole  of  the 
parallel  of  75°  N.  is  illuminated,  and  daylight  is  continuous  throughout 
the  twenty-four  hours.  No  part  of  the  parallel  of  75°  S.  is  illuminated 
at  this  time,  and  on  that  parallel  darkness  is  continuous.  When  the  sun 
is  vertical  at  A,  in  latitude  23 J°  N.,  the  circle  of  illumination  is  repre- 
sented by  a-a.  While  the  sun  appears  to  move  from  position  B  to 
position  A  and  back  again  to  B,  the  parallel  of  75°  N.  is  continuously 
illuminated,  while  the  parallel  of  75°  S.  at  the  same  time  is  continuously 
in  darkness.  When  the  sun  appears  to  move  from  the  position  where  its 
rays  are  vertical  at  B  to  the  position  where  its  rays  are  vertical  at  D,  a 
part  of  each  parallel  of  75°  is  illuminated,  and  during  this  time,  therefore, 
there  is  light  and  darkness  in  the  course  of  the  twenty-four  hours.  When 
the  sun's  rays  are  vertical  between  B  and  C,  more  than  half  of  the  paral- 
lel of  75°  N.  is  illuminated,  and  less  than  half  of  the  parallel  of  75°  S. 
When  the  sun  is  vertical  at  C  the  half  of  each  parallel  of  75°  (and  of  all 
other  parallels)  is  illuminated,  and  days  and  nights  are  equal.  While 
the  sun  appears  to  be  passing  from  C  to  D  less  than  half  of  the  parallel 
of  75°  N.  is  illuminated,  and  more  than  half  of  the  parallel  of  75°  S. 
During  this  time,  therefore,  nights  are  longer  than  days  in  latitude  75° 
N.,  and  days  are  longer  than  nights  in  latitude  75°  S.  When  the  sun  is 
in  a  position  where  its  rays  are  vertical  at  D,  the  circle  of  illumination  is 
d-d.  At  this  time  all  of  the  parallel  of  75°  N.  is  in  darkness,  and  all  of 
the  parallel  of  75°  S.  is  in  light.  This  condition  continues  while  the  sun 
appears  to  move  on  from  the  position  where  its  rays  are  vertical  at  D 
to  the  position  where  its  rays  are  vertical  at  E,  and  back  again. 


536  PHYSIOGRAPHY 

parallel  of  75°  N.  will  be  in  darkness  during  any  part  of  the  twenty- 
four-hour  day  (Fig.  536).  When  the  sun's  rays  are  vertical  in 
any  latitude  between  15°  S.  and  15°  N.,  a  part  of  the  parallel  of 
75°  N.  will  be  illuminated,  and  all  points  on  that  parallel  will  have 
alternating  light  and  darkness  in  the  course  of  the  twenty-four- 
hour  day.  (See  also  explanation  below  Fig.  536.) 

Here,  too,  there  is  a  natural  fourfold  division  of  the  year:  one 
(summer)  when  daylight  is  continuous,  one  (winter)  when  dark- 
ness is  continuous,  one  (spring)  when  there  is  alternating  day  and 
night  with  the  days  lengthening,  and  one  (autumn)  when  there 
is  alternating  day  and  night  with  the  nights  lengthening.  In 
other  words,  summer,  according  to  this  subdivision  of  the  year,  is 
the  time  during  which  the  sun  appears  to  move  from  15°  N.  to 
23£°  N.  (B  to  A,  Fig.  536)  and  back  again  to  15°  N.  Autumn  is 
the  time  during  which  the  sun  appears  to  pass  from  a  position 
where  its  rays  are  vertical  15°  N.  to  a  position  where  its  rays  are 
vertical  15°  S.  (B  to  D).  Winter  is  the  time  during  which  the  sun 
appears  to  pass  from  15°  S.  to  23J°  S.  (D  to  E)  and  back  again  to  15° 
S.,  and  spring  the  time  when  the  sun  is  passing  from  15°  S.  to  15° 
N.  (D  to  £). 

It  will  be  noted  that  the  lengths  of  the  several  seasons  denned 
in  this  way  are  not  the  same.  In  latitude  75°  the  summer  would 
be  as  long  as  the  winter,  and  the  spring  as  long  as  the  autumn; 
but  the  spring  and  autumn  would  be  nearly  twice  as  long  as  the 
summer  and  winter,  for  during  each  of  the  former  the  sun  moves 
through  30°,  and  during  each  of  the  latter  but  17°.  Not  only  this, 
but  the  lengths  of  the  several  seasons  would  vary  with  the  latitude. 
In  latitude  85°  the  summer  and  winter  would  be  longer  than  in 
latitude  75°,  and  the  springs  and  autumns  correspondingly  shorter. 

There  is  a  prevalent  idea  that  in  polar  regions  there  is  a  day 
of  six  months  and  a  night  of  six  months  each  year;  but  it  will  be 
seen  from  the  above,  as  well  as  from  what  has  been  stated  before, 
that  this  notion  is  not  correct.  There  is  a  six-months  day  and  a 
six-months  night  at  the  poles  only. 

Effect  of  varying  distance  of  the  sun.  Since  the  orbit  of 
the  earth  is  an  ellipse,  the  distance  of  the  earth  from  the  sun  varies 
in  the  course  of  a  year.  On  this  account,  the  amount  of  heat 
which  the  earth  receives  daily  varies  a  little,  being  somewhat 
greater  when  the  earth  is  nearer  the  sun,  somewhat  less  when  it 
is  farther  from  it.  But  the  variations  in  the  amount  of  heat  re- 


TEMPERATURE  OF  THE  AIR  537 

ceived  by  the  earth,  because  of  its  varying  distance  from  the  sun, 
are  of  relatively  little  importance  in  comparison  with  effects  which 
result  from  the  inclination  of  the  axis.  At  the  present  time,  the 
northern  hemisphere  has  its  summers  when  the  earth  is  farthest 
from  the  sun  (aphelion),  and  its  winters  when  it  is  nearest  (peri- 
helion). The  southern  hemisphere,  on  the  other  hand,  has  its 
summers  when  the  earth  is  nearest  the  sun  and  its  winters  when 
it  is  farthest  from  it.  This  condition  of  things  is  reversed  every 
10,500  years.  At  the  present  time,  the  southern  hemisphere 
receives  more  heat  from  the  sun  in  a  day  at  the  time  of  the  winter 
solstice  in  the  northern  hemisphere,  than  the  northern  hemisphere 
does  at  the  time  of  its  summer  solstice.  The  difference  is  consid- 
erable. 

Effect  of  altitude  on  temperature.  High  altitudes  are  colder 
than  low  ones,  and  the  average  rate  of  decrease  of  temperature  is 
about  1°  F.  for  330  feet  (1°  C.  for  594  feet)  of  rise,  for  altitudes 
where  observations  are  common.  It  varies,  however,  from  time  to 
time  and  from  place  to  place,  being  especially  influenced  by  the 
temperature  of  the  surface  beneath  it.  The  rate  of  decrease  of 
temperature  for  the  first  100  feet  or  so  of  rise  at  the  bottom  of  the 
atmosphere  is  much  more  rapid  where  the  land  or  water  is  warm. 

The  average  decrease  of  temperature  with  increase  of  altitude 
is  about  800  times  as  rapid  as  its  decrease  with  increase  of  latitude. 
In  other  words,  one  mile  of  ascent  in  the  air  means  about  the 
same  decrease  of  temperature  as  a  poleward  movement  of  800 
miles. 

When  air  rises  it  expands,  because  there  is  less  weight  of  air 
above  it  tending  to  compress  it.  As  a  gas  expands  it  is  cooled, 
and  as  it  is  compressed  it  is  warmed.  Dry  air  should  be  cooled 
about  1°  F.  for  every  183  feet  it  rises  (1°  C.  for  329  feet).  .Moist 
air  cools  much  less  rapidly  with  expansion,  for  reasons  which  will 
appear  later  (p.  572).  Conversely,  air  is  warmed  as  it  descends 
and  becomes  denser.  The  presence  of  moisture  makes  much  less 
difference  in  the  case  of  descending  air,  which  is  warmed  at  about 
the  same  rate  as  dry  air  is  cooled  during  its  ascent. 

High  altitudes  are  colder  than  low,  primarily  because  the  air 
is  thinner;  but,  in  the  case  of  isolated  elevations,  also  because  of  the 
more  complete  exposure. 

Since  the  air  is  thinner,  it  (a)  absorbs  less  heat  from  the  direct 
rays  of  the  sun,  chiefly  because  there  is  less  carbonic-acid  gas,  less 


538  PHYSIOGRAPHY 

water  vapor,  and  less  dust;  and  (6)  being  thinner,  it  is  less  effective 
in  retaining  the  heat  radiated  from  the  earth  below. 

In  sunny  days  in  summer  the  sunny  sides  of  bare  mountain 
surfaces,  when  free  from  snow,  get  very  warm.  If  the  air  re- 
mained in  contact  with  the  warm  rock  surface  for  long  periods  of 
time,  it  would  be  notably  warmed;  but  since  it  is,  as  a  rule,  moved 
on  quickly,  especially  about  isolated  elevations  of  notable  height, 
it  is  not  greatly  heated  before  it  passes  on,  and  the  new  air  by 
which  it  is  replaced  is  much  colder  than  air  which  has  been  resting 
directly  on  the  land. 

On  the  other  hand,  there  are  likely  to  be  many  cloudy  days 
in  the  mountains,  and  the  clouds  shelter  the  rocks  from  the  sun. 
This  tends  to  reduce  the  average  temperature  of  the  mountain, 
as  compared  with  that  of  low  land. 

Again,  where  mountains  are  sufficiently  high  and  not  too  steep 
to  retain  snow  throughout  the  year,  their  surfaces  are  never  warmed 


FIG.  537. — Diagram  to  show  that  the  sun's  rays  may  fall  less  obliquely  on  a 
mountain  slope  than  on  the  plain  adjacent.  Under  these  circu.nstances 
they  have  greater  heating  power,  so  far  as  the  surface  ot  the  land  is  con- 
cerned, on  the  mountain  than  on  the  plain. 

above  a  temperature  of  33°  F.,  the  melting-point  of  snow  (p.  525). 
The  temperature  has  been  observed  in  balloons  up  to  elevations 
of  about  30,000  feet,  where  it  was  found  to  be  -54°  F.  This  is 
doubtless  much  colder  than  it  would  be  at  the  top  of  a  mountain 
30,000  feet  high,  and  very  much  colder  than  it  would  be  on  a  pla- 
teau at  that  elevation.  The  temperature  has  been  recorded  by 
self-registering  thermometers  in  balloons  set  up  to  altitudes  of 
ten  miles,  where  the  temperature  was  — 104°. 

It  is  to  be  noted  that  land  surfaces  at  high  altitudes  may  be 
heated  quite  as  effectively  by  the  sun  as  land  surfaces  at  low 
altitudes.  That  this  is  the  case  is  shown  by  familiar  experiences 
in  high  mountain  regions,  where  the  surface  of  the  rock  may  be 
very  warm  though  the  air  is  cool.  A  mountain  surface  such  as 
that  shown  in  Fig.  537  may  receive  the  sun's  rays  much  more 


pq 


TEMPERATURE  OF  THE  AIR  539 

perpendicularly  than  a  flat  surface.  The  rock  is  correspondingly 
heated  while  the  sun  shines;  but  as  the  sun  goes  down,  the  heated 
rock  surface  cools  readily,  and  may,  during  the  night,  become 
much  cooler  than  the  surface  of  the  lower  land. 

It  is  to  be  noted  that  only  the  equatorward  sides  (the  southern 
sides  in  the  northern  hemisphere  and  the  northern  sides  in  the 
southern  hemisphere)  of  mountains  receive  the  sun's  rays  more 
perpendicularly  than  a  flat  surface.  The  poleward  slopes  of  moun- 
tains (outside  tropical  latitudes  and  sometimes  within  them)  receive 
the  sun's  rays  much  more  obliquely  than  flat  surfaces,  and  they 
receive  them  fewer  hours  per  day.  This  serves  to  reduce  the  average 
temperature  of  mountain  regions. 

Representation  of  Temperature  on  Maps 

It  is  desirable  to  have  some  method  of  representing  not  only 
the  general  distribution  of  temperature  over  the  earth,  but  various 
other  facts  concerning  temperature  and  its  variations.  Maps 
showing  such  phenomena  are  thermal  maps  or  charts.  The  princi- 
ple of  thermal  charts  is  simple. 

Isotherms.  A  line  may  be  drawn  on  the  surface  of  the  earth 
connecting  points  having  the  same  temperature.  Such  a  line  is 
an  isotherm.  An  isotherm  connecting  places  having  the  same 
average  temperature  for  the  year  is  an  annual  isotherm..  An 
isotherm  connecting  places  which  have  the  same  summer  or  the 
same  winter  temperature  is  a  seasonal  isotherm.  Similarly  there 
may  be  monthly  isotherms,  daily  isotherms,  etc.  A  map  show- 
ing the  distribution  of  isotherms  for  a  year,  a  season,  a  month,  or 
a  day,  is  an  isothermal  map  or  chart. 

The  line  of  highest  temperature  about  the  earth  is  the  thermal 
equator.  This  line  is  not  straight,  and  in  general  it  lies  a  little 
north  of  the  geographic  equator. 

Isothermal  charts.  Fig.  538  shows  the  annual  isotherms. 
It  shows  an  isotherm  of  80°  enclosing  a  considerable  area  in  the 
tropical  region  extending  from  the  Americas  eastward  to  north- 
ern Australia.  This  isotherm  shows  that  all  points  enclosed  by 
it  have  an  average  temperature  of  more  than  80°.  There  are  two 
isotherms  of  70°,  one  north  of  the  equator  and  one  south  of  it. 
All  points  between  the  isotherm  of  70°  and  the  isotherm  of  80° 
have  an  average  annual  temperature  of  more  than  70°  and  less 
than  80°.  In  the  Pacific,  all  points  between  the  two  70°  isotherms 


540  PHYSIOGRAPHY 

have  a  temperature  of  more  than  70°  and  less  than  80°.  The 
map  also  shows  two  isotherms  of  50°,  one  in  the  northern  hemi- 
sphere and  one  in  the  southern.  All  points  between  the  isotherms 
of  50°  and  70°  have  an  average  temperature  between  these  limits. 
The  warmer  portion  of  these  zones  in  either  hemisphere  is  the 
portion  near  the  higher  isotherm,  that  is,  nearer  the  equator. 

The  chart  expresses  the  general  fact  that  the  temperatures  are 
higher  in  the  equatorial  regions  and  lower  toward  the  poles,  and 
this  shows  that  there  is  a  relationship  between  isotherms  and  lati- 
tude. The  reason  for  this  relationship  has  already  been  explained. 

Fig.  539  shows  the  isotherms  for  the  month  of  January.  As 
compared  with  the  preceding  map,  this  shows  that  the  zone  of 
highest  temperature,  and  all  isotherms,  have  been  shifted  to  the 
south.  The  fact  that  the  sun  is  shining  vertically  some  distance 
south  of  the  equator  at  this  season,  seems  to  be  a  sufficient  reason 
for  the  change.  This  conclusion  may  be  tested  by  referring  to  the 
isothermal  chart  for  July  (Fig.  540),  for  if  the  conclusion  be  right, 
the  thermal  equator  and  all  isotherms  should  there  be  found  farther 
north  than  in  Fig.  538  or  Fig.  539.  Fig.  540  shows  this  to  be  the 
case. 

Fig.  539  shows  that  the  thermal  equator  is  mostly  south  of  the 
geographic  equator  in  January,  and  Fig.  540  shows  that  the  thermal 
equator  is  wholly  north  of  the  geographic  equator  in  July.  In  the 
former  case  it  is  in  latitude  20°  S.  (nearly)  in  South  Africa,  and  in 
the  latter  in  latitude  40°  N.  (nearly)  in  southwestern  Asia.  In  both 
charts  it  is  farther  from  the  equator  on  land  than  on  sea.  In  Africa, 
the  thermal  equator  is  fully  40°  farther  north  in  July  than  in  Janu- 
ary, and  in  the  Americas  the  shifting  is  still  greater. 

A  comparison  of  Figs.  539  and  540  shows  that  the  range  of  tem- 
perature between  January  and  July  is  greater  in  high  latitudes  than 
in  low.  Thus  in  the  southern  part  of  Hudson  Bay  it  is  80°;  at 
Montreal  about  50°;  in  Florida  less  than  30°;  and  at  the  equator 
in  South  America,  less  than  10°.  The  same  charts  show  that  the 
range  is  greater  in  the  interiors  of  continents  than  on  coasts  or  over 
the  sea  in  the  same  latitude. 

The  general  distribution  of  atmospheric  temperature  in  latitude 
is  shown  in  Fig.  541. 

What  determines  the  positions  and  courses  of  isotherms? 
1.  A  relationship  between  isotherms  and  parallels  is  suggested  further 
by  the  fact  that  the  isotherms  have  a  general  east-west  direction. 


TEMPERATURE  OF  THE  AIR 


541 


Some  of  them  are  notably  irregular,  but  none  of  them  runs  north  and 
south,  or  anywhere  nearly  north  and  south,  for  any  considerable 
distance.  Some  of  them  have  a  nearly  straight  east-west  course, 
and,  in  all,  the  east-west  direction  is  the  general  one.  Since,  how- 
ever, the  isotherms  do  not  follow  the  parallels  exactly,  it  Is  clear 
that  latitude  is  not  the  only  factor  which  determines  their  position. 

Northern  Hemisphere Southern  Hemisphere 


^ 


Mean  Temperature  f< 


.-  Globe 


I: 


Fia.  541. — Figure  showing  distribution  of  atmospheric  temperature  in  lati- 
tude for  the  year,  for  January,  and  for  July;  also  the  mean  temperature 
of  the  year  for  the  globe.  The  figures  at  the  left  are  Fahrenheit ,  those  at 
the  right  Centigrade.  The  numbers  at  the  top  represent  degrees  of 
latitude. 

Some  other  cause  or  causes  besides  the  length  of  day  and  the  angle 
of  the  sun's  rays  must  therefore  influence  temperature,  and  so  the 
position  of  the  isotherms. 

2.  From  Figs.  538,  539,  and  540  it  is  seen  that  the  isotherms  are 
straightest  where  there  is  least  land,  and  most  crooked  where  there 
is  much  land.  This  suggests  that  the  land  and  water  have  some- 
thing to  do  with  their  positions.  Following  this  idea,  it  is  to  be 


542  PHYSIOGRAPHY 

noted  that,  on  the  January  chart,  there  is  an  area  in  South  Africa, 
and  another  in  north  Australia,  surrounded  by  the  isotherm  of  90°. 
Both  of  these  areas  are  on  land,  and  there  is  no  corresponding  area 
over  the  sea.  It  is  to  be  noted  also  that  the  areas  where  the  tem- 
perature is  above  80°  are  wider  on  the  land,  and  in  the  vicinity  of 
land,  than  on  the  open  sea;  and  furthermore,  that  in  the  widest 
ocean  there  is  no  area  where  the  January  temperature  reaches  an 
average  of  80°.  All  these  facts  tend  to  confirm  the  conclusion  that 
the  sea  and  the  land  influence  the  position  of  the  isotherms. 

Following  this  idea  still  further,  it  is  seen  that  the  isotherms  of 
this  map  (Fig.  539)  frequently  bend  somewhat  abruptly  in  passing 
from  water  to  land,  or  vice  versa.  Thus  the  isotherm  of  40°  in  the 
northern  hemisphere  turns  abruptly  to  the  south  when  it  reaches 
North  America,  and  again  on  the  coast  of  Europe.  In  the  southern 
hemisphere,  the  isotherms  of  80°  and  70°  make  abrupt  turns  at  the 
west  coast  of  Africa  and  on  or  near  the  west  coast  of  South  America. 
This  tends  to  confirm  the  conclusion  that  the  relation  of  land  and 
water  has  something  to  do  with  the  position  of  isotherms.  It  will 
be  seen  later  that  ocean  currents  have  something  to  do  with  the 
peculiar  courses  of  the  isotherms  here  referred  to. 

So  far  as  this  chart  (Fig.  539)  is  concerned,  it  will  be  seen  that 
the  isotherms  south  of  the  equator  bend  poleward  on  the  land  in 
passing  from  west  to  east,  while  those  north  of  the  equator  bend 
equatorward. 

The  land  and  the  sea  are  affected  differently  by  the  sun's  rays 
(p.  530).  The  land  is  heated  more  readily  than  the  sea  in  the 
summer,  and  therefore  becomes  warmer.  The  land  also  gives  up 
its  heat  much  more  readily  than  the  sea,  and  becomes  cooler  in 
winter.  The  fact  that  an  isotherm,  for  example  the  January  iso- 
therm of  40°  in  the  northern  hemisphere,  bends  equatorward  in 
crossing  the  northern  continents,  shows  that  the  land  is  cooler  than 
the  water  in  the  same  latitude,  for  the  isotherm,  in  crossing  the  con- 
tinent, bends  toward  the  equator  to  find  the  same  temperature 
which  it  had  on  the  water.  In  the  southern  hemisphere,  on  the 
other  hand,  where  it  is  summer,  the  corresponding  isotherm,  on 
reaching  the  land,  bends  toward  the  pole  in  order  to  find  a  temper- 
ature like  that  of  the  sea. 

All  these  phenomena  clearly  indicate  that  the  position  of  the 
land  and  the  sea  has  something  to  do  with  causing  the  isotherms 
to  depart  from  the  parallels. 


TEMPERATURE  OF  THE  AIR  543 

If  the  preceding  inferences  are  correct,  the  July  isotherms 
should  be  in  contrast  with  the  January  isotherms.  The  former 
should  bend  poleward  on  the  continents  in  the  northern  hemisphere, 
and  equatorward  in  the  southern.  In  Fig.  540,  which  shows  the 
July  isotherms,  it  is  seen  that  every  isotherm  crossing  North  Amer- 
ica bends  poleward  on  the  land,  while  those  crossing  the  southern 
continents  bend  equatorward.  The  reason  is  that  this  is  the 
season  when  the  lands  of  the  northern  hemisphere  are  warmer  than 
the  seas  of  the  same  latitude,  and  when  the  lands  of  the  southern 
hemisphere  are  cooler  than  the  seas  about  them. 

It  will  be  noted  that  the  irregularities  of  the  isotherms  of  the 
northern  hemisphere  in  July  are  much  greater  than  those  of  the 
southern  hemisphere  in  January.  This  is  probably  because  there 
is  much  more  land  in  the  northern  hemisphere  than  in  the  southern, 
and  the  larger  land  areas  have  a  greater  effect  on  the  isotherms  than 
the  smaller  ones. 

These  facts  seem  to  confirm  the  inference  that  land  and  water 
influence  the  position  of  the  isotherms;  but  does  the  distribu- 
tion of  land  and  water  account  for  all  the  irregularities  of  the 
isotherms? 

If  the  unequal  heating  of  land  and  sea  were  the  only  factor 
concerned  in  deflecting  the  isotherms  from  the  parallels,  the  bends 
of  the  isotherms  should  be  as  pronounced  on  the  east  sides  of  the 
continents  as  on  the  west.  This  is  not  the  case,  as  shown  by  Figs. 
538  and  539.  Again,  the  January  isotherm  of  50°  near  the  west 
coast  of  North  America  bends  chiefly  on  the  land,  not  at  the  coast. 
On  the  eastern  side  of  the  continent  the  bend  of  the  isotherm  of  30° 
is  chiefly  on  the  sea,  not  at  the  coast.  Other  isotherms  have  similar 
courses.  We  infer,  therefore,  that  though  land  and  water  have  much 
to  do  with  the  irregularity  of  the  isotherms,  other  factors  also  are 
involved. 

3.  The  peculiarities  just  cited  may  be  explained  in  part  by  the 
winds.  The  prevailing  winds  in  the  middle  latitudes  of  North 
America  are  from  the  west,  and  the  westerly  winds  tend  to  carry  the 
temperature  of  the  sea  (warmer  in  winter)  over  onto  the  land  on 
the  western  side  of  the  continent,  and  the  temperature  of  the  land 
(cooler  in  winter)  over  onto  the  sea  on  its  eastern  side.  This 
appears  to  afford  a  partial  explanation  of  the  bends  of  the  iso- 
therms of  30°  and  50°  in  the  northern  hemisphere  in  January;  but 
it  does  not  afford  an  explanation  of  the  remarkable  northward 


544  PHYSIOGRAPHY 

loop  of  the  isotherm  of  30°  over  the  eastern  side  of  the  North 
Atlantic,  nor  of  the  lesser  one  over  the  corresponding  part  of  the 
Pacific. 

Other  illustrations  of  the  effects  of  winds  are  furnished  by  the 
west  coast  of  the  United  States.  Thus  in  July  (Fig.  540)  the  land 
is  warmer  than  the  sea,  and  the  cooler  temperature  of  the  latter  is 
carried  over  to  the  former.  The  winds  therefore  make  it  clear 
why  the  bends  in  the  isotherms  here  are  on  the  land,  rather  than 
at  the  coast  or  on  the  sea. 

On  the  whole,  the  influence  of  the  winds  on  the  position  of  the 
isotherms  is  less  clear  from  these  charts  than  the  influence  of  land 
and  sea.  This  is  partly  because  the  winds  are  inconstant,  and 
their  effects  at  one  time  tend  to  counteract  their  effects  at  another, 
and  the  maps  show  only  averages. 

4.  The  great  bend  in  the  isotherm  of  40°  in  the  North  Atlantic  in 
January  is  not  explained  by  the  relations  of  land  and  sea,  or  by 
winds.  It  is  due  to  a  warm  current  of  ocean  water  flowing  north- 
eastward, in  the  direction  of  the  pronounced  loop  of  the  isotherm. 
The  same  isotherm  is  held  off  the  eastern  coast  of  North  America 
by  a  cold  current  which  flows  southward  along  the  east  side  of  the 
continent.  Ocean  currents  are,  therefore,  a  fourth  cause  of  the 
irregularities  of  isotherms. 

The  amount  of  heat  carried  northward  by  the  ocean  currents 
of  the  Atlantic  and  Pacific  is  very  great.  Croll1  has  estimated 
that  conveyed  from  the  tropics  by  the  Gulf  Stream  to  be  equal 
to  two-fifths  of  that  received  by  the  Arctic  regions  from  the  sun. 
It  has  been  estimated  that  the  temperature  of  England  is  raised 
10°  F.,  that  of  Norway  16°,  and  that  of  Spitzbergen  19°  by  the 
warm  poleward  movement  of  waters  in  the  North  Atlantic.  These 
figures  have  been  called  into  question  and  are  very  likely  too 
high;  but  there  can  hardly  be  a  reasonable  doubt  that  the  north- 
ward movement  of  relatively  warm  water  helps  to  ameliorate  the 
temperature  of  northwestern  Europe,  especially  in  winter.  The 
tempering  influence  of  the  poleward  drift2  of  warm  water  is  indi- 
rect. The  air  over  the  water  is  warmed  and  made  moist,  and 

1  Climate  and  Time,  p.  27. 

2 The  term  "Gulf  Stream"  is  of  doubtful  propriety  as  applied  to  the 
poleward  movement  of  water  in  the  high  latitudes  of  the  North  Atlantic. 
The  "current"  is  very  indefinite  north  of  the  latitude  of  Newfoundland. 


TEMPERATURE  OF  THE  AIR  545 

it  is  this  warmed  and  moistened  air,  carried  over  to  the  land,  which 
raises  the  temperature  of  northwestern  Europe. 

It  should  be  noted  that  the  milder  climate  of  northwestern 
Europe,  as  compared  with  northeastern  North  America,  is  not  due 
wholly  to  the  poleward  drift  of  warm  waters.  Even  if  there  were 
no  Gulf  Stream,  the  climate  of  northwestern  Europe  would  be 
much  more  temperate  than  that  of  the  corresponding  latitudes  of 
North  America,  because  the  ocean,  whence  the  winds  of  winter 
blow  to  that  part  of  Europe,  is  warmer  than  the  land  whence  the 
winter  winds  blow  to  the  corresponding  latitudes  on  the  west  side 
of  the  Atlantic.  Similarly  the  heat  of  summer  is  less  extreme 
in  northwestern  Europe  than  in  northeastern  North  America. 

5.  Other  minor  causes  of  irregularities  in  isotherms  are  found  in 
topographic  relations,  in  the  character  of  the  surface,  the  amount  of 
moisture,  etc.  A  basin  region  shut  in  by  mountains  gets  hotter 
in  summer  than  a  region  not  so  surrounded,  partly  because  the  air 
is  warmed  by  heat  reflected  and  radiated  in  from  all  sides,  as  well 
as  by  heat  reflected  and  radiated  from  the  bottom,  and  partly 
because  the  enclosing  mountains  prevent  free  circulation  of  the 
air.  There  is  less  evaporation  from  a  dry  surface  than  from  a 
moist  one,  and  since  evaporation  cools  the  surface  notably,  a  dry 
surface  will  be  warmer  than  a  moist  one,  if  other  conditions  are 
the  same.  The  color  of  the  soil,  the  presence  or  absence  of  vegeta- 
tion, etc.,  also  affect  the  absorption  and  radiation  of  heat. 

Topographic  relations  have  much  to  do  with  the  high  tempera- 
ture (90°  and  above)  in  the  southwestern  part  of  the  United  States 
in  July.  The  dryness  of  the  soil  and  of  the  air  above  it  also  tends 
to  raise  the  temperature.  Aridity  also  helps  to  make  the  tempera- 
ture high  in  the  high-temperature  area  (90°  and  above)  in  northern 
Africa  (July)  and  Australia  (January). 

Altitude  has  a  pronounced  effect  on  temperature,  as  already 
pointed  out;  but  a  study  of  Figs.  538  to  540  seems  to  show  no 
relation  between  isothermal  lines  and  surface  relief.  The  reason 
is  that  on  isothermal  charts  all  isothermal  lines  are  represented 
as  at  sea-level.  This  is  done  by  making  allowance  for  altitude  at 
the  average  rate  of  1°  F.  for  about  330  feet.  Thus,  if  the  tempera- 
ture of  a  place  at  an  altitude  of  3300  feet  is  60°,  it  is  put  down  on 
the  chart  as  70°  (60° +10°).  If  the  place  were  6600  feet  above 
sea-level,  20°  F.  would  be  added  to  the  temperature  recorded  by 
the  thermometer.  Isothermal  charts,  therefore,  are  intended  to 


546  PHYSIOGRAPHY 

show  the   temperature   as   it  would  be  if  the  land 'were  at  sea- 
level. 

Thermal  charts  may  be  made  to  show  many  other  features.  A  chart 
may  be  made  to  show  the  departure  of  the  temperature  of  each  place,  from 
the  temperature  normal  to  its  latitude.  Such  departure  is  abnormal  tem- 
perature. Lines  connecting  places  having  the  same  abnormal  temperature 
are  is-abnormal  or  is-anomalous  lines.  They  may  be  made  for  the  year, 
for  any  season,  or  for  any  month  (Figs.  542  and  543).  Charts  may  be  made 
showing  the  lines  of  equal  annual  range  of  temperature  (Fig.  544),  and 
they  may  also  be  made  to  show  the  average  maximum  temperatures  (Fig. 
545)  and  the  average  minimum  temperatures  (Fig.  548).  The  former  are 
obtained  by  averaging  the  highest  temperatures  of  successive  years,  and 
the  latter  by  averaging  the  lowest  temperatures  of  successive  years.  The 
absolute  maximum  and  minimum  for  any  place  would  be  the  highest  and 
lowest  temperatures,  respectively,  ever  recorded  for  that  place.  Fig.  547 
shows  the  absolute  maximum  temperature  to  be  more  than  120°  in  the 
Sahara,  and  but  little  less  in  New  South  Wales  and  the  southwestern  part 
of  the  United  States.  The  lowest  temperature  recorded  is  in  northeastern 
Asia. 

Isothermal  surfaces.  A  surface  might  be  drawn  connecting 
all  points  having  the  same  temperature.  The  annual  isothermal 
surface  of  30°,  for  example,  would  be  at  sea-level,  where  the  iso- 
therms of  30°  appear  in  Fig.  538.  One  of  these  isotherms  is  north 
of  the  equator  and  one  south  of  it.  Equatorward  from  these  lines, 
in  either  hemisphere,  the  isothermal  surface  would  rise  above  sea- 
level.  The  temperature  at  sea-level  in  the  northern  part  of  South 
America  is  about  80°.  Its  temperature  is  therefore  about  50°  above 
that  of  the  isothermal  surface  of  30°.  That  surface  is  here  about 
50  times  330  feet,  or  16,500  feet,  above  sea-level.  Where  the  iso- 
therm of  50°  (Fig.  538)  crosses  North  America,  the  temperature  at 
sea-level  is  20°  too  high.  To  find  the  temperature  of  30°  in  this 
latitude  we  must  rise  high  enough  into  the  air  to  get  a  reduction  of 
20°;  that  is,  20  times  330  feet,  or  6600  feet. 

North  of  the  isotherm  of  30°  in  the  northern  hemisphere  (Fig. 
538)  the  temperature  at  sea-level  is  less  than  30°.  To  find  a  tem- 
perature of  30°  in  this  latitude,  therefore,  we  must  go  beneath  sea- 
level. 

The  rate  of  increase  of  temperature  below  sea-level  is  not  the 
same  as  above  it,  and  it  is  not  the  same  for  the  land  as  for  the  sea. 
Beneath  the  land  the  rate  of  increase  is  about  1°  F.  for  60  to  75 
feet.  (The  observed  rates  of  increase  beneath  the  surface  vary 


TEMPERATURE  OF  THE  AIR 


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TEMPERATURE  OF  THE  AIR 


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TEMPERATURE  OF  THE  AIR 


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TEMPERATURE  OF  THE  AIR 


553 


from  about  1°  F.  for  17  feet,  to  1°  for  more  than  100  feet.)  To  find 
the  isothermal  surface  of  30°  F.  where  the  isotherm  of  20°  F.  crosses 
the  continent,  we  should  have  to  go  down  far  enough  to  gain  10°  F. 
This  would  be  600  feet  if  the  rate  of  increase  is  1°  for  60  feet,  or 
1000  feet  if  the  rate  be  1°  for  100  feet. 

The  proper  conception  of  isothermal  surfaces  will  be  of  impor- 
tance when  we  come  to  consider  the  circulation  of  the  atmosphere. 


FIG.  548. — Isothermal  chart  cf  the  United  States  for  the  year. 
(U.  S.  Weather  Bureau.) 

Fig.  554  shows  sections  of  the  isothermal  surfaces,  along  the 
meridian  of  100°  in  January  and  July.  These  sections  are  based 
on  the  data  of  Figs.  539  and  540. 

It  will  be  seen  from  the  above  that  isothermal  lines  are  the  lines 
where  the  corresponding  isothermal  surfaces  touch  the  level  of  the 
earth  which  corresponds  to  sea-level. 

Daily  Range  of  Temperature 

It  has  been  found  by  experience  that  the  average  daily  tem- 
perature of  a  place  may  be  found  by  averaging  the  temperatures 
of  7  A.M.,  2  P.M.,  and  10  P.M.  It  is  found  that  the  daily  range  is 
less  above  the  bottom  of  the  atmosphere  than  at  the  bottom,  since 
the  lower  air  is  heated  much  by  contact  with  the  land  during  sunny 


554 


PHYSIOGRAPHY 


130'  lir          110'          105'          100-  95-  90*  85"  80'  «•  70'  65" 


FIG.  549. — Isothermal  chart  of  the  United  States  for  January. 
tU.  S.  Weather  Bureau.) 


125'  120-  115'  110'  105'          100"  95*  90*  65'  8O'  K'  TV 


10-  105"  100'  95'  90-  95'  80'  75'  70" 


FIG.  550. — Isothermal  chart  of  the  United  States  for  April. 
(U.  S.  Weather  Bureau.) 


TEMPERATURE  OF  THE  AIR 


555 


FIG.  551. — Isothermal  chart  of  the  United  States  for  July. 
(U.  S.  Weather  Bureau.) 


• 125' 120'  US'  110°  105'  100*  95'  90°  65°  80°  75"  70' 


FIG.  552. — Isothermal  chart  of  the  United  States  for  October. 
(U.  S.  Weather  Bureau.) 


556 


PHYSIOGRAPHY 


TEMPERATURE  OF  THE  AIR 


557 


days,  and  also  because  the  denser  air  below  is  heated  more  than  the 
rarer  air  above  by  direct  insolation.  It  is  also  found  that  the  daily 
range  is  greater  when  the  air  is  dry  than  when  it  is  moist,  for  in 
the  former  case  less  of  the  heat  radiated  from  the  land  is  absorbed 
by  the  air.  The  daily  range  is  greater  far  from  the  sea  than  in 
proximity  to  it,  for  the  sea  gets  neither  so  warm  nor  so  cold  as  the 
land.  Other  things  being  equal,  the  daily  range  is  greatest  when 
days  and  nights  are  nearly  equal. 

The  daily  range  of  temperature  is  often  as  much  as  40°  or  50°  F. 
in  dry,  interior  regions,  and  in  the  Sahara  it  is  sometimes  70°. 

The  temperature  of  the  day  is  highest  somewhat  after  noon,  for 
somewhat  the  same  reason  that  the  greatest  heat  of  summer  follows 
the  time  of  greatest  heating.  To  understand  this  point  it  should 

ISOTHERMAlg,,p 


FIG.  554. — Curves  of  the  isothermal  surface  of  30°  F.  A.  Along  the  meridian 
of  100°  W.,  corresponding  to  Fig.  539.  B.  Same,  corresponding  to  Fig. 
540.  The  numbers  below  the  horizontal  line  indicate  latitude. 

be  understood  that  the  land  surface  and  the  air  just  above  it  are 
losing  heat  by  radiation  all  the  time,  and  receiving  heat  by  insola- 
tion only  when  the  sun  shines. 

Let  us  suppose  the  day  to  be  twelve  hours  long.  When  the  sun 
rises,  the  daily  heating  begins,  and  the  rate  of  heating  increases  as 
the  sun  climbs  above  the  horizon.  The  land  and  the  air  receive 
most  heat  when  the  sun  is  highest,  that  is  at  noon.  After  noon  the 
heating  becomes  less  and  ends  at  sunset.  Fig.  555,  A,shows  the  curve 
of  insolation. 

When  cooling  by  radiation  exceeds  heating,  the  temperature 
falls.  This  is  the  case,  as  a  rule,  at  night,  for  radiation  goes  on 
after  insolation  ceases.  The  temperature  becomes  lowest  about 
sunrise,  when  radiation  without  insolation  has  been  going  on  longest 


558 


PHYSIOGRAPHY 


The  land  and  the  lower  air  continue  to  radiate  heat  after  sunrise. 
but  both  are  then  heated,  and  heated  as  a  rule  faster  than  they  are 
cooled  by  radiation,  for  the  temperature  rises.  As  the  temperature 
rises,  radiation  increases  (Fig.  555,  B);  but  it  does  not  commonly 
keep  pace  with  insolation,  for  the  temperature  continues  to  rise  till 
some  time  after  noon.  The  fact  that  the  temperature  then  begins 


12      246 


10      12      2       4        6       8      10     12 


122468101224681012 


Midnight 

12      2       4       6 


C 

Fio.  555. 

A.  Curve  of  insolation.  B.  Curve  of  radiation. 

C.  Curves  of  insolation  and  radiation  combined.  The  maximum  tempera- 
ture of  the  day  occurs  at  the  higher  crossing,  the  minimum  temperature 
of  the  day  just  to  the  left  of  the  lower  crossing  of  these  two  lines. 

to  fall  shows  that  radiation  then  exceeds  insolation.  Fig.  555,  C 
shows  the  curve  of  insolation  in  its  relation  to  the  curve  of  radiation. 
The  average  daily  range  of  temperature  by  months  for  six 
places  is  shown  in  Fig.  556.  San  Diego,  Phrenix,  Shreveport,  and 
Charleston  are  in  about  the  same  latitude  (about  33°).  All  are  in 
the  zone  of  westerly  winds.  San  Diego,  on  the  Pacific  coast,  has 
an  average  daily  range  of  about  14°  F.  Phosnix,  which  is  inland, 
much  higher,  and  in  a  dry  region,  has  an  average  daily  range  of 
about  33°.  Shreveport,  which  is  inland,  but  low,  and  in  a  region 


TEMPERATURE  OF  THE  AIR 


559 


of  abundant  moisture,  has  an  average  daily  range  of  about  17°. 
Charleston,  on  the  eastern  coast,  has  a  daily  range  of  about  14°. 
Tampa  and  San  Antonio  are  farther  south,  and  both  are  affected 
somewhat  by  the  trades.  Tampa  is  on  the  coast,  while  San  Antonio 


40 
35 
30 

25 
"0 

Jan.       Feb.     March    April     May      June      July       Aug.      Sept.      Oct.      Nov.     Dec.   Average 

,  >^ 

o, 

S., 

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.S.C. 

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^ 

FIG.  556. — Curve  showing  the  average  daily  range  of  temperature  for  cer- 
tain type  stations,  for  each  month.  The  figures  at  the  right  show  the 
average  daily  range  for  each  station.  The  great  range  at  Phrenix  is  the 
result  of  high  altitude  and  aridity.  (U.  S.  Weather  Bureau.) 

is  inland.     The  former  has  an  average  daily  range  of  about  19°, 
while  the  latter  has  a  range  of  about  21°. 

The  Seasonal  Range  of  Temperature 

The  seasonal  range  of  temperature  is  affected  by  various  condi- 
tions, such  as  (1)  latitude,  (2)  position  with  reference  to  land  and 
sea,  (3)  prevailing  winds,  (4)  presence  of  snow  during  the  warmer 
season,  and  (5)  humidity. 

1.  In  general  the  seasonal  range  of  temperature  increases  with 
the  latitude  (compare  Figs.  539  and  540),  because  the  range  of  in- 
solation increases  with  the  latitude.     This  range  is  greatest  at  the 
poles,  where  there  is  six  months  of  insolation  and  six  months  free 
from  it.     The  great  range  of  seasonal  temperature  to  which  the 
poles  would  be  entitled  by  their  latitude  is  greatly  modified  by  (2) 
and  (4)  of  the  preceding  paragraph. 

2.  Islands  have  a  lesser  range  of  temperature  than  continental 
lands  in  the  same  latitude,  and  coasts  have  a  lesser  range  than 


560 


PHYSIOGRAPHY 


interiors,  because  the  range  of  sea  temperature  is  less  than  the  range 
of  land  temperature  (Fig.  557).  A  striking  and  rather  extreme  illus- 
tration of  the  difference  between  the  range  of  temperature  on  an 
island  and  inland  is  afforded  by  Thorshavn  (Faroe  Islands,  Lat. 
62°  N.),  where  the  annual  range  is  7.9°  C.,  and  Yakutsk,  in  the 
same  latitude,  where  the  range  is  61.6°  C. 

3.  A  coast  to  which  the  prevailing  winds  blow  from  the  ocean 
has  a  less  range  of  temperature  than  a  coast  to  which  the  prevailing 
winds  blow  from  the  land.  Thus  the  range  of  temperature  is  less 
on  the  Pacific  coast  of  the  United  States  than  on  the  Atlantic  in  the 


Fio.  557. — Diagram  illustrating  the  difference  between  continental  and 
oceanic  temperatures,  the  former  indicated  by  the  full  line  and  the  latter 
by  the  dotted  line.  The  letters  stand  tor  the  months.  The  numbers  are 
the  degrees  above  and  below  the  average  annual  temperature  of  the  place. 
(After  Hann.) 

same  latitude,  the  winds  being  chiefly  from  the  west  in  both  cases. 
Hann  has  shown  that  in  Europe,  between  the  latitudes  of  47°  and 
52°,  the  temperature  changes  from  west  to  east  are  as  follows: 
With  every  10°  of  longitude  there  is  a  decrease  of  3.1°  C.  in  winter, 
an  increase  of  0.7°  C.  in  summer,  and  a  decrease  of  1.3°  C.  in  the 
mean  annual  temperature. 

4.  The  presence  of  snow  during  the  warm  season,  as  in  high 
latitudes  and  high  mountains,  prevents  a  high  temperature  in  sum- 
mer, even  though  insolation  be  strong  (p.  525). 

The  annual  range  of  temperature  is  of  much  importance  on  vari- 
ous human  affairs.  It  has  some  effect  on  vegetation,  and  so  on  all 
industries  connected  with  the  soil.  The  range  of  temperature,  or 
more  exactly  the  temperature  of  winter,  has  some  effect  on  trans- 
portation, especially  transportation  by  means  of  water.  Naviga- 
tion ceases,  for  example,  on  the  Great  Lakes,  because  ice  forms 
about  their  borders  in  winter.  The  lower  limit  of  temperature  also 


TEMPERATURE  OF  THE  AIR  561 

affects  some  phases  of  mining.  Placer  mining,  for  example,  is  sus- 
pended in  winter  in  high  latitudes  and  high  altitudes,  not  only 
because  the  gravel  and  sand  are  frozen,  but  also  because  the 
water  needed  in  the  mining  is  frozen.  Other  effects  of  great 
seasonal  range  will  be  readily  suggested. 

Effect  of  Atmospheric  Temperature  on  Atmospheric  Movement 

When  air  is  heated  it  expands  and  becomes  lighter,  volume 
for  volume.  If  we  think  of  the  air  over  a  given  area  as  shut  in 
from  its  surroundings  on  all  sides,  but  not  shut  in  above,  it  would 
expand  upward  when  heated.  The  result  would  be  that  its  surface 
would  rise  above  that  of  its  surroundings.  If  its  surface  became 
higher  than  that  of  its  surroundings,  the  upper  part  of  the  air 
would  spread  (run  over)  sideways,  much  as  water  would  under 
the  same  circumstances.  If  some  of  the  air  at  the  top  of  a  heated 
column  runs  over,  the  pressure  of  the  air  at  the  bottom  of  the 
heated  column  is  less  than  that  at  the  bottom  of  the  surrounding 
air,  and,  if  the  air  of  the  surrounding  area  were  not  shut  off,  it  would 
move  in  from  the  area  of  greater  pressure  (where  the  air  is  denser) 
to  the  area  of  less  pressure  (where  the  air  is  lighter).  The  result 
would  be  a  horizontal  movement  at  the  bottom  of  the  atmosphere 
(Fig.  534) ;  that  is,  a  wind.  Unequal  heating  of  the  air  is,  therefore, 
a  cause  of  air  movements,  and  since  the  air  is  being  unequally  heated 
constantly,  it  follows  that  unequal  heating  is  a  constant  cause  of 
atmospheric  movement.  Some  of  the  movements  are  horizontal 
and  some  vertical;  some  are  in  the  lower  part  of  the  air  and  some 
in  the  upper. 

The  unequal  heating  of  the  air  is  the  immediate  cause  of  certain 
familiar  winds  and  breezes. 

1.  Land-  and  sea-breezes.  During  a  sunny  summer  day  the 
land  near  a  lake  or  sea  becomes  warmer  than  the  water.  The  result 
is  that  the  air  over  the  land  becomes  sensibly  warmer  than  that 
over  the  water  on  a  hot  day.  The  expanded  lower  air  over  the 
land  crowds  the  air  above  it,  and  so  increases  the  pressure  above 
the  bottom  of  the  air.  The  result  is  that  the  pressure  above  the 
bottom  of  the  air  over  the  land  is  greater  than  that  at  the  same 
level  over  the  sea.  There  is,  as  a  result,  (1)  a  movement  of  air 
from  the  land  to  the  water  somewhat  above  the  bottom  of  the 
air.  This  movement  diminishes  the  pressure  at  the  bottom  of 


562  PHYSIOGRAPHY 

the  air  on  land,  and  increases  the  pressure  at  the  bottom  of  the 
air  over  the  sea.  This  gives  rise  to  (2)  a  breeze  from  sea  to  land 
at  the  bottom  of  the  atmosphere.  This  is  the  sea  (or  lake)  breeze. 
At  night,  the  air  over  the  land  cools  and  contracts  below,  and 
pressure  above  becomes  less  than  at  the  same  level  over  the  sea. 
Above  the  bottom  of  the  atmosphere,  therefore,  air  flows  in  (3) 
from  sea  to  land.  This  increases  the  pressure  at  the  bottom 
of  the  atmosphere  on  land,  and  decreases  that  at  the  bottom 
over  the  sea.  A  breeze  therefore  sets  seaward  (4)  from  the  land 
to  the  sea  at  the  bottom  of  the  atmosphere.  This  is  the  land- 
breeze. 

The  sea-breeze  is  best  developed  in  middle  and  low  latitudes 
during  the  hot  part  of  the  day  in  summer.  When  it  has  the  same 
direction  as  the  prevailing  wind,  it  occasionally  develops  such 
strength  that  business  is  interrupted  and  people  forced  to  seek 
shelter.  This  is  sometimes  the  case  at  Valparaiso.  Along  cer- 
tain coasts  fishermen  put  to  sea  in  the  early  morning  with  the 
land-breeze,  and  return  at  night  with  the  sea-breeze.  Land- 
and  sea-breezes  will  be  referred  to  again  in  connection  with  atmos- 
spheric  circulation. 

2.  Monsoons.     Some  lands  near  the  sea  become  so  much  heated 
in  summer  that  the  sea  (from-sea)  wind  continues  during  the  hot 
season,  not  merely  through  the  hot  part  of  the  day,  while  the 
land  (from-land)  wind  holds  sway  during  the  winter.     This  is  the 
case,  for  example,  in  India.     Such  winds,  which  change  their  direc- 
tions with  the  seasons,  are  monsoon  winds. 

The  monsoon  winds  of  the  Indian  Ocean  exerted  a  great 
influence  on  the  early  trade  of  India.  European  sailing-vessels 
formerly  timed  their  outward  voyages  so  as  to  take  advantage  of 
the  southwest  monsoon,  and  their  return  voyages  so  as  to  take 
advantage  of  the  northeast  monsoon. 

3.  Mountain  and  valley  breezes.     At  night  the  air  of  the  moun- 
tain tops  becomes  cold,  because  of  its  prompt  radiation  of  heat. 
It  thus  becomes  denser  than  the  air  below,  and  descends,  giving  the 
mountain  (that  is,from-mountain)  breeze.   The  downward  flow  (or 
blow)  of  the  air  is,  however,  not  confined  to  mountain  valleys,  but 
affects  the  slopes  of  the  mountains  generally.     In  the  morning, 
especially  on  sunny  days,  the  air  next  the  land  becomes  heated, 
and  most  at  the  lower  levels.     It  therefore  expands  upward,  and 
expands  more  over  the  lowland  than  over  the  mountains.     The 


TEMPERATURE  OF  THE  AIR  563 

result  is  that  air  moves  toward  the  mountains  at  this  time  of  day, 
and  strikes  them  at  such  an  angle  as  to  be  deflected  upward.  This 
is  the  valley  (really  toward-mountairi)  breeze.  There  is  also  an  up- 
ward movement  of  the  ah*  from  the  mountain  slope.  This  air 
tends  to  go  straight  up,  but  is  crowded  over  against  the  mountain 
by  the  mountain-ward  movement,  and  so  strengthens  the  moun- 
tain breeze.  At  the  top  of  the  mountain,  horizontal  winds  take 
away  the  air  which  tends  to  accumulate  there  as  a  result  of  the 
valley  breeze. 

Mountain  and  valley,  and  land  and  sea  breezes,  and  monsoon 
winds,  are  not  the  only  ones  due  to  differences  of  atmospheric 
temperature,  but  they  afford  the  simplest  illustrations  of  air  move- 
ments due  to  this  cause. 

Vertical  movements  and  temperature.  It  has  been  stated 
already  that  when  air  rises  it  expands,  and  that  as  it  expands  it 
becomes  cooler;  and,  conversely,  that  when  air  descends  it  becomes 
denser  and  warmer.  These  changes  of  temperature  have  an  im- 
portant bearing  on  the  condensation  and  precipitation  of  atmos- 
pheric moisture,  and  will  be  considered  in  connection  with  that 
topic. 


CHAPTER  XV 
THE   MOISTURE   OF   THE  AIR 

THE  atmosphere  always  contains  some  water  vapor,  even  in 
the  driest  deserts.  We  can  neither  see  nor  smell  water  vapor,  nor 
can  we  recognize  it  as  such  by  feeling,  though  air  with  much  water 
vapor  has  a  different  feeling  from  air  with  little. 

The  presence  of  moisture  in  the  air,  under  ordinary  conditions, 
is  proved  by  various  familiar  phenomena.  If  a  pitcher  of  ice 
water  is  allowed  to  stand  in  a  warm  room,  drops  of  water  often 
appear  on  the  outside  of  it.  This  water  could  have  come  only 
from  the  air.  Again,  if  a  vessel  of  warm  air  be  closed  and 
its  temperature  lowered  sufficiently,  the  inside  of  the  vessel  will 
become  coated  with  droplets  of  water.  The  amount  of  reduction 
of  temperature  necessary  to  bring  about  this  result  is  great  or  slight, 
according  as  the  amount  of  water  vapor  in  the  air  is  small  or  large, 
or,  more  exactly,  it  depends  upon  the  amount  which  is  in  the  air 
compared  with  that  which  the  space  occupied  by  the  air  is  capable 
of  holding.  Water  vapor  sometimes  condenses  into  water  in  the 
air,  and  then  becomes  visible  as  clouds  or  fog. 

Water  vapor  may  be  looked  upon  as  an  atmosphere  by  itself, 
for  it  is  distributed  very  much  as  it  would  be  if  there  were  no  other 
atmosphere.  Water  vapor  is  five-eighths  as  dense  as  dry  air;  that 
is,  a  cubic  foot  of  water  vapor  would  weigh  five-eighths  as  much 
as  a  cubic  foot  of  dry  air  under  the  same  conditions  of  temperature 
and  pressure.  The  water  vapor  of  the  air  displaces  some  of  the 
oxygen,  nitrogen,  etc.,  and  so  makes  the  air  lighter. 

Function  of  atmospheric  moisture.  The  function  of  the  mois- 
ture in  the  atmosphere  is  a  most  important  one.  Without  it  no 
life  could  exist.  In  addition  to  furnishing  the  rain,  the  snow,  and 
all  the  water  upon  which  land  life  depends,  it  serves  a  most  im- 
portant function  in  connection  with  temperature,  as  already  in- 

564 


THE  MOISTURE  OF  THE  AIR  565 

dicated.  It  appears  to  be  the  most  important  constituent  of  the 
air  in  the  absorption  both  of  the  heat  radiated  from  the  sun  and 
of  that  radiated  from  the  earth.  It  increases  the  average  tem- 
perature at  the  bottom  of  the  atmosphere  and  it  reduces  the 
extremes  of  heat  and  cold  which  would  exist  if  the  air  were 
altogether  dry. 

Sources  of  water  vapor:  evaporation.  It  is  a  familiar  fact 
that  a  moist  surface  exposed  to  the  air  soon  becomes  dry,  and  that 
water  left  standing  in  an  open  dish  will  presently  disappear.  Any 
fluid,  such  as  ink,  which  contains  much  water,  also  dries  up  if 
left  uncovered.  These  familiar  experiences  illustrate  what  is 
taking  place  all  the  time  wherever  moist  surfaces  are  exposed  to 
the  air.  They  are  constantly  losing  water  to  the  atmosphere. 
We  conclude  therefore  that  the  water  vapor  of  the  air  is  being  derived 
constantly  from  all  exposed  moist  surfaces.  The  conversion  of 
liquid  water  into  water  vapor  is  evaporation.  It  consists  of  the 
passage  of  molecules  from  the  surface  of  a  liquid  or  a  solid  into 
the  vaporous  condition.  The  molecules  of  a  liquid,  for  example, 
are  in  active  movement.  If  they  move  with  sufficient  velocity 
when  near  the  surface  of  the  liquid,  they  may  pass  out  of  the  range 
of  the  attraction  of  the  other  molecules  of  the  liquid,  in  which  case 
they  become  vapor. 

Evaporation  also  takes  place  from  land  surfaces  which  seem 
dry,  for  even  here  the  rock,  subsoil,  etc.,  beneath  the  surface  is 
more  or  less  moist,  and  moisture  is  continually  passing  from  be- 
neath up  into  the  atmosphere.  Evaporation  also  takes  place  from 
snow  and  ice,  even  though  the  temperature  is  far  below  that  of 
melting.  This  is  shown  by  the  fact  that  snow  and  ice  slowly  dis- 
appear in  a  temperature  below  32°  F.  A  wet  cloth,  put  into  a 
very  low  temperature,  say  0°  F.,  freezes  stiff;  but  if  it  remains  in 
the  same  temperature,  it  presently  becomes  dry.  The  ice  in  it  has 
evaporated. 

All  animal  respiration  also  furnishes  water  to  the  atmosphere. 
This  is  readily  shown  in  winter,  when  the  water  vapor  of  the  breath 
condenses,  and  so  becomes  visible,  in  the  cold  atmosphere.  The  water 
breathed  out  is  not  seen  in  summer,  or  in  a  warm  room,  because  it 
condenses  at  lower  temperatures  only.  Plants  also  breathe  out 
moisture,  and  the  amount  given  off  from  thrifty  growing  vegetation 
is  very  great.  Water  vapor  is  also  given  off  by  active  volcanoes 
(p.  368). 


566  PHYSIOGRAPHY 

On  the  whole,  water  surfaces  (oceans,  lakes,  rivers,  etc.)  yield 
more  water  vapor  than  equal  areas  of  land  surface.  The  oceans 
must  be  looked  upon  as  the  ultimate  source  of  most  of  the  water 
vapor.  But  for  this  great  reservoir,  the  waters  of  the  land  would 
presently  be  exhausted.  On  the  whole,  the  ocean  receives  water 
from  rivers,  springs,  and  rains  about  as  fast  as  it  loses  it  by  evapora- 
tion, so  that  the  amount  of  water  in  the  ocean  remains  nearly  con- 
stant from  year  to  year. 

On  the  average,  30  to  40  inches  of  rain-water  are  estimated  to 
fall  from  the  air  each  year  on  land;  that  is,  enough  to  make  a  layer 
30  to  40  inches  deep  if  spread  over  all  the  land.  The  amount  of  water 
evaporated  each  year  must  be  about  the  same  as  the  amount  which 
is  precipitated.  If  the  precipitation  on  the  oceans  is  equal  to  that 
on  the  lands,  square  mile  for  square  mile,  and  if  all  were  taken  from 
the  oceans  and  not  returned,  the  oceans  would  be  dried  up  in  less 
than  4000  years,  or,  according  to  the  larger  figure  (40  inches),  in  less 
than  3000  years.  If  this  amount  of  water  were  evaporated  from  the 
lakes  of  the  earth,  it  would  probably  exhaust  them  in  less  than  one 
year. 

The  energy  necessary  to  evaporate  this  amount  of  water  is  very 
great.  Assuming  that  the  average  amount  of  rainfall  is  60  inches 
instead  of  30,  Strachey  has  estimated  that  the  energy  necessary  to 
evaporate  this  amount  of  water  and  lift  it  3000  feet  (the  average 
height  from  which  rain  falls)  would  be  equal  to  300,000  million  horse- 
power constantly  in  operation. 

Rate  of  evaporation.  Fig.  558  shows  the  amount  of  evaporation 
in  inches  of  water  which  there  would  be  from  a  free  water  surface 
in  various  parts  of  the  United  States.  The  evaporation  is  seen  to 
be  highest  in  the  warmer  and  drier  parts  of  the  country. 

Several  conditions  affect  the  rate  of  evaporation.  The  principal 
ones  are  (1)  the  amount  of  water  vapor  in  the  atmosphere,  (2)  the 
temperature  of  the  atmosphere,  and  (3)  the  strength  of  the  wind. 

(1)  The  greater  the  amount  of  water  vapor  in  the  atmosphere, 
the  less  readily  does  new  vapor  form  and  rise  into  it.  The  pressure 
of  the  water  vapor  above  the  surface  from  which  evaporation  is 
taking  place  seems  to  be  the  controlling  factor.  If  it  is  sufficiently 
great  there  will  be  no  evaporation,  at  least  in  the  sense  that  there 
will  be  no  increase  of  water  vapor  in  the  air.  Such  evaporation 
as  takes  place  will  be  balanced  by  condensation  on  the  evaporating 
surface. 


THE  MOISTURE  OF  THE  AIR 


507 


568  PHYSIOGRAPHY 

(2)  Other  things  being  equal,  the  warmer  the  water  surface  the 
faster  the  evaporation.    This  is  illustrated  by  familiar  experiences. 
Water  on  a  hot  stove  evaporates  sooner  than  water  in  a  cool  place, 
and   water   in  the  sun  evaporates,  in  general,  much  more  rapidly 
than  in  the  shade. 

(3)  The   stronger  the  wind   the  more  rapid   the  evaporation. 
The  reason  appears  to  be  as  follows:  When  the  air  is  still,  the  space 
just  above  a  body  of  water  or  a  moist  surface  becomes  well  charged 
with  water  vapor,  and  this  tends  to  retard  evaporation;   but  when 
the  air  is  in  movement,  the  water  vapor  is  carried  away  about  as 
fast   as   it   is  formed,  so  that  new  and  often  drier  air  continually 
comes  in  over  the  surface  from  which  evaporation  is  taking  place. 
If  the  water  vapor  formed  were  moved  away  as  rapidly  by  some 
other  means,  evaporation  wrould  go  on  just  as  readily  as  when  the 
wind  blows. 

(4)  Evaporation  is  also  influenced  by  pressure  of  the  air,  being 
diminished  slightly  by  increase  of  pressure. 

The  function  of  the  atmosphere  in  evaporation.  The  air  influ- 
ences evaporation  by  its  movement,  as  just  noted,  and  also  because 
it  affects  the  temperature  above  the  land  and  water  (p.  526) ;  but 
evaporation  is  not  dependent  on  the  air.  It  would  go  on  in  a  vacuum 
at  a  given  temperature  rather  more  rapidly  than  in  air  at  the  same 
temperature. 

Evaporation  takes  up  heat.  Evaporation  cools  the  surface  from 
which  it  takes  place.  If  the  hand  be  moistened,  it  feels  cool  as  it 
dries,  and  the  faster  the  evaporation,  as  when  the  wind  blows,  the 
more  distinct  is  the  cooling.  This  is  why  moist  clothing  seems  cooler 
in  the  wind  than  in  still  air,  even  when  the  temperature  is  the  same. 
It  takes  about  1000  times  as  much  heat  to  evaporate  a  given  amount 
of  water  as  it  would  take  to  raise  its  temperature  1°  F.  The  evap- 
oration from  forested  regions  in  moist  tropical  lands  is  so  great  that 
the  temperature  there  is  often  much  lower  than  \vould  be  expected 
from  the  insolation. 

Amount  of  water  vapor  in  the  air.  The  amount  of  water  vapor 
in  the  air  varies  greatly  from  place  to  place,  and  even  in  the  same 
place  from  time  to  time.  Various  attempts  have  been  made  to 
estimate  the  amount  in  the  air  at  one  time,  but  the  results  are  far 
apart.  Its  amount  has  been  estimated  as  high  as  1  per  cent,  of 
the  weight  of  the  atmosphere  (p.  512).  This  would  be  equivalent 
to  about  4  inches  of  water  if  it  were  precipitated.  This  is  probably 


THE  MOISTURE  OF  THE  AIR 


569 


much  above  the  average  amount,  though  very  warm  air  may  con- 
tain locally  much  more  than  1  per  cent,  of  water. 

The  following  table  (p.  570)  presents  an  estimate  of  the  amount 
of  water  vapor  which  the  lower  part  of  the  atmosphere  is  capable 
of  holding  under  different  conditions  of  temperature.  Since  only 
about  one  twenty-fifth  of  the  water  vapor  is  above  30,000  feet,  this 
table  shows  about  twenty-four  twenty-fifths  of  all  the  atmosphere 
may  hold  at  these  temperatures. 

Some  idea  of  its  amount  is  gained  in  another  way.  One  cubic  foot 
of  air  at  0°  F.  is  capable  of  containing  one  half  grain  of  water  vapor, 
at  60°  F.,  5  grains,  and  at  80°  F.,  11  grains.  The  weight  of  air  in  a 
room  40X40X15  feet,  at  a  temperature  of  60°  F.  and  under  ordi- 
nary pressure,  is  about  1800  pounds.  The  weight  of  water  it  is 
capable  of  containing  is  nearly  20  pounds.  This  would  nearly  fill 
a  common  water-pail. 

Distribution  of  water  vapor.  So  soon  as  the  water  vapor  passes 
into  the  air  it  is  distributed,  partly  by  winds  and  partly  by  diffu- 
sion. Evaporation  at  one  point,  therefore,  tends  to  moisten  the 
air  everywhere,  though  first  and  most  in  the  region  where  the  evapo- 
ration takes  place. 

The  amount  of  water  vapor  in  the  air  diminishes  rapidly  upward, 
largely  because  of  the  low  temperature,  as  shown  in  the  following 
table: 


Altitude. 

Water  vapor. 

Air  density. 

Feet. 

0 

1.00 

1.00 

13,000  + 

0.24 

0.61 

30,000- 

0.04 

0.32 

Atmospheric  moisture  and  atmospheric  movements.  Since 
water  vapor  makes  the  air  lighter,  and  since  movements  result  when 
the  air  of  one  place  becomes  lighter  than  that  of  another,  inequality 
in  the  amount  of  moisture  in  the  air  in  different  places  is  a  cause  of 
atmospheric  movement. 

Saturation.  The  amount  of  water  vapor  in  the  air  at  any  place 
at  any  time  depends  on  the  temperature  and  on  the  available  sup- 
ply of  water.  The  warmer  the  air  the  more  the  moisture  which  it 
can  hold. 

When  there  is  all  the  water  vapor  in  the  air  which  is  possible 
at  a  given  temperature,  the  air  is  said  to  be  saturated.  It  is  cus- 


570 


PHYSIOGRAPHY 


ternary  to  speak  of  the  air  as  being  saturated;  yet  it  is  in  reality 
not  the  air  which  is  saturated,  but  the  space  which  the  air  occupies. 
The  amount  of  water  vapor  necessary  to  saturate  a  given  space 
depends  on  the  temperature  of  the  space,  and  is  essentially  the  same 
whether  air  is  present  or  not.  It  is  also  sometimes  said  that  the 
water  vapor  is  saturated.  In  spite  of  its  inaccuracy,  the  expression 
saturation  of  air  is  in  such  common  use  that  it  is  likely  to  be  retained. 
Humidity  and  Dew-point.  The  amount  of  moisture  which  the 
ah*  contains  is  its  absolute  humidity.  The  percentage  of  moisture 
which  air  contains  at  any  temperature,  in  comparison  with  what 
it  might  contain  at  that  temperature,  is  known  as  its  relative  humidity 
(Fig.  559).  When  air  contains  half  the  moisture  which  it  might 
contain  it  is  said  to  have  a  relative  humidity  of  50.  When  it  is 
saturated  with  moisture,  its  humidity  is  100.  Air  is  commonly  said 
to  be  "dry"  when  its  relative  humidity  is  low,  and  "moist"  when 
its  relative  humidity  is  high.  The  average  relative  humidity  of  air 
over  the  land  is  probably  about  60  per  cent.,  and  that  over  the  ocean 
about  85  per  cent.,  so  that  the  amount  of  water  actually  in  the  atmos- 
phere is  less  than  that  which  might  be  calculated  from  the  table. 
The  part  of  our  country  which  is  productive,  agriculturally,  without 
irrigation,  is  chiefly  where  the  relative  humidity  is  more  than  65. 


Height  of  column  of 

Depth  of  water  which  would  be  in  saturated  air  below  the  levelo 
indicated,  for  the  following  dew-points  at  sea-level. 

air  above  the 

ground. 

80°  F. 

70°  F. 

60°  F. 

50°  F. 

Feet. 

Inches. 

Inches. 

Inches. 

Inches. 

6,000 

1.3 

1.0 

0.7 

0.5 

12,000 

2.1 

1.5 

1.1 

0.8 

18,000 

2.5 

1.8 

1.3 

0.9 

24,000 

2.7 

2.0 

1.4 

1.0 

30,000 

2.8 

2.1 

1.5 

1.1 

The  relative  humidity  of  the  air  in  dry  regions  is  much  greater 
than  is  popularly  supposed.  It  is  rarely  so  low  as  half  that  of  regions 
moist  enough  to  be  productive.  Thus  at  Yuma,  Ariz.,  the  average 
relative  humidity  for  the  year  is  42.9  per  cent.,  with  a  mean  monthly 
minimum  of  34.7  per  cent.  The  corresponding  figures  for  Santa  Fe 
are  44.8  and  28.7;  for  Pueblo,  46.2  and  37.6.  In  Death  Valley, 
Cal.,  the  average  relative  humidity  for  five  months  was  23. 

Any  reduction  of  temperature  of  saturated  air  causes  some 
of  its  moisture  to  be  condensed.  The  temperature  at  which  air 


THE  MOISTURE  OF  THE  AIR 


571 


572 


PHYSIOGRAPHY 


begins  to  allow  its  water  vapor  to  condense  is  the  dew-point.  Air 
which  is  saturated  is  therefore  at  the  dew-point.  It  will  be  seen 
that  the  dew-point  is  not  a  fixed  temperature,  but  is  influenced  by 
the  amount  of  water  vapor  in  the  air.  When  this  amount  is  large, 


Mdt 


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FIG.  560. — Graph  showing  the  relations  between  temperature,  wind  velocity, 
and  humidity,  at  Blue  Hill  Observatory,  Massachusetts.  (Cox,  U.  S. 
Weather  Bureau.) 

the  temperature  of  the  dew-point  is  relatively  high;  when  the  amount 
is  small,  the  temperature  of  the  dew-point  is  relatively  low. 

Air  may  be  brought  to  the  dew-point  in  various  ways:  (1)  It 
may  be  carried  (by  wind)  where  the  temperature  is  lower,  as  to 
a  higher  latitude  or  altitude;  (2)  it  may  be  cooled  by  having  cooler 
air  brought  to  it,  as  by  a  cold  wind;  (3)  it  may  be  cooled  by  radia- 
tion, or  (4)  by  expansion. 

Condensation.  When  the  temperature  of  condensation  is  above 
32°,  the  vapor  condenses  into  visible  water,  which  usually  takes 
the  form  of  little  droplets.  If  the  temperature  of  condensation  is 
less  than  32°,  the  water  crystallizes  as  it  condenses,  and  takes  the 
form  of  ice  particles. 

Condensation  and  temperature.  When  the  water  vapor  of  the 
air  is  condensed,  an  amount  of  heat  equal  to  that  absorbed  in  its 
evaporation  is  set  free.  This  is  why  rising  moist  air  is  not  cooled 


THE  MOISTURE  OF  THE  AIR  573 

so  rapidly  as  rising  dry  air  (p.  537).  Dry  air  is  cooled  about  1°  F. 
for  every  183  feet  of  rise,  but  saturated  air  at  68°  F.  must  rise  nearly 
twice  as  much  to  be  cooled  1°  F.  This  slower  rate  of  cooling  is 
because  of  the  heat  set  free  by  the  condensation  of  moisture. 

Dew  and  frost.  It  sometimes  happens  that  the  temperature  of 
the  surface  of  the  land,  or  of  the  objects  upon  it,  becomes  lower 
than  the  temperature  of  the  surrounding  air.  This  is  especially 
likely  to  be  the  case  in  the  clear  nights  of  late  summer  and  autumn. 
If  the  temperature  of  the  grass  blades,  for  example,  becomes  lower 
than  the  dew-point  of  the  surrounding  air  at  night,  moisture  from 
the  surrounding  air  will  be  condensed  on  them.  Such  moisture  is 
dew.  Dew  does  not  fall,  but  condenses  on  the  surface  of  solid  ob- 
jects. A  good  illustration  of  dew  is  often  furnished  by  the  moisture 
which  gathers  on  the  outside  of  a  pitcher  of  ice-water  on  a  summer's 
day.  The  temperature  of  the  pitcher  is  below  the  dew-point  of  its 
surroundings,  and  moisture  from  the  air  therefore  condenses  on  it. 
Dew  forms  on  clear  nights  rather  than  cloudy  ones,  because  the  heat 
of  the  day  is  radiated  more  readily  from  the  land  and  the  bottom  of 
the  air  when  there  are  no  clouds.  Dew  is  formed  on  still  nights 
more  than  on  windy  ones,  because  the  wind  tends  to  move  away  the 
air  which  is"  approaching  its  dew-point,  supplying  other  air  in  its 
place,  and  the  incoming  air  is  often  warmer  than  that  moved  on. 

When  the  temperature  of  the  dew-point  is  below  32°  F.,  the 
moisture  which  condenses  on  solid  objects  condenses  as  frost  in- 
stead of  dew.  Frost  is  not  frozen  dew,  any  more  than  snow  is 
frozen  rain.  It  stands  in  the  same  relation  to  dew  that  snow  does 
to  rain.  In  the  autumn,  frost  is  more  likely  to  occur  in  valleys  and 
on  low  flats  than  on  adjacent  hills,  because  the  colder  air  settles  to 
the  lower  levels. 

Dew,  and  sometimes  frost,  may  form  on  the  under  sides  of  ob- 
jects. If  a  pan  be  placed  bottom  up  on  the  ground,  there  will  be 
dew  on  the  inside  of  it  in  the  morning  as  often  as  on  the  outside. 
There  is  often  dew  on  the  under  side  of  a  flat  stone  when  there  is 
none  on  its  top.  Even  in  a  desert,  a  rubber  blanket  spread  on 
the  ground  at  night  will  often  be  wet  on  the  under  side  in  the 
morning.  The  explanation  is  as  follows:  The  air  in  the  ground  has 
some  moisture.  During  the  day,  when  the  sun  shines,  this  air  is 
warmed.  At  night,  the  air  above  cools  much  more  quickly  than 
the  air  in  the  ground.  The  cooler  heavier  air  above  then  sinks 
into  the  ground,  displacing  and  crowding  up  the  warmer  air  below 


574 


PHYSIOGRAPHY 


with  its  water  vapor.  On  reaching  the  cool  pan  or  the  cool  rubber 
blanket,  some  of  the  moisture  is  condensed.  If  the  air  in  the 
ground  had  more  moisture  than  that  above  ground,  water  vapor 
would  pass  up  from  below,  even  if  the  air  with  which  it  is  associated 
did  not.  IK  the  daytime  the  rising  moisture  would  not  condense 
on  the  pan  or  blanket,  because  they  would  be  warmer  than  the 
water  vapor  from  below,  if  the  sun  were  shining;  but  at  night 
their  temperature  may  be  low  enough  to  cause  condensation. 

Clouds  and  fog.    The  water  droplets  and  the  ice  particles  con- 
densed  from  the  water  vapor  of  the  air  take  the  form  of  clouds 


FIG.  561. — Fog  streaming  in  from  the  Pacific  Ocean. 
(U.  S.  Weather  Bureau.) 


Coast  of  California. 


if  the  condensation  takes  place  without  precipitation  above  th- 
bottom  of  the  atmosphere,  and  the  form  of  fog  (above  32°  F.)  or 
frost  (below  32°  F.)  if  in  the  lower  part  of  the  atmosphere.  Fogs 
and  air-frost  are  the  same  as  clouds,  except  that  the  former  are 
lower.  Fog  is,  indeed,  but  a  cloud  resting  on  the  surface  of  the 
land.  If  moisture  condenses  and  the  particles  remain  suspended 
in  the  air  about  the  top  of  a  mountain,  there  is,  to  the  observer  on 
the  plain  or  in  the  valley  below,  a  cloud  about  the  mountain;  but 
if  the  observer  were  to  climb  up  into  the  cloud,  it  would  then  appear 
to  be  fog.  Fogs  are  often  formed  when  the  warmer  air  over  a  lake 
in  autumn  blows  over  the  colder  land,  or  when  the  air  over  warmer 


THE  MOISTURE  OF  THE  AIR 


575 


water  from  one  part  of  the  ocean  (e.g.,  a  warm  ocean  current)  blows 
over  colder  water.  They  also  often  form  in  valleys  at  night,  espe- 
cially in  autumn,  when  the  night  temperatures  are  much  lower  than 


FIG.  562. — Morning  fog  over  vallevs.     Mount  Tamalpais,  Cal. 
(U.  S.  Weather  Bureau.) 

those  of  the  day.     The  cooler  air  settles  in  the  valleys,  which  are 
therefore  more  likely  to  have  fogs  than  the  uplands  are. 

Fogs  occasionally  lead  to  shipwreck  on  sea,  and  interrupt  busi- 
ness operations  on  land.  A  persistent  and  dense  fog  in  London, 
December  10  to  17,  1905,  was  estimated  to  have  cost  the  city 


FIG.  563. — Fog  waves.     Coast  of  California.     (U.  S.  Weather  Bureau.) 

$1,750,000  per  day  in  one  way  and  another,  largely  through  suspen- 
sion of  business.  Such  estimates  are,  however,  to  be  taken  with 
reserve;  since  much  of  the  suspended  business  is  transacted  later. 


576  PHYSIOGRAPHY 

A  heavy  fog  facilitated  Washington's  retreat  to  New  York  after 
the  battle  of  Long  Island. 

The  droplets  of  water  in  clouds  and  fogs  must  be  very  small  to 
remain  suspended  in  the  air.     It  has  been  estimated  that  they  are 


FIG.  564. — Fog  rising  and  turning  to  cloud.     Mount  Tamalpais,  Cal. 
(U.  S.  Weather  Bureau.) 

often  about  1/3000  of  an  inch  in  diameter,  but  there  is  doubtless 
great  variation. 

Clouds  also  affect  temperature  by  hindering  radiation.  In  gen- 
eral, cloudiness  lowers  the  summer  temperatures  of  intermediate 
latitudes,  raises  their  winter  temperatures,  and  gives  them  a  higher 
average  temperature. 

Forms  of  clouds.  Clouds  assume  many  forms.  Among  the 
more  common  are  the  cumulus,  the  stratus,  the  nimbus,  and  the 
cirrus  clouds.  Between  these  more  distinct  forms  there  are  all 
gradations,  giving  rise  to  the  names  cirro-cumulus,  cirro-stratus, 
cumulo-stratus,  etc. 

Cumulus  clouds  are  thick  clouds,  the  upper  surfaces  of  which 
are  more  or  less  dome-shaped,  with  irregular  and  fleecy  protuber- 
ances. Their  bases  are  nearly  horizontal.  They  appear  to  be 
formed  by  ascending  convection  currents,  and  their  plane  bases  seem 
to  mark  the  level  at  which  condensation  takes  place  as  the  air  rises. 
They  appear  especially  in  clear,  hot  weather,  and  most  commonly 
begin  to  form  in  mid  or  late  forenoon,  after  insolation  has  estab- 


THE  MOISTURE  OF  THE  AIR 


577 


lished  convection  currents.  They  grow  as  the  heat  of  the  day 
increases,  and  normally  attain  their  greatest  size  at  about  the  hour 
of  maximum  heat.  As  evening  approaches,  they  commonly  grow 
smaller.  They  are  frequently  dissipated  before  sundown,  but 


FIG.  565. 


FIG.  566. 


FIG.  565. — Cumulus  (wool-pack)  clouds.  (Photo,  from  Cloud  Chart,  Hydro- 
graphic  Office,  Dept.  of  the  Navy.) 

FIG.  566. — Cumulus  clouds  of  the  fair-weather  type.  (U.  S.  Weather 
Bureau.) 


FIG.  567. 


FIG.  568. 


FIG.   567. — Spring  cumulus  clouds  of  the   rain  type.        (U.   S.   Weathei 

Bureau.) 
FIG.    568. — Cumulus  clouds  at  Santa  F6,  New  Mexico.     (U.  S.  Weather 

Bureau.) 

sometimes  pass  into  other  forms  of  cloud  (Figs.  565-573  and  Fig. 
576). 

Stratus  clouds  are  horizontal  sheets  of  lifted  fog.  When  the 
sheet  is  broken  by  wind  or  mountains,  it  is  sometimes  called  fracto- 
stratus. 


578 


PHYSIOGRAPHY 


Nimbus  or  rain-clouds  consist  of  thick  layers  of  dark  clouds 
without  definite  shape,  and  with  ragged  edges,  from  which  con- 
tinued rain  or  snow  generally  falls  (Fig.  573). 


FIG.  569.  FIG.  570. 

FIG.  569. — Cumulus  clouds;    thunder-heads  in  process  of  active  growth. 

(U.  S.  Weather  Bureau.) 
FIG.  570. — Tumbled  cumulus  clouds.     (U.  S.  Weather  Bureau.) 


FIG.  571.  FIG.  572. 

FIG.  571. — Cumulus  clouds,  broken  and  wind-torn.     (U.  S.  Weather  Bureau.) 
FIG.  572. — Alto-cumulus  clouds,  wave  form.      (U.  S.  Weather  Bureau.) 

Cirrus  clouds  are  detached,  delicate,  and  fibrous.  They  are 
often  described  as  having  the  form  of  feathers.  They  are  generally 
white,  and  sometimes  arranged  in  belts.  They  are  usually  high  and 
thin,  and  often  of  particles  of  snow  or  ice  (Figs.  574-576). 


THE  MOISTURE  OF  THE  AIR 


579 


Precipitation.  The  condensation  of  the  water  vapor  of  the  air 
leads  to  rain,  snow,  or  hail,  if  the  products  of  condensation  fall. 
Whether  precipitation  really  takes  place  after  the  formation  of 
clouds  depends  on  many  conditions.  To  give  rain  or  snow,  the 


FIG.  573.  FIG.  574. 

FlG.   573. — Cumulo-nimbus    clouds.        (From    Cloud    Chart,    Hydrographic 

Office,  Dept.  of  the  Navy.) 
FIG.  574. — Cirrus  clouds.     (From  Cloud  Chart,  Hydrographic  Office,  Dept. 

of  the  Navy.) 


FIG.  575.  FIG.  576. 

FIG.  575. — Cirro-stratus  clouds.      (U.  S.  Weather  Bureau.) 
FIG.  576. — Cirro-cumulus  clouds;  mackerel  sky.      (U.  S.  Weather  Bureau.) 

particles  of  water  or  snow  in  the  cloud  must  be  heavy  enough  to 
fall;  and  if  they  are  to  reach  the  bottom  of  the  atmosphere,  they 
must  not  pass  through  air  which  is  dry  enough  and  warm  enough 
to  evaporate  them  before  they  reach  the  bottom  of  the  atmosphere. 


580  PHYSIOGRAPHY 

Whether  precipitation  takes  the  form  of  rain  or  snow  depends  not 
only  on  the  temperature  of  condensation,  but  also  on  the  tempera- 
ture of  the  air  over  the  place  where  the  precipitation  takes  place. 
Precipitation  which  starts  as  snow  may  become  water  before  it 
reaches  the  bottom  of  the  air.  It  often  snows  on  a  mountain  when 
it  rains  in  the  valley  below.  Precipitation  which  starts  as  water 
rarely  freezes  as  it  descends,  though  some  hail  may  be  regarded  as 
frozen  rain. 

Since  condensation  follows  cooling,  and  since  precipitation  fol- 
lows condensation,  sufficient  cooling  (below  dew-point)  of  the  air 
may  cause  precipitation.  It  follows  that  there  may  be  rain  (or 
snow)  (1)  when  air  is  blown  up  a  cold  mountain-side,  (2)  when  it  is 
blown  poleward  (or,  in  general,  from  a  warmer  to  a  cooler  place) 
without  rising,  (3)  when  it  rises  by  convection,  both  because  (a)  it 
is  cooled  by  being  brought  to  cooler  air,  and  (6)  because  it  expands ; 
(4)  when  cooler  air  is  brought  to  warmer  air.  Rains  due  to  (1)  are 
not  rare  in  mountain  regions,  and  rains  due  to  (3)  are  common 
where  convection  currents  are  strong,  as  in  the  region  of  tropical 
calms,  where  precipitation  occurs  almost  daily  during  the  hottest 
part  of  the  day. 

The  distribution  of  rainfall  is  dependent,  in  large  measure,  on  the 
winds,  and  will  be  considered  later. 

Rain-making.  Various  attempts  have  been  made  to  produce 
rain  by  means  which  may  be  called  artificial.  The  methods  em- 
ployed  have  been  various,  but  the  results  have  been  uniformly 
unsuccessful.  The  method  most  tried  has  been  that  of  producing 
explosions  of  one  sort  or  another  in  the  air  well  above  the  land- 
If  there  were  cloud  particles  in  abundance  in  the  air,  such  dis- 
turbances  might  perhaps  have  the  effect  of  causing  them  to  unite 
and  so  to  become  large  enough  to  fall;  but  the  amount  of  rain- 
fall which  can  be  thus  produced,  under  the  most  favorable 
conditions,  is  probably  too  small  to  be  of  consequence.  Other 
methods  which  have  been  tried  or  suggested  seem  equally  futile. 

Summary.  The  air  is  constantly  taking  up  moisture  from  all 
moist  surfaces.  This  moisture,  in  the  form  of  invisible  vapor,  is 
diffused  and  blown  everywhere.  When  it  reaches  a  temperature 
which  is  low  enough  (the  dew-point),  the  moisture  is  condensed. 
If  it  condenses  in  the  upper  air,  it  may  fall  as  rain  or  snow,  or  it 
may  remain  suspended  in  the  air  in  the  form  of  a  cloud  and  be 
evaporated  again.  If  it  condenses  on  the  surface  of  solid  objects 


THE  MOISTURE  OF  THE  AIR  581 

at  the  bottom  of  the  atmosphere,  it  forms  dew  or  frost.  Water 
vapor  is  thus  in  constant  circulation,  and  all  land  life  depends  upon 
it.  Some  of  the  water  which  is  precipitated  out  of  the  atmosphere 
falls  on  the  surface  from  which  it  was  evaporated,  but  much  of  it 
falls  in  places  far  distant  from  those  whence  it  was  evaporated. 


CHAPTER  XVI 
ATMOSPHERIC   PRESSURE 

THAT  the  air  is  substantial  and  has  weight  is  shown  by  the 
familiar  phenomena  cited  on  page  506.  Its  downward  pressure 
or  weight  has  already  been  stated  to  be,  on  the  average,  nearly 
15  pounds  to  the  square  inch  at  sea-level.  Differences  in  atmos- 
pheric pressure  are  the  primary  cause  of  atmospheric  movements,  or 
winds,  and  winds  are  of  so  much  significance,  in  one  way  and  an- 
other, that  it  is  convenient  to  have  some  standard  method  of 
measuring  and  recording  atmospheric  pressures. 

The  pressure  of  the  atmosphere  is  measured  by  the  barometer. 

The  barometer.  The  principle  of  the  ordinary  barometer  is 
as  follows:  A  tube  more  than  30  inches  long,  closed  at  one  end,  is 
filled  with  mercury,  and  the  tube  is  then  placed,  open  end  down, 
in  a  dish  of  mercury  (Fig.  577).  The  mercury  in  the  tube  will 
sink  until  its  upper  surface  reaches  a  level  about  30  inches  above 
the  level  of  the  mercury  in  the  dish,  if  the  place  of  the  experiment 
be  near  sea-level.  The  mercury  remains  at  this  level  in  the  tube 
because  the  pressure  of  the  air  on  the  mercury  in  the  dish  is  suf- 
ficient to  balance  the  downward  pressure,  or  weight,  of  the  mercury 
in  the  tube.  Since  the  pressure  of  the  air  at  sea-level  holds  the 
mercury  in  the  tube  up  about  30  inches  or  760  millimetres,  the 
pressure  of  the  air  at  sea-level  is  said  to  be  30  inches  or  760  milli- 
metres. If  the  atmospheric  pressure  becomes  less,  the  mercury 
in  the  tube  falls,  and  if  the  atmospheric  pressure  becomes  greater, 
the  mercury  in  the  tube  rises. 

At  elevations  above  sea-level  the  pressure  becomes  less  be- 
cause more  of  the  air  is  left  below,  and  the  higher  the  ascent,  the 
less  the  pressure,  as  shown  in  the  following  table. 

582 


ATMOSPHERIC  PRESSURE 


583 


Altitude  above  Barometric  pressure 

sea-level  in  feet.  in  inches. 

0 30 

1,800 28 

3,800 26 

5,900 24 

8,200 22 

10,600 ; 20 

13,200 18 

16,000 16 

Altitude  above  sea-level  may  be  measured  by  means  of  baro- 
metric pressure;  but  since  mercurial  barometers  are  not  conve- 
niently carried  and  are  easily  broken,  another 
form  of  barometer,  the  aneroid  barometer,  has 
been  devised  for  this  purpose. 

Air  pressures  unequal.  The  general  facts  set 
forth  in  previous  chapters  make  it  clear  that  the 
pressure  of  the  atmosphere  must  vary  from  point 
to  point,  and  from  time  to  time  at  the  same  point. 
Some  of  the  reasons  are  as  follows: 

1.  The  temperature  of  the  surface  on  which 
the  air  rests  is  unequal,  and  increase  of  tempera- 
ture makes  the  air  lighter.    Not  only  this,  but  the 
temperature  in  a  given  place  varies  from  time  to 
time.     It  follows  that  the  pressure  of  the  air  at  a 
given  place  varies  from  time  to  time. 

2.  A  cubic  foot  of  dry  air  at  a  temperature  of 
68°  weighs  523.72  grains  under  a  pressure  of  30 
inches.     A  cubic  foot  of  saturated  air  under  the 
same  conditions  weighs  4.26  grains  less  (p.  564). 
On  the  whole,  the  amount  of  moisture  in  the  air  is 
greater  in  warm  regions  than   in  cold  ones,  and 
greater  over  the   sea  and  moist  lands  than  over 
dry  regions.     Since  the  amount  of  moisture  in  the 
air  at  a  given  place  varies  from  time  to  time,  the 
pressure  is  being  constantly  disturbed. 

If  temperature  and  moisture   were   the   only 


IG.  577.  — Dia- 
gram to  illus- 
trate the  princi- 
ple of  the  bar- 
o  m  e  t  e  r.  The 
pressure  of  the 
air  at  A  main- 
tains the  mer- 
cury at  /  in  the 
tube  when  there 
is  no  air  in  the 
tube  above  B. 


•factors  controlling  air  pressure,  it  should  be  least  in  low  latitudes, 
where  it  is  warmest  and  where  there  is  abundant  moisture.  In 
other  words,  it  should  be  least  where  the  isotherms  are  highest, 


584  PHYSIOGRAPHY 

especially  over  moist  regions,  and  greatest  in  cold  regions,  where 
the  air  is  relatively  dry.  Since  the  distribution  of  atmospheric 
pressure  does  not  correspond  with  these  general  rules,  as  we 
shall  see,  and  since  changes  of  pressure  in  a  given  region  take 
place  independently  of  changes  in  temperature  and  moisture  in 
that  region,  it  follows  that  factors  other  than  temperature  and 
moisture  influence  atmospheric  pressure. 

Representation  of  Pressure  on  Maps  and  Charts 

Isobars.  Lines  may  be  drawn  on  the  surface  of  the  earth  con- 
necting points  where  the  atmospheric  pressures  are  equal.  Such 
lines  are  isobars.  A  map  showing  lines  of  equal  pressure  is  known 
as  an  isobaric  map  or  chart .  An  isobaric  chart  for  the  year,  that  is, 
an  annual  isobaric  chart,  shows  isobars  connecting  points  having 
the  same  average  pressure  throughout  the  year.  There  may  be 
isobaric  charts  for  the  several  seasons  and  for  the  several  months, 
and  there  may  be  charts  for  any  shorter  period.  The  daily  weather 
maps  are  daily  isobaric  charts. 

Fig.  578  represents  an  isobaric  chart  for  the  year.  The  figures 
on  the  lines  indicate  the  average  pressure  for  the  year  in  inches. 
The  isobars  of  30  inches  or  more  are  full  lines;  those  of  less  than 
30  inches  are  dotted  lines.  A  few  suggestions  will  help  in  the  in- 
terpretation of  the  map.  In  the  southern  hemisphere,  the  isobar 
of  30  inches  encloses  a  belt  extending  almost  around  the  world. 
It  is  interrupted  only  in  the  vicinity  of  Australia.  Every  point 
within  the  area  enclosed  by  this  isobar  has  an  average  atmospheric 
pressure  of  more  than  30  inches.  Every  point  within  the  isobar 
of  30.10  inches  has  an  average  annual  pressure  of  more  than  30.10 
inches,  while  every  point  between  the  isobars  of  30.00  and  30.10 
has  an  average  annual  pressure  of  between  30.00  and  30.10  inches, 
etc.  Between  the  two  adjacent  isobars  of  29.90  in  the  equatorial 
part  of  the  Atlantic,  the  pressure  is  less  than  29.90,  but  not  so  low 
as  29.80.  If  the  pressure  sank  to  the  latter  figure,  there  would  have 
been  isobars  of  29.80  inches. 

It  will  be  noted  that  the  pressure  within  the  areas  enclosed  by 
the  isobars  of  30.10  in  the  South  Atlantic  is  more  than  30.10,  while 
the  pressure  between  the  adjacent  29.90  isobars  of  the  mid-Atlantic 
is  less  than  29.90.  In  explanation  of  the  difference,  it  is  to  be  noted 
that  as  the  30. 10  isobar  is  approached  from  without,  the  pressure  is 
increasing;  and  that  as  the  29.90  isobars  are  approached  from  pole- 


ATMOSPHERIC  PRESSURE 


585 


586 


PHYSIOGRAPHY 


ward,  the  pressure  is  decreasing.    By  the  application  of  this  principle 
the  interpretation  will  be  seen. 

In  the  interpretation  of  isobaric  charts  another  point  should  be 
understood.  The  pressure  of  the  atmosphere  diminishes  with  in- 
creasing elevation,  as  indicated  in  a  general  way  by  the  table  on 
page  583.  It  is  shown  in  greater  detail  in  the  following  table, 
which  shows  the  height  of  a  column  of  air,  at  different  temperatures, 
corresponding  to  0.1  of  an  inch  of  pressure: 

HEIGHT  OF  AN  AIR  COLUMN  CORRESPONDING  TO  0.1  OF  AN  INCH  BAROMETRIC 
PRESSURE,  AT  VARIOUS  TEMPERATURES. 


Air  Pressure   in 
Inches. 

Average  Temperature    in   Degrees  Fahrenheit. 

20° 

30° 

40° 

50° 

60° 

70° 

80° 

22  

Feet. 
116 
111 
106 
102 
98 
94 
91 
88 
85 

Feet. 
119 
114 
109 
105 
101 
97 
93 
90 
87 

Feet. 
122 
116 
111 
107 
103 
99 
95 
92 
89 

Feet- 
124 
119 
114 
109 
105 
101 
98 
94 
91 

Feet. 
127 
124 
116 
112 
107 
103 
100 
96 
93 

Feet. 
130 
124 
121 
114 
110 
106 
102 
98 
95 

Feet. 
132 
126 
121 
116 
112 
108 
104 
100 
97 

23    

24   

25..  

26  

27     

28  

29          

30  

If,  for  example,  one  ascends  95  feet  from  sea-level,  where  the 
temperature  is  70°  F.  and  the  pressure  30  inches,  the  pressure  of 
the  atmosphere  is  reduced  by  0.1  of  an  inch.  At  a  level  where  the 
pressure  is  but  28  inches  (1800  feet  above  sea-level;  see  p.  583),  102 
feet  of  ascent  would  be  necessary  to  reduce  the  pressure  0.1  of  an 
inch. 

It  will  be  recalled  that  the  temperatures  shown  oti  an  isothermal 
chart  are  not  those  actually  observed,  but  that  allowance  is  made 
for  altitude  above  sea-level.  Similarly,  the  pressures  shown  on  an 
isobaric  chart  are  not  those  actually  observed  on  the  land.  They 
are  the  pressures  which  would  exist  if  there  were  no  elevations 
above  sea-level.  Before  being  recorded  on  an  isobaric  chart,  the 
observed  atmospheric  pressure  of  a  place  95  feet  above  sea-level, 
when  the  temperature  is  70°  F.,  has  0.1  of  an  inch  added  to  it  if  the 
observed  pressure  was  30  inches.  If  the  temperature  were  lower, 
0.1  of  an  inch  would  be  added  for  a  lesser  height,  since  colder  air  is 
heavier.  Thus  at  a  temperature  of  40°  F.,  89  feet  of  rise 'makes  a 
difference  of  0. 1  of  an  inch  in  the  pressure  of  the  atmosphere. 


ATMOSPHERIC  PRESSURE 


587 


Isobaric  surfaces.  An  isothermal  surface  connects  places  having 
the  same  temperature.  So  an  isobaric  surface  connects  places  hav- 
ing the  same  pressure,  that  is,  the  same  amount  of  air  above.  If, 
for  example,  one  place  at  sea-level  has  a  pressure  of  30  inches,  and 
another  a  pressure  of  29.80  inches,  the  isobaric  surface  of  30  inches 
would  lie  beneath  sea-level  where  the  pressure  is  but  29.80  at  sea- 
level.  If  the  temperature  of  the  place  be  70°,  it  would  be  necessary 
to  descend  about  190  feet  below  sea-level,  at  the  place  where  the 


FIG.  579. — A  series  of  isobaric  lines  showing  diminishing  pressure  toward  the 

center. 


29.70" 
~  -28.80-"' 

--29.90- — ' 
-30.90 ' 


FIG.  580. — Section  through  the  area  represented  in  Fig.  579,  showing  the 
position  of  isobaric  surfaces.  As  the  pressure  toward  the  center  of  the 
area  shown  in  Fig.  579  diminishes,  the  isobaric  surface  bends  downward. 
It  will  be  seen  that  isobaric  lines  are  the  lines  where  isobaric  surfaces 
cut  sea-level.  " 

pressure  is  29.80  inches  at  the  surface,  to  find  the  level  where  the 
pressure  is  30  inches.  If  the  observed  pressure  at  another  place  at 
sea-level  be  30.10  inches,  the  isobaric  surface  of  30  inches  would 
rise  above  sea-level  there.  These  relations  are  shown  in  Figs.  579 
and  580.  The  former  is  a  series  of  isobaric  lines,  with  pressures 
varying  from  30.00  to  29.70  inches;  the  latter  is  a  vertical  sec- 
tion through  such  an  area,  to  show  the  isobaric  surfaces.  From 
these  figures  it  is  seen  that  the  isobaric  lines  (Fig.  579)  are  the  lines 
where  the  isobaric  surfaces  cut  the  plane  of  sea-level. 


588  PHYSIOGRAPHY 

If  a  surface  of  water  had  the  form  shown  in  Fig.  580,  the  water 
from  the  higher  parts  would  flow  to  the  lower  parts  until  the  sur- 
face became  level.  The  air,  which  is  more  fluid  than  water,  behaves 
in  a  similar  way,  and  moves  down  every  isobaric  surface  which 
has  slope.  Such  movements  are  winds.  When  the  isobaric  slope 
is  great,  or,  in  other  words,  when  the  isobaric  gradient  is  high,  the 
wind  is  strong;  when  the  isobaric  gradient  is  low,  the  wind  is 
gentle;  and  when  there  is  no  isobaric  gradient,  that  is,  when 
the  isobaric  surface  is  level,  there  is  no  wind.  The  strong  wind  is 
strong  for  much  the  same  reason  that  a  swift  river  is  swift;  the 
gentle  wind  is  gentle  for  much  the  same  reason  that  a  slow  river 
is  sluggish. 

Isobaric  charts  have  their  highest  value  in  showing  the  direc- 
tion and  the  strength  of  winds,  and  winds  are  determined  by  iso- 
baric surfaces.  In  order  to  know  about  the  winds  of  a  given  place, 
we  must  compare  the  pressures  of  adjacent  areas  at  the  same  level. 
For  example,  it  is  not  the  difference  in  pressure  between  the  top  of 
Pike's  Peak  and  Denver,  as  measured  at  the  two  places  by  a  barom- 
eter, which  is  of  consequence  in  determining  winds  between  these 
places,  but  it  is  the  pressure  at  the  top  of  Pike's  Peak,  as  compared 
with  the  pressure  at  the  same  elevation  over  Denver,  which  is  sig- 
nificant. In  Fig.  581  it  is  the  relation  of  the  pressures  at  A  and  B, 
not  that  between  A  and  D,  which  is  significant.  If  the  isobaric 
surface  at  A  extends  as  a  plane  to  B,  there  will  be  no  wind  between 
the  two  places,  because  the  isobaric  surface  has  no  gradient. 

To  determine  what  the  winds  are  to  be,  therefore,  we  must  com- 
pare pressures  at  the  same  level.  This  is  why  all  isobars  are  reduced 
to  sea-level,  on  isobaric  charts. 

The  courses  of  isotars.  Returning  now  to  Fig.  578,  several 
points  are  readily  seen.  (1)  The  isobars  in  general  have  an  east- 


FlG.  581. — It  is  the  atmospheric  pressure  at  the  same  level  in  adjacent  areas 
which  determines  movements  of  air. 

west  course,  though  many  of  them  are  irregular;  (2)  they  are  in 
general  higher  in  low  latitudes  than  in  high  latitudes;  (3)  they  are 
highest  in  the  latitudes  just  outside  the  tropics;  (4)  they  are  more 


ATMOSPHERIC  PRESSURE  589 

regular  in  the  southern  hemisphere  than  in  the  northern;  and  (5) 
they  are,  on  the  whole,  more  irregular  on  the  land  than  on  the  sea. 

Isobars  and  parallels.  Though  many  of  the  isobars  are  very 
irregular,  their  general  courses  are  east-west,  and  none  of  them 
have  a  north-south  course  for  any  considerable  distance.  In  this 
respect  they  correspond  in  a  general  way  with  isotherms  (Fig.  538). 
Furthermore,  the  extra- tropical  belts  of  high  pressure  have  an  east- 
west  course,  and  are  therefore  essentially  parallel  to  the  parallels. 

We  have  now  to  inquire  why  the  isobars  follow,  in  a  general 
way,  the  parallels. 

It  has  already  been  seen  that  isotherms  tend  to  follow  parallels. 
Is  it  the  latitude,  or  the  distribution  of  temperature  which  is  largely 
determined  by  latitude,  which  influences  the  pressure,  and  therefore 
determines  the  position  of  the  isobars?  Or  is  there  some  other 
cause  which  controls  or  influences  their  position? 

Low  latitudes  have  higher  temperatures  than  high  latitudes;  and 
increase  of  temperature  expands  the  air,  and  so  makes  it  lighter. 
If,  therefore,  temperature  controls  the  position  of  isobars,  they 
should  be  lowest  at  the  equator  and  highest  at  the  poles.  Fig.  578 
shows  not  only  that  this  is  not  the  case,  but  that  pressures  are  dis- 
tributed in  apparent  defiance  of  temperature.  The  isobars  are 
highest  neither  where  it  is  coldest  nor  where  it  is  warmest;  they 
are  highest  neither  in  the  lowest  nor  in  the  highest  latitudes.  It 
is  clear,  therefore,  that  neither  latitude  nor  temperature,  nor  both 
together,  control  the  position  of  isobars. 

It  does  not  follow,  however,  that  these  factors  have  no  effect 
on  atmospheric  pressure;  and  if  the  principles  thus  far  developed 
be  correct,  atmospheric  temperature  must  affect  atmospheric  pressure. 
The  only  inference,  therefore,  which  we  are  warranted  in  making 
at  this  stage  is  that  latitude  and  temperature  are  not  the  chief  factors 
which  determine  the  distribution  of  atmospheric  pressure,  and  therefore 
of  isobars.  It  will  be  seen  in  the  sequel,  however,  that  temperature 
is  really  the  fundamental  factor,  though  its  effect  is,  in  part,  indirect. 

Relation  of  isobars  to  land  and  water.  Let  us  see  if  further 
study  of  the  isobaric  charts  will  throw  additional  light  on  the  dis- 
tribution of  atmospheric  pressure. 

The  isobars  are  much  more  regular  in  the  southern  hemisphere, 
where  there  is  much  water,  than  in  the  northern  hemisphere,  where 
there  is  less  water  and  more  land.  In  this  respect  they  have  some 
relation  to  isotherms.  (Compare  Fig.  538.) 


590  PHYSIOGRAPHY 

The  map  (Fig.  578)  also  shows  that  the  high-pressure  belt  in  either 
hemisphere  centering  about  latitudes  a  little  above  30°  is  somewhat 
regular  in  the  southern  hemisphere,  where  water  is  abundant,  but 
very  irregular  in  the  northern  hemisphere,  where  there  is  more  land. 
It  is  suggested  therefore  that  the  distribution  of  land  and  water  may 
influence  the  position  of  isobars.  It  will  be  remembered  that  this 
was  one  of  the  factors  influencing  the  position  of  isotherms,  because 
the  land  is  warmer  than  the  sea  in  the  same  latitude  in  summer,  and 
cooler  in  winter,  and  anything  which  influences  temperature  should 
influence  pressure  also. 

If  temperature  influences  the  position  of  the  isobars,  this  influ- 
ence should  appear  on  seasonal  or  monthly  isobaric  charts. 

Isobars  and  Temperature.  The  isobaric  map  for  January  (Fig. 
582)  shows  that  the  high-pressure  belt  is  much  expanded  in  iho 
northern  hemisphere  (winter),  especially  on  the  land  (compare  Fig. 
578),  and  much  contracted  in  the  southern  hemisphere  (summer). 
Since  the  pressure  is  high  (above  30  inches)  over  a  greater  area  in 
the  hemisphere  which  has  winter,  the  map  suggests  that  the  low  tern  - 
perature  of  this  hemisphere  at  this  season  may  be  a  cause  of  tho 
widened  area  of  high  pressure. 

This  inference  may  be  tested  further  from  this  chart.  The  belt 
of  high  pressure  in  the  northern  hemisphere  (Fig.  582)  is  much  wider 
on  land  than  on  the  sea.  Since  the  land  is  cooler  than  the  sea  durinj; 
January,  the  inference  that  high  pressure  goes  with  low  tempera- 
ture seems  to  be  supported.  In  the  southern  hemisphere,  Janu- 
ary is  a  summer  month,  and  the  land  is  warmer  than  the  sea.  If 
increasing  temperature  causes  low  pressure,  the  pressure  should  there 
be  lower  on  the  land  than  in  the  sea.  The  map  shows  this  to  be 
the  case.  This  chart  therefore  seems  to  show  that  high  tempera- 
ture tends  to  reduce  the  pressure,  for  (1)  the  width  of  the  high-pressure 
belt  is  greater  in  the  hemisphere  which  has  winter;  (2)  the  width 
of  the  high-pressure  belt  is  greater  on  the  cooler  land  than  on  the 
less  cool  sea  in  the  hemisphere  which  has  winter;  and  (3)  the  pres- 
sure over  the  land  is  less  than  that  over  the  sea  in  the  hemisphere 
where  the  land  is  warmer  than  the  sea. 

This  inference  may  also  be  tested  by  the  isobaric  chart  for  July 
(Fig.  583).  At  that  time  of  year,  the  high-pressure  belt  in  the 
southern  hemisphere  (winter)  should  be  expanded,  especially  on 
land,  while  that  in  the  northern  hemisphere  should  be  contracted, 
especially  on  the  land.  Fig.  583  shows  this  to  be  the  case.  We 


ATMOSPHERIC  PRESSURE 


591 


- 


•A   3t 


592  PHYSIOGRAPHY 

therefore  return  with  increased  confidence  to  the  conclusion  that 
high  temperature  reduces  the  pressure,  while  low  temperature  increases 
it.  The  charts  furnish  much  more  evidence  in  support  of  the  same 
conclusions.  Some  of  them  are  the  following: 

1.  Figs.  582  and  583  show  that  the  atmospheric  pressure  changes 
from   season   to  season  in  the  same  place.     Thus  in  January  the 
pressure  over  the  larger  part  of  the  United  States  exceeds  SO  inches, 
while  in  July  it  falls  short  of  30  inches.     Similarly  the  pressure  in 
southern  Africa  exceeds  30  inches  in  July  (^winter),  and  falls  short 
of  it  in  January  (summer).    The  pressure  over  much  of  Asia  exceeds 
30  inches  in  January,  and  falls  short  of  it  in  July.     Other  illustra- 
tions of  the  same  sort  may  be  found  on  the  maps.     In  many  cases, 
therefore,  increased  temperature  goes  urith  decreased  pressure,  £s  shewn 
by  Figs.  582  and  £83. 

2.  It  will  be  seen  from  Figs.  582  and  583  that  the  difference 
between  the  pressure  in  January  and  July  is  greater  in  Asia  than 
elsewhere,  being,  at  the  maximum,  nearly  an  inch.     In  North  America 
and  southern  Africa  it  is  about  0.40  of  an  inch,  while  in  Europe  and 
South  America  it  is  still  less.      The  seasonal  range  of  pressure  is 
greater  on  large  land  areas  than  on  small  ones.     This  is  in  keeping, 
it  will  be  noticed,  with  seasonal  changes  of  temperature  (compare 
Figs.  539  and  540),  and  is  another  confirmation  of  the  close  rela- 
tionship between  isobars  and  isotherms. 

3.  Again,  it  is  to  be  noted  that  the  center  of  the  high-pressure 
belt  in  the  northern  hemisphere  in  January  is  in  latitude  SG°  cr  a 
little  less,  with  great  expansions  of  the  belt  to  the  northward  on 
land.    The  centre  of  the  high-pressure  belt  in  the  southern  hemi- 
sphere at  the  same  time  is  in  latitude  about  35°.     In  July,  en  the 
other  hand,  the  centre  of  the  high-pressure  belt  in  the  northern 
hemisphere  is  in  latitude  about  3o°,  and  in  the  southern,  in  latitude 
about  30°.     That  is,  the  centres  of  the  high-pressure  belts  shift  in  har- 
mony with  the  apparent  motion  of  the  sun. 

4.  It  is  to  be  noted,  also,  that  the  seasonal  variation  of  pressure 
on  the  sea  is  not,  in  general,  so  great  as  that  on  land.     The  seasonal 
change  of  temperature  is  also  less  on  the  sea  (compare  Figs.  £39 
and  540). 

5.  The  high-pressure  (more  than  30  inches)  belt  in  each  hemi- 
sphere is  not  only  greatly  contracted  in  the  summer  (July  in  the 
northern  hemisphere  and  January  in  the  southern,  Figs.  582  and  5S3), 
but  it  is  interrupted  in  each  hemisphere  on  land.    This  suggests 


ATMOSPHERIC  PRESSURE 


593 


594  PHYSIOGRAPHY 

that  the  relations  of  sea  and  "land  influence  pressure.  Since  sea 
and  land  influence  temperature,  their  influence  on  pressure  may 
be  only  a  result  of  their  influence  on  temperature. 

A  relationship  between  temperature  and  isobars  is  clear,  but 
it  is  also  clear  that  temperature  does  not  afford  a  full  explanation 
of  the  distribution  of  pressures  as  shown  by  the  isobars.  The  expla- 
nation of  the  high  pressures  outside  the  tropics,  and  the  low  pres- 
sures in  high  latitudes,  a  feature  which  appears  on  all  the  charts, 
is  not  found  in  temperature. 

Isobars  and  humidity.  We  have  seen  (p.  564)  that  water  vapor 
makes  the  air  lighter.  Are  the  isobars  lowest  over  the  oceans  in 
warm  latitudes,  where  the  air  contains  on  the  average  most  moisture? 
Figs.  578,  582,  and  583  show  that  this  is  not  the  case.  It  is  rea- 
sonable to  conclude,  therefore,  that  the  amount  of  moisture  in  the 
air  is  not  the  chief  factor  controlling  the  isobars,  though  atmospheric 
moisture  must  influence  atmospheric  pressure. 

Inequalities  of  temperature  and  moisture  in  the  air  are  the  only 
factors  thus  far  studied  which  might  affect  the  isobars;  and  since 
they  do  not  explain  the  most  striking  feature  in  the  distribution  of 
atmospheric  pressure,  namely,  the  high  pressures  in  low  latitudes, 
we  conclude  that  something  besides  temperature  and  moisture  must 
be  involved  in  their  explanation. 

The  high-pressure  belts.  The  explanation  of  the  high  pressure 
in  low  latitudes  rather  than  in  high,  and  the  explanation  of  the 
highest  pressures  just  outside  the  tropics,  is  not  found  on  the  isobaric 
charts.  These  larger  features  of  pressure-distribution  are  probably 
to  be  explained  by  the  general  circulation  of  the  atmosphere  under 
the  influence  of  rotation.  Several  factors  bear  upon  this  point. 

1.  In  the  equatorial  zone,  the  air  is  heated  and  expanded.  As 
it  rises  by  expansion,  it  must  flow  to  north  and  south.  If  the 
expansion  affected  the  atmosphere  all  the  way  from  bottom  to 
top,  there  would  be  outflow  from  the  top  of  the  atmosphere  in 
the  equatorial  zone  in  either  direction,  for  the  same  reason  that 
outflow  would  take  place  from  the  top  of  a  mound  or  ridge  of  water 
if  such  existed.  But  the  expansion  of  the  air  by  heating  is  chiefly 
in  the  lower  part  of  the  air.  As  the  lower  ah*  expands,  it  pushes  up 
the  air  above  it.  The  pressure  of  the  air  at  the  bottom,  before  out- 
flow takes  place  above,  is  not  diminished,  but  the  pressure  at  a  point 
above,  say  at  the  upper  limit  of  the  effective  heating,  is  increased, 
because  a  larger  part  of  the  air  is  now  crowded  up  above  that  level 


ATMOSPHERIC  PRESSURE 


595 


This  is  illustrated  by  Figs.  584  and  585.  The  former  shows  the 
crowding  of  the  air  above  the  zone  of  heating,  and  the  latter  the 
resulting  isobaric  slopes.  Except  at  the  bottom  of  the  atmosphere, 
the  isobaric  surfaces  slope  downward  on  either  hand  from  the  equa- 


FIG.  584. — Expansion  of  the  lower  air  as  a  result  of  heating,  crowds  the  air 
above,  and  so  increases  its  density  and  pressure,  as  compared  with  the 
density  and  pressure  of  air  at  the  same  level  outside  the  heated  area. 

torial  zone,  and  air  always  flows  down  an  isobaric  surface.  Over 
the  heated  equatorial  zone,  therefore,  the  expanded  air  rises,  and 
at  some  level  above  the  bottom  of  the  atmosphere  it  flows  pole- 
ward down  the  isobaric  surface  (Fig.  586).  This  is  the  case  in  spite 


Level 


FIG.  585. — The  condition  of  things  represented  in  Fig.  584  gives  rise  to  move- 
ments of  air. 

of  the  low  pressure  at  the  bottom  of  the  atmosphere  in  equatorial 
latitudes. 

When  some  of  the  air  flows  out  poleward  from  the  equatorial 
belt,  the  pressure  at  the  bottom  of  the  equatorial  belt  is  diminished, 
because  the  amount  of  air  above  is  diminished.  At  the  same  time 
the  pressure  on  both  sides  of  the  equatorial  belt  is  increased,  because 
the  amount  of  air  is  there  increased.  Furthermore,  when  the  air 
of  the  equatorial  belt  expands,  it  pushes  laterally  as  well  as  upward, 
and  so  tends  to  compress  the  air  outside  the  belt  where  the  expan- 
sion takes  place. 

Both  the  outward  flow  and  the  outward  crowding  of  the  air 
in  the  equatorial  belt  tend  to  increase  the  pressure  of  the  air  out- 
side the  zone  of  principal  heating,  but  they  do  not  make  it  ap- 
parent why  the  zones  of  greatest  pressure  should  be  in  latitude 
30°  or  a  little  above. 


596 


PHYSIOGRAPHY 


2.  When  the  pressure  in  the  equatorial  belt  is  diminished  by 
the  outflow  of  air  above,  a  barometric  slope  is  established  toward 
the  equator  from  either  side  at  the  bottom  of  the  atmosphere,  as 
shown  by  the  lower  arrows.  Fig.  585,  even  when  the  barometric  slope 
is  from  the  equator  in  the  upper  air.  Thus  a  system  of  convective 
circulation  is  established.  In  the  long  run,  the  outflow  of  air  from 
the  equatorial  belt  toward  the  poles  will  be  equaled  by  the  inflow 
of  air  from  higher  latitudes  on  either  side  to  the  equatorial  belt. 
The  poleward-flowing  upper  air  descends  as  it  reaches  higher  and 
higher  latitudes,  and  it  takes  the  place  of  the  air  which  moves 
equatorward.  On  the  whole,  the  amount  of  ascending  poleward- 
flowing  air  from  the  equatorial  zone,  equals  the  amount  of  descend- 


60° 


40° 


ALTITUDE  OF   2000  FEET 
20°    8EA  LEVEL  20°  40° 


30         INCHES 


FIG.  586. — Slope  of  isobaric  surfaces  along  meridians  at  various  altitudes. 

(After  Waldo.) 

ing  equatorward-flowing  air  from  high  latitudes.  There  must> 
therefore,  be  a  vertical  plane  in  the  atmosphere  in  each  hemisphere, 
on  the  equatorward  side  of  which  as  much  air  ascends  as  descends 
on  the  poleward  side.  This  vertical  plane  should  be  near  latitude 
30°,  for  this  parallel  divides  the  surface  of  the  hemisphere,  and 
therefore  the  volume  of  air  in  each  hemisphere,  into  two  nearly 
equal  parts.  This  is  regarded  as  a  cause  of  the  high-pressure  belts 
at  30°  in  both  hemispheres. 

3.  Given  the  high-pressure  belts  in  extra-tropical  latitudes, 
the  circulation  of  the  bottom  air  which  follows  helps  to  main- 
tain them.  The  air  moving  poleward  from  these  belts  of  high  pres- 
sure turns  to  the  right  in  the  northern  hemisphere,  and  to  the 
left  in  the  southern,  becoming  westerly  winds  in  both  hemispheres. 
In  both,  this  turning  causes  these  winds  to  crowd  on  the  equator- 
ward  side  of  their  lines  of  movement.  This  tends  to  maintain  and 


ATMOSPHERIC  PRESSURE  597 

increase  the  pressure  in  the  high-pressure  belts,  by  crowding  on 
their  poleward  sides. 

Given  the  extra-tropical  belts  of  high  pressure,  the  numerous 
irregularities  and  changes  of  pressure  from  season  to  season,  as 
shown  by  the  isobaric  charts,  may  be  explained  chiefly  by  varia- 
tions in  temperature. 

Permanent  areas  of  low  pressure.  Fig.  578  shows  areas  of 
low  pressure  in  the  North  Pacific  and  North  Atlantic  oceans.  These 
areas  of  low  pressure  are  still  more  pronounced  on  the  January 
chart  (Fig.  582),  and  but  feebly  marked  on  the  July  chart  (Fig. 
583).  No  corresponding  areas  of  low  pressure  are  known  in  the 
southern  hemispheres.  No  explanation  of  these  areas  of  low 
pressure  is  here  attempted. 

Temporary  and  local  variations  of  pressure.  There  are  many 
variations  of  pressure  not  shown  on  seasonal  or  even  on  monthly 
isobaric  charts,  though  they  appear  on  daily  weather  maps.  These 
will  be  studied  in  the  next  chapter.  There  are  even  variations  of 
pressure  which  do  not  appear  on  the  daily  maps.  Chief  of  them 
are  the  daily  variations,  presumably  caused  by  the  daily  varia- 
tions of  temperature.  Thus  there  are  daily  maxima  at  about  10  A.M. 
and  10  P.M.,  and  daily  minima  at  about  4  P.M.  and  4  A.M.  These 
daily  changes  range  from  0.01  to  0.15  of  an  inch,  the  range  being 
greatest  in  low  latitudes.  No  satisfactory  explanation  of  these 
variations  has  been  given. 


CHAPTER  XVII 
GENERAL  CIRCULATION   OF  THE  ATMOSPHERE 

Prevailing  and  Periodic  Winds 

INEQUALITIES  of  atmospheric  pressure  involve  atmospheric  move- 
ments. Since  atmospheric  pressures  are  unequal,  and  since  proc- 
esses are  constantly  in  operation  which  renew  the  inequalities, 
movements  are  continuous.  Unequal  insolation  is  the  most  im- 
portant factor  in  disturbing  the  equilibrium  of  the  air,  and  so  in 
generating  air  movements,  and  in  determining  their  initial  direc- 
tion; but  the  rotation  of  the  earth  has  much  influence  in  directing 
them,  once  they  are  started.  Since  the  greater  insolation  is  always 
in  the  same  general  zone,  and  since  the  rotation  of  the  earth  is 
always  in  the  same  direction,  the  ah*  movements  generated  and 
directed  by  insolation  and  rotation  are  systematic,  and  result  in 
a  general  circulation  of  the  atmosphere. 

It  is  to  be  borne  in  mind  that  the  movement  of  air  is  always 
from  a  region  of  greater  pressure  to  one  of  less  pressure  at  the  same 
level,  or,  in  other  words,  always  down  a  barometric  or  isobaric  slope. 
The  familiar  saying  that  "the  wind  bloweth  where  it  listeth"  is 
true  only  in  the  sense  that  the  air  always  listeth  to  blow  down  the 
steepest  accessible  isobaric  gradient,  and  that  where  there  is  no 
gradient,  it  listeth  not  to  blow. 

The  General  Effect  of  Unequal  Insolation 

If  the  air  were  in  equilibrium  over  the  whole  earth  at  a  uniform, 
low  temperature,  and  if  it  could  then  be  heated  by  the  sun  for  a 
time  without  involving  horizontal  movement,  the  effect  would  be 
to  raise  its  surface  over  all  areas  where  its  temperature  was  in- 
creased, and  to  raise  it  most  where  it  was  heated  most,  that  is,  in 
the  low  latitudes  (Fig.  586).  As  indicated  in  the  last  chapter 
(p.  595),  the  result  would  be  the  establishment  of  a  barometric 

598 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       599 

gradient  from  the  equatorial  region  toward  the  poles  (Fig.  586), 
and  this  is  the  condition  necessary  for  poleward  movements  of 
air. 

Since  the  air  in  low  latitudes  is  always  being  heated  more 
effectively  than  that  in  higher  latitudes,1  movement  should  be  essen- 
tially constant,  above  the  bottom  of  the  air,  from  the  equatorial 
zone  to  the  polar  zones  in  both  hemispheres.  These  poleward 
movements  of  air  lessen  the  pressure  at  the  bottom  of  the  atmos- 
phere in  low  latitudes,  because  air  has  moved  away  from  that  zone. 
As  the  pressure  is  thus  lessened  in  the  equatorial  region,  a  baro- 
metric gradient  is  established  toward  the  equator  at  the  bottom  of 
the  atmosphere  (Fig.  587),  and  air  must  then  come  in  from  higher 
latitudes.  Here,  then,  we  have  two  elements  of  a  general  circu- 
lation: a  poleward  movement  in  the  upper  air,  and  an  equator- 
ward  movement  in  the  lower  air,  and  the  causes  which  generate 
these  movements  are  constantly  in  operation. 

It  should  perhaps  be  noted  that  quite  apart  from  circulatory 
movements,  there  would  be  lateral  crowding  by  the  expanding  air 
of  low  latitudes  (Fig.  584).  In  so  far 
as  this  is  effective,  it  would  reduce 
the  mass  of  air  above  any  point  of 
the  surface  where  the  air  was  ex- 
panding. It  would  also  tend  to  in- 
crease the  amount  of  air  over  areas 
poleward  from  the  zone  of  heating, 
and  so  would  tend  to  establish 
equatonvard  gradients  in  the  lower 
part  of  the  air.  The  result  would  be 
to  increase  the  equatorward  isobaric  FlQ  587._Diagram  showing  the 
slope  at  the  bottom  of  the  atmos-  general  system  of  circulation 
i  which  would  be  established  by 

unequal  heating,  as  a  result  of 

From    unequal    heating    alone,      differences  in  latitude. 

therefore,  there  is  a  constant  ten- 
dency to  the  movement  of  ah-  (1)  from  low  latitudes  toward  the 
poles  above  the  bottom  of  the  atmosphere,  and  (2)  a  compensa- 
tory movement  from  the  higher  latitudes  toward  the  equator. 
These  are  the  most  fundamental  facts  in  the  general  circulation  of 

1  High  latitudes  sometimes  receive  more  heat  per  day  than  low  latitudes 
(see  p.  525),  but  the  air  of  high  latitudes  is  never  so  effectively  heated, 
because  of  the  abundance  of  ice,  snow,  ice-cold  water,  and  frozen  ground. 


600  PHYSIOGRAPHY 

the  atmosphere.  They  involve  vertical  as  well  as  horizontal  move- 
ments of  air.  The  vertical  movements  are  (1)  upward  in  low  lati- 
tudes, where  the  air  (a)  expands  upward,  and  (b)  is  crowded  up- 
ward by  the  cooler  and  heavier  air  which  flows  in  below,  and  (2) 
downward  in  higher  latitudes.  The  system  of  circulation  which 
would  be  established  by  the  greater  heating  of  the  low  latitudes, 
taken  by  itself,  is  somewhat  as  shown  in  Fig.  587. 

The  general  poleward  movement  of  air  from  low  latitudes 
seems  to  be  clearly  established  by  observation,  but  its  return  to 
low  latitudes  is  much  less  clearly  indicated  in  observed  winds. 
Of  its  return  there  can  be  no  question,  but  how  and  where  it  is 
effected  is  not  well  understood,  for  outside  the  low  altitudes  of  the 
low  latitudes  (the  trade-wind  zones)  no  persistent  equatorward 
movements  of  air  are  recorded.  Much  air  moves  equatorward  in 
the  aperiodic  atmospheric  disturbances,  and  perhaps  the  return  is 
chiefly  effected  through  them.  These  aperiodic  movements  will  be 
studied  in  the  next  chapter. 

It  is  to  be  noted  that  the  isobaric  gradients  at  the  bottom  of 
the  atmosphere  in  low  latitudes  do  not  correspond  with  those  in 
the  upper  air  (Fig.  586) ;  yet  these  apparently  inharmonious  gradi- 
ents co-exist.  The  reasons  for  each  have  been  given;  their  co- 
existence means  that  the  tendency  to  the  poleward  slope  is  so 
strong  that,  except  in  the  lower  part  of  the  atmosphere,  it  is  not  over- 
come by  the  causes  which  develop  the  equatorward  gradient  at 
the  bottom  of  the  atmosphere. 

But  for  the  influence  of  rotation  and  the  unequal  heating  of  land 
and  water  areas  in  the  same  latitude,  the  atmospheric  movements 
just  outlined  would  tend  to  follow  meridians.  Rotation  affects  the 
course  of  the  atmospheric  movements  in  more  ways  than  one.  It 
not  only  deflects  all  currents  to  the  right  in  the  northern  hemi- 
sphere, and  to  the  left  in  the  southern,  but  it  appears  to  be 
responsible,  in  part  at  least,  for  the  concentration  of  the  high 
pressures  of  extra-tropical  latitudes  into  belts  near  the  tropics 
(p.  596) ;  and  these  belts  of  high  pressure  have  an  important  influ- 
ence on  the  course  of  circulation  at  the  bottom  of  the  atmosphere, 
and  interfere  with  the  simplicity  of  circulation  outlined  above. 

Effect  of  the  Extra-tropical  Belts  of  High  Pressure 

In  each  high-pressure  belt  (Fig.  578)  the  isobaric  surfaces  are 
bowed  up  in  the  lower  part  of  the  atmosphere  (Fig.  586),  and  from 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       601 

each  there  is  a  barometric  gradient  both  to  north  and  south.  From 
each  of  these  belts,  therefore,  there  should  be  a  flow  of  air  both 
southward  and  northward  at  the  bottom  of  the  atmosphere.  If  no 
other  factors  were  involved,  the  movements  of  the  lower  air  should 


60 

FIG.  588. — Diagram  representing  the  general  movements  which  would  take 
place  in  the  lower  air  if  there  were  no  rotation. 

be  those  shown  in  Fig.  588;  and  if  forces  were  in  operation  to  con- 
stantly renew  the  high-pressure  belt,  these  movements  of  air  would 
be  constant.  At  the  center  of  the  high-pressure  belt,  there  would 
be  little  horizontal  movement  of  the  air.  The  narrow  zone  in  this 
position  is  the  zone  of  tropical  calms. 

It  will  be  observed  that  the  poleward  flow  in  the  lower  part  of 
the  atmosphere  from  the  high-pressure  belts  would  be  in  much  the 
same  direction  as  the  poleward  flow  of  upper  air  from  the  equa- 
torial zone,  while  the  equatorward  flow  of  lower  air  from  the  high- 
pressure  belts  would  be  opposed  in  direction  to  the  flow  in  the  upper 
air  of  the  same  latitude. 

After  the  poleward  gradients  in  the  larger  part  of  the  atmos- 
phere (Fig.  586),  the  barometric  slopes  in  the  lower  air  from  the 
high-pressure  belts  are  perhaps  the  most  important  fact  in  the 
general  circulation  of  the  atmosphere. 

The  High-latitude  Areas  of  Low  Pressure 

The  permanent  areas  of  low  pressure  over  the  northern  oceans 
(Figs.  578,  582,  and  583)  constitute  another  permanent  factor  in  the 


602  PHYSIOGRAPHY 

atmospheric  circulation.  Their  influence  is  less  commonly  recog- 
nized than  that  of  the  high-pressure  belts,  but  it  is  perhaps  of  more 
than  minor  importance.  To  these  areas  there  must  be  a  constant 
inflow  of  air,  and  from  them  it  rises  and  flows  out  above,  thus 
modifying  the  general  course  of  the  circulation,  and  helping  to 
destroy  its  simplicity.  It  is  perhaps  significant  that  the  great 
centers  of  glaciation  in  the  glacial  period  lay  on  the  continents  to 
the  east  of  these  areas  of  permanent  low  pressure. 

According  to  the  outline  given  above,  the  atmospheric  circula- 
tion in  one  hemisphere  appears  to  be  measurably  independent  of 
that  in  the  other.  This,  however,  is  less  true  than  would  appear 
from  the  statements  already  made.  The  average  pressure  for  the 
northern  hemisphere  for  January  has  been  estimated  at  29.99  inches, 
and  that  for  the  southern  hemisphere  at  the  same  time  29.79  inches. 
The  average  pressures  for  July  are  estimated  at  29.87  inches  in  the 
northern  hemisphere  and  29.91  inches  for  the  southern.  It  has 
been  calculated  that,  to  bring  about  the  condition  which  exists  in 
January,  some  32,000,000  tons  of  air  must  have  been  shifted  from 
the  southern  hemisphere  into  the  northern  since  the  preceding 
summer.  This  transfer  is  probably  effected  because  the  low  tem- 
perature of  the  extensive  land  areas  in  the  northern  hemisphere  so 
reduces  the  temperature  and  increases  the  density  of  the  air  over 
great  areas  in  that  hemisphere,  that  the  north-poleward  gradient 
in  the  upper  air  is  increased,  and  the  crest  of  the  barometric  sur- 
face (Fig.  586)  shifted  south  of  the  equator.  In  other  words,  the 
wind  equator  and  the  thermal  equator  are  then  south  of  the  geographic 
equator.  The  shifting  of  the  thermal  equator,  and  therefore  of  the 
wind  equator,  is  shown  in  Figs.  539  and  540,  respectively.  The 
corresponding  shifting  of  the  wind  zones  is  illustrated  by  Fig.  589. 

These  three  factors,  namely,  (1)  the  poleward  gradients  in 
the  upper  air  of  low  latitudes,  (2)  the  gradients  in  the  lower  air 
from  the  high-pressure  belts  in  extra-tropical  latitudes,  and  (3)  the 
gradients  in  the  lower  air  toward  the  areas  of  low  pressure  in 
high  latitudes,  are  the  principal  ones,  named  in  the  order. of  their 
importance,  in  the  general  circulation  of  the  atmosphere. 

Direction  of  Winds 

Once  wind  is  started,  its  direction  may  be  influenced  by  various 
factors.  Chief  among  them  is  the  rotation  of  the  earth,  which 
affects  the  course  of  all  winds  except  such  as  blow  in  the  plane  of 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       603 

the  equator.     The  farther  they,  go,  the  more  are  their  directions 
changed. 

A  generalized  diagram  of  the  observed  winds  of  the  lower  air  is 
shown  in  Fig.  590.     This  figure  represents  the  winds  blowing  out 


iUfeTRpPIOAt-B^LTV^V       \  ^*S. 
Sj^UARY'tlMiT  ~">r;U~ 


FIG.  589. — Diagram  illustrating  the  FIG.  590. — Generalized  diagram  of  wind 
shifting  of  wind  zones.  (After  directions  at  the  bottom  of  the  atmos- 
Davis.)  phere. 

from  the  extra-tropical  belts  of  high  pressure,  and  following  more 
or  less  systematic  courses.  The  poleward  winds  from  the  high- 
pressure  belts  are  turned  toward  the  east  in  both  hemispheres,  and 
so  become  westerly  winds  (southwesterly  in  the  northern  hemisphere 
and  northwesterly  in  the  southern).  The  winds  blowing  toward  the 
equator  in  the  lower  air  from  the  belts  of  high  pressure  become 
easterly  (northeasterly  in  the  northern  hemisphere  and  south- 
easterly in  the  southern)  and  are  known  as  trade-winds.  The 
zone  along  the  thermal  equator  where  the  northeasterly  and  south- 
easterly trades  meet,  and  where  ascending  currents  of  air  are  more 
pronounced  than  horizontal  movements,  is  known  as  the  zone  of 
equatorial  calms.  The  position  of  the  zone  of  calms  shifts  a  little 
with  the  sun,  its  center  remaining  near  the  thermal  equator.  (Com- 
pare Figs.  539  and  540.) 

The  trade-winds  are  remarkably  persistent,  and  have  long  been 
known  and  utilized  by  navigators. 

The  westerly  winds  of  middle  latitudes  and  the  trades  of  low 


604 


PHYSIOGRAPHY 


latitudes  are  the  prevailing  winds  at  the  bottom  of  the  atmosphere, 
and  are  sometimes  called  the  planetary  winds. 

The  explairiation  of  the  deviation  of  the  winds  from  meridional 
courses,  always  to  the  right  in  the  northern  hemisphere  £  nd  always 
to  the  left  in  the  southern,  is  the  same  as  that  underlying  the  change 
in  the  direction  of  the  swinging  pendulum,  and  is  illustrated  by 
Fig.  591.  This  figure  may  be  taken  to  represent  the  northern 
hemisphere  as  seen  from  above  the  North  Pole.  The  curved 
arrows  show  the  direction  of  rotation.  The  arrow  at  n  represents 
a  wind  starting  poleward.  The  arrows  o,  p,  and  q  represent  suc- 
cessive directions  of  the  wind  as  it  advances.  Their  departure 


FIG.  591. — Diagram  illustrating  deflection  of  winds  to  the  right  in  the  north- 
ern hemisphere.  The  deflection  is  to  the  left  in  the  southern  hemi- 
sphere, for  the  same  reason. 

from  meridians  is  to  the  right,  and  the  departure  becomes  more 
pronounced  as  the  latitude  becomes  higher.  The  arrow  w  repre- 
sents a  wind  blowing  westward,  or  contrary  to  the  direction  of 
rotation.  Since  the  motion  of  the  air  in  the  direction  of  the 
arrow  is  much  less  rapid  than  the  rotation  of  the  earth  in  the 
opposite  direction,  the  arrows  x,  y,  and  z  represent  progressions 
backward.  Similarly  the  arrow  s,  near  the  pole,  represents  a  wind 
blowing  southward,  and  the  arrows  t,  u,  and  v  represent  the  suc- 
6essive  directions  which  such  a  wind  would  have,  the  departures 
from  the  meridians  being  still  to  the  right.  The  arrow  e  repre- 
sents a  wind  starting  eastward,  and  the  arrows  /,  g,  and  h,  the 
successive  directions  of  the  wind.  The  wind  here  progresses 
forward,  because  its  direction  corresponds  with  the  direction 
of  rotation.  A  corresponding  diagram  might  be  made  for  the 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE      605 

southern  hemisphere  which  would  snow,  in  a  similar  way,  the 
deflection  of  winds  to  the  left  of  meridians. 

By  referring  to  Fig.  586,  it  will  be  seen  that  the  trade-winds  can- 
not have  great  depth.  While  they  are  pronounced  at  the  surface, 
they  must  cease  at  some  relatively  slight  elevation  above,  for  the 
configuration  of  the  isobaric  surfaces  changes.  As  a  matter  of 
observation,  the  trade-winds  have  been  observed  to  cease  at  an 
elevation  of  about  10,000  feet  on  Teneriffe  (Canary  Islands,  Lat. 
28°).  Their  upper  limit  has  also  been  noted  on  various  mountains 
in  South  America  and  on  the  Hawaiian  Islands,  and  is  not  far  from 
the  above  figure. 

The  westerly  winds,  on  the  other  hand,  have  much  greater  depth. 
Figs.  592  and  593  show  the  directions  of  winds  in  the  United  States 
(1)  at  the  bottom  of  the  atmosphere,  and  (2)  in  the  upper  air,  as 
shown  by  the  movements  of  the  upper  clouds.  The  movements  of  the 
lower  part  of  the  air  are -very  different  in  the  two  cases,  but  the 
movements  indicated  by  the  upper  clouds  are  to  the  eastward  in  both. 

The  Circumpolar  Whirl 

The  circulation  in  each  hemisphere  is  often  looked  upon  as  a 
great  eddy  centering  at  the  pole.  If  this  were  the  true  view  of  the 
case,  it  would  account  for  the  low  pressure  in  high  latitudes  and 
the  high  pressure  in  low  latitudes,  and  the  pressure  should  decrease 
steadily  to  each  pole. 

Land  and  Water  Circulation 

While  the  winds  at  the  bottom  of  the  atmosphere  tend  to  fall 
into  the  general  system  shown  in  Fig.  590,  the  simplicity  of  the 
system  is  interfered  with  by  various  disturbing  influences  which 
modify  the  system  of  planetary  winds.  Chief  of  these  disturbing 
factors  is  the  unequal  heating  of  the  atmosphere  over  land  and 
water.  This  not  only  interferes  with  the  direction  of  planetary  winds, 
but  is  itself  the  cause  of  winds. 

Monsoons  and  land-  and  sea-breezes  have  already  been  cited  as 
illustrations  of  the  effects  of  the  unequal  heating  of  land  and  water. 

The  monsoon  influence  is  probably  much  stronger  than  is  com- 
monly recognized,  for  it  overcomes  the  prevailing  winds  on  a  large 
scale.  Thus  in  winter,  Eurasia  is,  on  the  average,  a  centre  of  air 
dispersion  in  the  lower  air  (Fig.  582),  while  in  summer,  air  flows  in 


606 


PHYSIOGRAPHY 


FIG.  592. — Chart  showing  the  direction  of  air  movements  at  the  bottom  of  the 
atmosphere  (upper  figure),  at  the  horizon  of  the  lower  clouds  (middle 
figure),  and  at  the  level  of  the  upper  clouds  (lower  figure),  at  a  time  when 
the  pressure  is  high  about  Lake  Superior.  The  winds  are  westerly:  in  the 
upper  air,  without  reference  to  inequalities  of  pressure  in  the  lower  air. 
(U.  S.  Weather  Bureau.) 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       607 


FIG.  593. — Figure  showing  the  movements  of  the  air  when  atmospheric  pres- 
sure is  low  about  Lake  Superior.  It  will  be  noted  that  the  movements 
in  the  upper  air  (lowest  figure)  are  from  the  west  as  in  the  preceding  case. 


608 


PHYSIOGRAPHY 


toward  it,  though  most  of  its  area  is  in  the  zone  of  the  westerlies. 
The  same  influence  is  probably  of  great  importance  over  and  about 
every  large  land  area,  but  it  is  only  where  it  opposes  and  overcomes 
the  prevailing  wind  that  it  is  popularly  recognized. 

India  is  usually  cited  as  affording  the  best  illustration  of  the 
monsoon  influence.    This  country  is  in  the  latitude  of  the  northern 


*  *•  /  /  i  v\  \  r*> 

*  *  >  •.  \  ^  \  \  / 

•  •     *  '•  •<<    *•  \  * 

^t/  /    ,/T  i     *   •<:       I 


FIG.  594. — The  isobars  of  India  for    FIG.   595. — Figure  showing  the  direc- 
January.     (After  Bartholomew.)  tion  of  winds  in  India  in  winter. 

(After  Koppen.) 

trades,  where  easterly  (northeasterly)  winds  should  prevail.  In 
Fig.  594,  the  gradient  is  from  northeast  to  southwest,  and  the  direc- 
tion of  the  wind  (Fig.  595)  is  in  harmony  with  the  planetary  system; 


FIG.  596. — The   isobars  of  India  for 
August.     (After  Bartholomew.) 


FIG.  597. — The  winds  of  India  in  mid- 
summer.    (After  Koppen.) 


but  in  Fig.  596  the  isobaric  gradient  is  to  the  northward,  because 
the  land  is  warmer  than  the  sea  and  the  winds  blow  in  that  direc- 
tion (Fig.  597).  That  is,  the  planetary  (northeast)  wind  is  overcome 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       609 


during  the  hot  season  by  the  winds  which  result  from  the  seasonal 
change  of  temperature  which  establishes  a  seasonal  gradient.  At 
the  same  season,  the  low  pressure  north  of  India,  developed  by  the 
heat  of  summer,  counteracts  the  high  pressure  normal  to  this  lati- 
tude, and  the  prevailing  wind  is  displaced  by  seasonal  winds  blow- 
ing toward  the  area  of  low  pressure.  Figs.  598  and  599  show  the 


FIG.   598. — Isotherms   of    India  for 
January.     (After  Buchan.) 


FIG.   599. — Isotherms    of    India    for 
August.     (After  Buchan.) 


isotherms  for  the  same  region  at  the  corresponding  seasons,  and 
make  clear  the  relation  between  pressure  and  temperature. 

When  the  monsoon  blows  with  the  prevailing  wind,  as  in  western 
India  in  winter,  the  prevailing  wind  is  strengthened;    if  the  two 


FIG.  600. — Isobars  and  winds  in 
Spain  and  Portugal,  month  of 
January.  (After  Hann.) 


FIG.  601. — Isobars  and  winds  in 
Spain  and  Portugal,  month  of 
July.  (After  Hann.) 


tend  to  blow  in  opposite  directions,  as  in  western  India  in  summer, 
the  stronger  prevails.  Spain,  in  the  zone  of  westerly  winds,  affords 
an  excellent  example  of  the  same  thing.  Figs.  600  and  601  show 


610 


PHYSIOGRAPHY 


the  conditions  in  winter  and  summer.  In  winter  the  isotherms  over 
the  plateau  are  low,  and  the  isobars  high,  and  the  winds  blow  out 
from  its  center  instead  of  toward  it.  In  summer  the  case  is 
reversed. 

The  general  principle  of  the  monsoon  makes  itself  felt  about  the 
Great  Lakes.  At  Chicago,  which  is  in  the  zone  of  southwesterly 
winds,  northeast  winds  predominate  in  spring,  because  the  lake  is 
then  much  cooler  than  the  land,  and  the  winds  set  toward  the  land 
and  overcome  the  prevailing  winds  (Fig.  602).  Similar  diagrams 


January,  1904 


April,  1904 


Septenber,  1904 


FIG.  602. — Diagram  showing  the  direction  and  velocity  of  winds  in  Chicago 
during  January,  April,  July,  and  September,  1904.  The  time  during 
which  the  wind  blew  from  any  given  direction  is  shown,  relatively,  by 
the  length  of  the  lines.  The  relative  average  velocity  is  shown  by  the 
width  of  the  lines.  The  monsoon  influence  of  the  lake  is  seen  in  the 
preponderance  of  northeast  winds  in  April.  (Cox,  U.  S.  Weather  Bureau.] 

for  a  station  fifty  miles  from  the  lake  would  show  less  wind  from  the 
northeast  in  April.  The  same  thing  is  illustrated  by  Fig.  603,  which 
shows  wind  "roses"  for  Chicago  and  Key  West.  The  former  has  a 
prolongation  to  the  northeast,  indicating  primarily  the  landward 
winds  in  spring,  though  the  principal  direction  of  wind  otherwise 
is  southwest.  Key  West  is  in  the  zone  of  the  trade-winds,  and 
the  easterly  winds  greatly  predominate  over  all  others. 

The  principle  involved  in  the  daily  land-  and  sea-breezes  along 
coasts  (p.  561)  is  the  same1  as  that  of  the  monsoon,  but  the  resulting 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       611 

winds  are  more  local.  They  are  not  usually  felt  very  far  from  shore, 
and  do  not  extend  to  great  heights.  At  Coney  Island,  the  sea-breeze 
has  been  found  to  be  limited  to  a  height  of  about  500  feet  at  times 
when  it  has  been  determined.  At  slightly  higher  levels  the  air- 
currents  were  those  of  the  prevailing  winds.  At  some  other  places  sea- 
breezes  have  been  known  to  extend  up  1300  feet. 

On  the  coast  of  Massachusetts  the  sea-breeze  sometimes  starts 
as  early  as  eight  o'clock  in  the  morning,  though  more  commonly  not 
till  an  hour  or  two  later.  At  first  it  advances  inland  at  the  rate  of 
3  to  8  miles  per  hour,  and  later  more  slowly.  It  penetrates  inland 


Fia.  603.— Wind  "roses  "  for  Chicago  and  Key  West,  1902.  The  shaded  parts 
of  the  diagrams  shows  the  relative  duration  of  the  periods  when  the  wind 
blew  from  different  directions.  The  greater  the  distance  from  the  cross- 
ing-point of  the  radiating  lines,  the  longer  the  period.  The  influence  of 
the  lake,  giving  rise  to  lake  breezes  and  to  winds  of  monsoon  character, 
is  conspicuous  at  Chicago.  Key  West  is  in  the  zone  of  trade-winds. 
(Cox,  U.  S.  Weather  Bureau.) 

10  to  20  miles,  and  sometimes  gives  rise  to  thunder-storms.  On 
the  coast  of  southern  California,  the  land-  and  sea-breezes  persist 
throughout  the  year,  being  much  stronger  in  summer  than  in 
winter.  Land-breezes  are  generally  less  well  developed  than  sea- 
breezes. 

Breezes  corresponding  to  land-  and  sea-breezes  are  often  felt  about 
large  lakes. 

The  sea-breeze  is  of  consequence,  not  only  by  lowering  the  land 
temperature  in  hot  weather,  but  by  bringing  pure  air  to  the  land. 
This  is  of  much  importance  along  densely  populated  coasts.  The 
explanation  of  the 'sea-breezes  has  already  been  suggested  (p.  561). 


612  PHYSIOGRAPHY 

The  unequal  heating  of  high  and  low  lands  in  the  same  latitude 
also  causes  slight  and  temporary  departures  from  the  normal  planet- 
ary circulation  (p.  562). 

Besides  the  planetary  winds,  the  seasonal  winds,  and  minor  periodic 
winds,  whose  times  of  coming  and  going  are  more  or  less  regular, 
there  are  numerous  winds  which  blow  at  irregular  times,  and  whose 
coming  cannot  be  foretold  long  in  advance.  These  irregular  winds 
are  the  chief  cause  of  the  uncertain  elements  of  the  weather.  Some 
of  them  are  due  to  unequal  temperatures,  some  to  unequal  amounts 
of  atmospheric  moisture,  and  some  to  other  causes. 

Illustrations  of  aperiodic  winds  due  to  unequal  temperature  are 
whirlwinds  and  tornadoes,  both  of  which  are  due  to  strong  convec- 
tion currents  generated  by  excessive  local  heating,  and  some  larger 
whirls  of  air,  especially  tropical  cyclones.  These  will  be  referred  to 
in  the  next  chapter. 

Again,  just  as  \vaves  of  water  generated  by  the  wind  are  felt 
far  beyond  the  place  where  they  were  generated,  and  long  after  the 
wind  ceases  to  blow,  so  local  disturbances,  leading  to  the  flow  of 
air  from  one  place  to  another,  make  themselves  felt  far  beyond  the 
place  of  disturbance.  Movements  therefore  generate  movements. 

Summary.  We  may  now  recall  the  chief  points  thus  far  studied 
in  connection  with  atmospheric  circulation.  They  are  as  follows: 

(1)  Above  the  lower  part  of  the  atmosphere  there  is  a  pole- 
ward movement  of  the  air  from  low  latitudes. 

(2)  There  must  be  a  compensatory  movement  of  air  from  high 
latitudes  to  low;  but  outside  the  extra-tropical  belts  of  high  pres- 
sure, this  movement  is  not  well  defined. 

(3)  The  extra-tropical   high-pressure  belts   are  the  zones  from 
which  the  dominant  planetary  winds  at  the  bottom  of  the  atmos- 
phere start. 

(a)  These  planetary  winds  tend  to  blow  poleward  and  equator- 
ward  in  each  hemisphere,  from  the  belts  of  high  pressure. 

(6)  They  are  deflected  to  the  right  in  the  northern  hemisphere 
and  to  the  left  in  the  southern  hemisphere,  by  the  rotation  of  the 
earth,  thus  establishing  the  double  trade-wind  zone,  with  the  equa- 
torial calms  in  the  centre,  and  two  zones  of  westerly  winds,  with 
tropical  calms  on  the  equatorward  margin  of  each. 

(4)  The  simplicity  of  the  system  of  planetary  winds  is  interfered 
with  by  the  great  inequalities  of  temperature  between   land  and 
sea  in  the  same  latitude.    The  isobaric  gradients  established  by 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       613 

unequal  heating  may  be  higher  than  those  which  direct  the  planet- 
ary winds.  In  such  cases  the  planetary  winds  are  overcome  by 
seasonal  winds,  such  as  the  monsoons,  or  by  daily  breezes,  such  as 
land-  and  sea-breezes,  and  mountain  and  valley  breezes. 

Gradient,  velocity,  and  directions  of  wind.  The  slope  of  an 
isobaric  surface  is  its  gradient.  Gradient  is  differently  expressed 
in  different  countries.  In  England  the  barometric  gradient  is 
said  to  be  1  when  the  difference  of  pressure  is  0.01  of  an  inch  in 
17  miles.  In  the  United  States,  barometric  gradient  is  commonly 
denned  as  the  difference  in  pressure  at  the  same  level  between  two 
points  which  are  distant  from  each  other  the  length  of  1°  of  lati- 
tude. Thus  if  two  places  5°  apart  in  latitude  have  a  difference  of 
pressure  of  0.5  inch,  the  gradient  is  0.10  of  an  inch.  Stated 
mathematically,  30-29.50=  .50-7-5  =  0.10. 

The  greater  the  gradient,  the  greater  the  velocity  of  the  wind. 
On  the  isobaric  chart,  high  gradient  is  expressed  by  the  crowding 
of  isobaric  lines.  The  crowding  of  such  lines,  therefore,  means 
high  winds.  A  gradient  of  0.10  inch  means  a  wind  of  about  30 
miles  an  hour,  and  a  gradient  of  0.20  means  a  wind  of  about  55 
miles  an  hour.  These  figures  presume  a  plane  surface.  The  actual 
velocity  at  the  bottom  of  the  atmosphere  is  much  modified  by 
the  shape  of  the  surface.  The  rougher  the  surface,  the  less  the 
velocity.  Observations  have  shown  that  the  velocity  of  the  wind 
at  the  height  of  low  buildings  (say  40  to  80  feet)  on  land  is  only 
about  one-fourth  as  great  as  that  at  an  elevation  of  40  feet  on 
the  sea;  while  at  a  height  of  100  to  150  feet,  the  velocity  is 
half  that  over  the  sea  at  an  elevation  of  40  feet. 

In  general,  the  average  velocity  of  winds  is  greatest  in  latitude 
50°  or  thereabouts.  The  average  velocity  for  the  United  States 
has  been  estimated  at  about  9.5  miles  per  hour,  and  for  Europe, 
10.3.  The  velocity  is  greater  over  the  sea  than  over  the  land, 
largely  because  it  is  checked  on  land  by  friction  with  the  uneven 
surface,  with  vegetation,  buildings  etc.1  It  is  also  greater  in  the 
upper  air  than  in  the  lower,  for  the  same  reason.  The  following 
table  gives  the  velocity  of  the  wind  at  various  levels  above  the 
bottom.  It  is  based  on  observations  on  the  movement  of  clouds 
at  Blue  Hill  Observatory,  near  Boston. 

1  Hchr.l-oltz  has  calculated  that  if  the  whole  body  of  air  were  set  in 
motion  at  the  uniform  rate  of  20  miles  per  hour,  it  would  take  nearly  43,000 
years  to  slow  it  down  to  10  miles  as  a  result  of  friction. 


614  PHYSIOGRAPHY 

COMPUTED  EASTERLY  OR  WESTERLY  WIND  VELOCITIES  ALONG  A  MERIDIAN. 


E=easterly,  W  =  westerly.     Velocity  of  wind 
in  miles  per  hour  at  various  altitudes. 

Increase  in   easterly 
velocities  with  eack 
(about)   3300    feet 

Latitude. 

in  altitude. 

Sea-level. 

About 
3300  feet. 

About 
13,200  feet. 

Miles  per  hour. 

N.  Lat.  75° 

E.       2.7 

W.      0.2 

W.      9.2 

W.       +3.0 

70° 

E.       2.0 

W.      2.0 

W.    14.3 

W.           4.1 

65° 

W.     0.1 

W.     4.9 

W.    19.3 

W.          4.8 

60° 

W.      2.4 

W.      7.6 

W.    23.1 

W.          5.2 

55° 

W.      3.4 

W.     8.7 

W.    24.5 

W.          5.3 

50° 

W.      3.3 

W.     8.7 

W.    24.9 

W          5.4 

45° 

W.      3.0 

W.     8.5 

W.    25.0 

W.          5.5 

40° 

W.      1.6 

W.     7.2 

W.    24.0 

W.          5.6 

35° 

E.       0.7 

W.      5.0 

W.    22.4 

W.          5.8 

30° 

E.       5.3 

W.      0.6 

W.    18.2 

W.          5.9 

25° 

E.       8.9 

E.       3.1 

W.    14.4 

W.          5.8 

20° 

E.       9.4 

E.       3.8 

W.    13.0 

W.         5.6 

N.  Lat.  15° 

E.      7.8 

E.       4.3 

W.      6.1 

W.         3.5 

Equator     0° 

S.  Lat.  15° 

E.     15.6 

E.     10.5 

W.      4.8 

W.         5.1 

20° 

E.     13.0 

E.       8.2 

W.      6.4 

W.         4.8 

25° 

E.       6.4 

E.       1.7 

W.    12.5 

W.         4.7 

30° 

W.      2.4 

W.     7.0 

W.    21.0 

W.         4.7 

35° 

W.      7.7 

W.    12.3 

W.    26.1 

W.         4.6 

40° 

W.    11.6 

W.    16.2 

W.    30.0 

W.         4.6 

45° 

W.-  14.9 

W.    19.5 

W.   33.3 

W.         4.6 

50° 

W.    17.1 

W.   21.7 

W.    35.7 

W.         4.6 

55° 

W.    17.0 

W.    21.6 

W.    35.6 

W.         4.7 

a  Lat.  60° 

W.    13.6 

W.    18.2 

W.    32.2 

W.      +4.7 

This  table  shows  that  there  is  an  increase  of  velocity  with  in- 
crease of  altitude,  ranging  from  1  to  2  miles  per  hour  for  1000  feet. 
According  to  the  table,  the  trade-winds  do  not  reach  up  to  alti- 
tudes of  13,200  feet,  for  at  this  altitude  all  winds  are  represented 
as  blowing  to  the  east.  The  table  also  represents  them  as  extend- 
ing farther  from  the  equator,  especially  in  the  northern  hemi- 
sphere, in  low  altitudes,  than  higher  up. 


THE  GENERAL,  CIRCULATION  AND  PRECIPITATION 

Rainfall  is  of  the  utmost  importance  to  most  land  life,  both 
plant  and  animal.  This  is  shown,  in  a  general  way,  by  the  ab- 
sence of  forests  and  the  meagreness  of  herbaceous  vegetation  in 
arid  regions;  and  wherever  plant  life  is  scanty,  animal  life  is  also 
relatively  scarce.  Human  industries,  too,  are  much  affected  by 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       615 

the  amount  and  distribution  of  the  rainfall,  as  shown  by  the  fact 
that  no  arid  region  supports  a  dense  population.  Nevada,  almost 
all  of  which  receives  less  than  10  inches  of  rain  per  year,  had,  in 
1900,  only  one  inhabitant  for  each  two  and  a  half  square  miles. 
Only  3.4  per  cent,  of  the  population  of  the  United  States  lives  in  the 
third  of  the  country  where  the  rainfall  is  less  than  20  inches  per 
year.  The  best  of  soil  is  unproductive  unless  adequately  watered. 
Twenty  inches  of  rain  per  year  is  generally  considered  to  be  the 
minimum  for  general  agricultural  purposes,  but  something  depends 
on  the  latitude  and  something  on  the  seasonal  distribution  of  the 
rain.  The  warmer  the  climate,  the  more  the  rainfall  needed,  be- 
cause of  the  greater  evaporation ;  and  the  aggregate  amount  neces- 
sary is  less  if  it  falls  when  the  growing  crops  need  it  most.  If 
rainfall  could  be  ideally  distributed,  the  half  of  20  inches  would 
probably  be  adequate  in  the  middle  latitudes  of  the  United  States. 
Rain  or  snow  falling  at  times  when  plants  are  not  growing  is, 
however,  not  worthless,  for  some  of  it  remains  underground,  and 
is  available  for  plants  at  a  later  time.  The  secret  of  the  successful 
"dry  farming,"  which  is  just  now  attracting  much  attention,  con- 
sists in  so  treating  the  soil  that  the  water  which  falls  during  all 
parts  of  the  year  is  retained  in  the  soil  and  subsoil  till  the  growing 
season. 

Land  so  situated  that  it  may  be  irrigated  is  not  directly  depend- 
ent on  rain  and  snow;  but  the  water  used  in  irrigation  is  derived 
from  rainfall,  though  the  fall  is  often  far  from  the  place  where  the 
water  is  used.  Great  as  the  results  of  irrigation  are  likely  to  be 
in  our  own  country,  it  will  never  make  more  than  a  fraction  of  the 
arid  land  valuable  for  agricultural  purposes,  because  the  amount  of 
water  available  is  limited. 

The  distribution  of  rainfall  is  largely  influenced  by  the  winds, 
which  bear  moisture  from  the  places  where  it  is  evaporated,  to  the 
places  where  the  temperature  favors  its  condensation  and  precipi- 
tation. Prevailing  winds,  periodic  winds,  and  aperiodic  winds  all 
play  their  part  in  determining  where  rain  falls,  how  much  falls, 
and  at  what  times  of  the  year.  The  vertical  movements  of  the  air, 
too,  have  something  to  do  with  rainfall,  and  in  some  places  are 
more  important  than  the  horizontal  movements  to  which  the  name 
winds  is  usually  restricted. 

To  know  the  rainfall  (or  snowfall)  of  any  given  region,  it  is 
needful  to  know  (1)  what  winds  affect  it,  (2)  the  topography  of 


616 


PHYSIOGRAPHY 


the  surface  over  which  the  winds  have  already  blown  before  reach- 
ing it,  and  (3)  the  topographic  situation  and  relations  of  the  place 
itself. 

Rainfall  in  the  zones  of  the  trades.  In  the  trade-wind  zones 
the  winds  are  blowing  from  higher  to  lower  latitudes,  and  therefore, 
on  the  whole,  from  cooler  to  warmer  latitudes.  As  the  air  is 
warmed,  it  is  capable  of  taking  more  moisture.  So  long  as  the 
trades  blow  on  the  sea,  therefore,  they  would  not  ordinarily  give 
rain.  Where  they  blow  over  low  lands,  which  in  this  latitude  are 
warmer  than  the  sea,  they  take  moisture  rather  than  give  up  what 


FIG.  604. — Map  showing  the  precipitation  for  the  world. 

they  have.  On  the  sea  and  on  low  lands,  therefore,  the  trade- 
winds  are  "  dry  "  winds.  Sahara  and  considerable  parts  of  Australia 
are  essentially  desert,  largely  because  of  the  drying  influence  of 
the  trades. 

If,  however,  the  air  of  the  trades  is  forced  up  over  mountains, 
it  is  cooled,  and  some  of  its  moisture  may  be  condensed  and  may 
fall  as  rain  or  snow.  The  windward  sides  of  high  mountains  in  the 
trade-wind  zone  should  therefore  have  heavy  rainfall.  An  illustration 
is  afforded  by  the  east  side  of  the  Andes  Mountains  in  tropical 
latitudes,  where  the  rainfall  is  heavy  (Fig.  604).  Another  illus- 
tration is  afforded  by  the  volcanic  cones  of  the  Hawaiian  Islands. 
The  trade-winds  yield  little  rain  to  their  lower  slopes,  but  forced  up 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       617 

over  the  mountains,  they  yield  abundant  moisture  in  the  cooler 
altitudes.  The  level  of  the  rainfall  is  readily  seen  by  the  change 
in  vegetation. 

After  the  air  of  the  trades  passes  over  a  mountain  range,  it 
descends,  and  is  warmed  both  by  contact  with  the  warm  surface 
and  by  compression.  It  therefore  takes  up  moisture.  The  lee- 
ward sides  of  mountains  in  the  trade-wind  zones  should  therefore 
be  regions  of  little  precipitation.  The  west  slope  of  the  Andes 
Mountains  is  a  case  in  point  (Fig.  604).  A  high  mountain  range 
on  the  east  side  of  a  continent  in  the  zone  of  the  trades  would  tend 
to  make  all  the  area  to  the  west  of  it  dry,  unless  it  also  is  affected 
by  high  mountains. 

Since  the  trade-winds  shift  a  little  with  the  seasons  (Fig.  589), 
the  tracts  which  receive  rain  from  them  also  shift.  Tracts  which 
have  trade-wind  rains  at  one  season,  but  not  at  another,  often 
have  wet  and  dry  seasons,  and,  in  general,  the  dry  season  corre- 
sponds with  the  time  of  the  trade-winds. 

In  the  zone  of  equatorial  calms,  also  called  doldrums,  the  tem- 
perature is  high,  and  the  air,  warmed  by  the  sun  daily,  expands 
and  is  crowded  upward  by  the  cooler  air  which  comes  in  from  the 
xones  of  the  trades.  As  it  rises,  the  air  expands  and  cools,  and 
often  gives  up  some  of  its  moisture.  In  this  zone,  therefore,  there 
are  likely  to  be  daily  rains  from  cumulus  clouds  at  the  time  of  day 
when  the  upward  currents  are  strongest,  that  is,  in  the  afternoon. 
Since  the  doldrums  shift  north  and  south  a  few  degrees  yearly  with 
the  shifting  of  the  thermal  equator,  a  place  near  the  equator  which 
receives  the  daily  rains  during  one  season,  may  be  without  them 
at  another  time  of  the  year. 

In  the  zone  of  tropical  calms  (p.  601),  air  is  descending  rather 
than  rising,  and  so  yields  little  or  no  rain.  Like  the  equatorial 
calms,  these  extra-tropical  calms  shift  north  and  south  a  little  with 
the  sun.  They  are,  on  the  whole,  the  driest  latitudes  of  the  globe, 
crossing  Sahara,  Arabia,  Australia,  and  the  southern  part  of  South 
America. 

Rainfall  in  the  zones  of  the  prevailing  westerlies.  The  prin- 
ciples which  apply  to  the  trade-wind  zones  apply  also  in  the  zones  of 
the  westerly  winds.  These  winds  are,  on  the  whole,  blowing  from 
lower  to  higher  latitudes,  and  so  are  being  gradually  cooled.  They 
might  therefore  yield  some  moisture,  even  at  sea-level  or  on  low 
land,  and  especially  on  land  in  the  winter  season.  The  heat  of  the 


618  PHYSIOGRAPHY 

land  in  summer  often  prevents  condensation  and  precipitation  until 
the  air  has  moved  far  to  poleward.  When  such  winds  cross  moun- 
tains, they  yield  moisture  to  their  windward  slopes  and  summits, 
and  become  dry  on  the  leeward  slopes. 

Planetary  winds  alone  considered,  a  high  mountain  range  on 
the  west  side  of  a  continent  in  the  zones  of  westerly  winds  would 
make  all  the  low  land  to  the  east  of  it  dry. 

An  application  of  these  principles  will  help  us  to  understand 
the  rainfall  of  the  United  States,  so  far  as  it  is  dependent  on  planet- 
ary winds. 

Our  prevailing  winds  for  almost  all  the  country  are  from  the 
southwest.  Coming  on  to  the  land  from  the  Pacific  in  the  winter, 
these  winds  reach  the  cooler  land,  and  yield  moisture,  even  at  low 
levels.  This  gives  the  low  lands  of  California  their  wet  season. 
As  the  winds  blow  over  the  high  mountains  back  from  the  coast, 
they  yield  more  moisture,  so  that  all  the  area  west  of  the  crest  of 
the  first  high  range  is  well  supplied  with  rain  and  snow  in  the  winter 
season.  As  the  winds  blow  beyond  the  Sierras  and  Cascade  Moun- 
tains, the  air  descends  and  becomes  warmer,  and  therefore  dry. 
East  of  these  mountains  lie  the  semi-arid  lands  of  eastern  Oregon 
and  Washington,  and  the  Great  Basin  with  its  Great  Salt  Lake. 

When  these  winds  reach  the  higher  parts  of  the  Rocky  Mountains, 
which  are  often  higher  than  the  mountains  farther  west,  they  again 
yield  some  moisture.  But  farther  east,  all  the  way  to  the  Atlantic, 
these  winds,  taken  by  themselves,  would  remain  dry,  for  they  cross 
no  more  high  mountains,  and  they  do  not  generally  go  far  enough 
north  to  reach  a  temperature  as  low  as  that  of  the  mountains  they 
have  passed.  For  some  distance  east  of  the  mountains  the  rain- 
fall is  very  deficient;  but  east  of  central  Kansas  and  Nebraska  the 
lands  are  well  supplied  with  moisture.  Southeast  of  a  line  run- 
ning from  about  Galveston  to  Cleveland,  the  land  might  be  sup- 
plied with  moisture  by  the  southwesterly  winds  from  the  Gulf,  but 
there  is  abundant  rainfall  far  to  the  west  of  this  line.  It  is  therefore 
clear  that  some  factor  other  than  the  westerly  winds  is  involved  in  the 
precipitation.  This  factor  is  the  aperiodic  cyclonic  winds,  to  be 
studied  in  the  next  chapter.  Passing  over  the  country  from  east  to 
west,  the  cyclones  cause  moist  air  to  flow  northward  from  the  Gulf  to 
higher  and  cooler  latitudes.  This  change  in  latitude,  together  with 
the  cooling  of  the  air  as  it  rises  in  the  cyclone,  is  the  cause  of  the 
precipitation  which  redeems  the  central  and  eastern  mrte  of  fVip 


GENERAL  CIRCULATION  OF  THE  ATMOSPHERE       619 

United  States  from  the  aridity  which  affects  the   belt  next  east  of 
the  Rockies. 

The  winds  which  blow  from  the  Pacific  to  the  continent  in  sum- 
mer have  a  somewhat  different  effect  upon  rainfall,  though  the 
principles  involved  are  the  same.  At  this  time  of  year,  the  winds 
which  blow  from  the  Pacific  to  the  low  lands  of  central  and  southern 
California  find  a  temperature  on  the  land  higher  than  their  own 
These  winds  are  therefore  dry  in  this  region,  and  give  to  much  of 
California  its  dry  season.  Blowing  inland,  these  winds  reach  moun- 
tains so  high  that  the  temperature  is  low  enough  to  give  rise  to  con- 
densation and  precipitation. 

Farther  north  the  case  is  somewhat  different.  In  Washington, 
for  example,  the  mountains  near  the  coast  are  high  enough  to  occa- 
sion precipitation  even  in  summer.  In  Alaska,  where  some  of  the 
mountains  are  always  covered  with  snow,  precipitation  is  heavy  in 
the  summer,  and  at  high  altitudes  it  often  falls  as  snow  instead  of 
rain. 

Monsoon  winds  likewise  yield  moisture  when  they  blow  from 
warmer  to  cooler  regions.  In  general  they  blow  toward  warmer 
regions,  and  so  should  be  dry  winds;  but  once  started  they  are 
sometimes  forced  up  over  high  mountains,  and  precipitation  follows. 
The  heaviest  recorded  rainfall  on  the  southern  slopes  of  the 
Himalayas,  is  due  to  monsoon  winds.  Numerous  famines  in  India 
have  followed  the  failure  of  the  monsoon  rains.  The  famine  of 
1876-78  affected  58,000,000  people  directly,  and  is  estimated  to 
have  cost  5,000,000  lives.  As  in  the  case  of  the  planetary  winds,  it 
is  the  windward  sides  of  the  mountains  which  receive  the  heavy 
precipitation  from  the  monsoons.  It  is  clear,  therefore,  that  the 
windward  sides  of  high  mountains  are  places  of  heavy  rain-  and 
snowfall. 

Land  and  sea  (or  lake)  breezes  (daily)  rarely  yield  much  rain, 
though  they  often  give  rise  to  fogs  when  they  blow  from  the  warmer 
water  to  the  cooler  land.  Such  fogs  may  occasionally  be  seen,  as 
at  Chicago  in  the  late  autumn  or  early  winter,  sometimes  ad- 
vancing over  the  land  with  a  wall-like  front,  varying  from  a  few 
feet  to  many  scores  of  feet  in  height. 

Valley  breezes  sometimes  give  rise  to  heavy  showers,  as  already 
noted. 


CHAPTER  XVin 
WEATHER   MAPS 

Aperiodic  Changes  of  Pressure 

FIG.  605  is  a  weather  map  for  the  United  States  for  January  12, 
1899.  Like  other  weather  maps,  it  shows  (1)  the  distribution  of 
atmospheric  pressure,  (2)  the  direction  of  the  winds  in  various  parts 
of  the  country,  (3)  the  condition  of  the  air  with  reference  to  cloudi- 
ness, rainfall,  snowfall,  etc.,  at  all  points,  and  (4)  the  temperature. 

1.  Isobars.  The  full  lines  of  the  weather  map  are  isobars. 
The  map  shows  a  range  of  pressure  from  30.6+  inches  in  the  area 
centering  about  the  Hudson  River  Valley,  to  29.5—  in  the  area  cen- 
tering in  North  Dakota.  The  pressure  is  high  (over  30  inches)  in 
the  eastern  half  of  the  country,  and  low  (less  than  30  inches)  in 
the  western  interior,  and  high  again,  but  not  very  high,  in  an  area 
near  the  Pacific  coast. 

The  isobar  of  30.6,  in  the  eastern  part  of  the  United  States,  is 
a  closed  line.  On  either  side  of  it  is  the  isobar  of  30.5.  Since  the 
pressure  rises  as  the  isobar  of  30.6  is  approached  from  either  side, 
it  is  inferred  to  continue  to  rise  after  this  isobar  is  passed.  The 
area  inside  it  is  therefore  inferred  to  have  a  pressure  of  more  than 
30.6  inches,  but  not  so  much  as  30.7  inches,  else  another  isobar  would 
have  been  represented. 

Similarly,  all  points  between  the  isobars  of  30.6  and  30.5  have 
pressures  intermediate  between  those  indicated  by  those  figures. 
The  pressure  is  higher  near  the  former  isobar,  and  less  near  the  latter. 

The  center  of  this  high-pressure  area  is  marked  "high."  "  High" 
on  the  weather  map  means  an  area  where  the  pressure  is  distinctly 
higher  than  that  of  its  surroundings,  and  generally  exceeds  30  inches, 
and  the  word  is  placed  in  the  center  cf  such  an  area.  The  move- 
ments of  air  about  a  high  are  an  anticyclone. 

620 


WEATHER  MAPS 


621 


To  the  west  of  this  "high,"  ;the  pressure  decreases  steadily  to 
North  Dakota,  where  there  is  a  center  of  low  pressure,  marked  "low." 
"Low"  means  an  area  in  which  the  pressure  is  less  than  30  inches, 
and  on  the  map  the  word  is  placed  at  the  point  in  such  an  area  where 
the  pressure  is  lowest.  The  movements  of  air  about  a  "low"  con- 
stitute a  cyclone.  A  cyclone  is  one  type,  and  in  middle  latitudes 
the  most  important  type,  of  a  storm. 

The  isobar  of  29.5  about  the  low  in  North  Dakota  is  a  closed 
line.  Since  the  pressure  is  becoming  less  as  this  line  is  approached, 


S jmfcoll  Indicate :     QClear       9  Partly  Cloudy        0  Cloudy  H   Rain          ®  Snow 


FIG.  605.— Weather  map  of  the  United  States  for  January  12,  1899.  The 
full  lines  are  isobars,  the  dotted  lines  isotherms.  (U.  S.  Weather 
Bureau.) 

it  is  inferred  that  the  pressure  at  all  points  within  this  isobar  is  less 
than  29.5,  though  nowhere  so  low  as  ,29.4.  At  all  points  between 
the  isobars  of  29.5  and  29.6,  the  pressure  is  between  these  figures. 
West  of  the  "low"  the  pressure  increases.  The  pressure  in  the 
high  near  the  Pacific  coast  is  not  so  great  as  that  in  the  high  over 
the  Hudson  Valley. 

Most  weather  maps  show  both  lows  and  highs,  or  at  least 
one  of  each.  This  means  that  there  is  generally  at  least  one  area 
of  high  pressure  (anticyclone)  and  one  of  low  pressure  (cyclone)  at 
the  same  time  within  the  area  of  the  United  States.  Since  this  is 


622  PHYSIOGRAPHY 

the  case,  the  atmospheric  pressures  are  generally  somewhat  unequal 
in  different  parts  of  the  country. 

Weather  maps  are  made  by  the  Weather  Bureau,  a  branch  of  the 
national  Department  of  Agriculture.  They  are  prepared  in  vari- 
ous central  offices  of  the  country.  To  these  offices  the  facts  con- 
cerning the  pressure  and  the  temperature  of  the  air,  the  direction 
and  velocity  of  the  wind,  the  cloudiness,  and  the  precipitation,  are 
telegraphed  daily  from  numerous  points  or  "stations"  established 
and  maintained  by  the  Government. 

2.  Wind.  Wherever  barometric  pressures  are  unequal,  iso- 
baric  surfaces  are  uneven.  They  are  depressed  in  cyclones,  and 
elevated  in  anticyclones.  As  a  result,  there  must  be  winds  from 
anticyclones  to  cyclones.  On  January  12,  1899  (Fig.  605),  winds 
must  have  been  blowing  out  from  the  highs  in  the  east  and 
west  respectively,  and  toward  the  low  in  the  northwest,  on  the 
day  when  the  pressures  were  as  indicated  on  the  map.  The  arrows 
on  the  map  show  the  direction  of  the  winds,  which  blew  as  the 
arrows  fly,  as  reported  from  the  various  stations. 

It  will  be  seen  that  winds  do  not  blow  straight  out  from  the 
anticyclonic  centres,  nor  straight  in  toward  the  cyclonic  centers. 
They  doubtless  start  straight  out  from  each  high,  but  they  are 
deflected  toward  the  right,  as  most  of  the  arrows  about  the  anti- 
cyclones show.  Similarly,  the  winds  which  blow  toward  the  cy- 
clonic centers  do  not  blow  straight  toward  them,  but  are  deflected  a 
little  to  the  right,  as  most  of  the  arrows  about  the  lows  show. 
In  the  southern  hemisphere  the  deflections  would  be  to  the  left. 
Fig.  606  shows  the  theoretic  circulation  about  highs  and  lows: 
A,  northern  hemisphere ;  B,  southern  hemisphere. 

It  will  be  noted  that  two  arrows  in  the  western  high  (Fig.  605) 
are  directed  toward  the  center  of  the  high-pressure  area.  They 
probably  mean  that  there  are  subordinate  centres  of  lesser  pressure 
within  the  general  area  of  the  anticyclone,  and  the  winds  blow 
toward  them.  If  this  is  the  case,  the  subordinate  lows  are  too 
weak  to  be  shown  by  the  isobars,  which  represent  differences  of  0. 1 
inch. 

Something  as  to  the  strength  of  the  winds  at  various  points  may 
be  inferred  from  the  map.  The  distance  from  the  center  of  the 
high  in  the  east,  Fig.  605,  to  Lake  Michigan  is  about  800  miles. 
The  difference  in  pressure  is  about  0.5  inch.  The  gradient  is 
therefore  about  1  (English  system).  This  means  a  wind- velocity 


WEATHER  MAPS 


623 


of  about  12  miles  per  hour — a  fresh  breeze — between  these 
points.  The  velocity  of  the  wind  blowing  from  Michigan  to  North 
Dakota  is  about  the  same.  The  velocity  of  the  wind  from  Texas 
to  North  Dakota  is  much  less.  In  general,  where  isobars  are 
crowded,  the  gradient  is  high  and  the  winds  strong.  Where  they 
are  widely  separated,  the  gradient  is  low  and  the  air-flow  gentle. 
The  winds  in  cyclonic  storms  occasionally  attain  a  velocity  of  40 
to  60  miles  an  hour;  but  the  average  is  much  less,  and  the  cyclonic 


FIG.  606. — Diagram  showing  the  deflection  of  air  currents  about  highs  and 
lows.     A,  northern  hemisphere ;  B,  southern  hemisphere. 

(not  tornadic,  p.  667)  wind  which  is  violent  enough  to  be  destruc- 
tive is  rare. 

The  circulation  of  air  about  a  cyclone  is  vertical  as  well  as 
horizontal:  the  air  currents  move  in  toward  the  center  of  the 
storm,  and  spirally  up  at  the  same  time.  This  upward  movement 
is  of  great  consequence  in  its  effect  on  precipitation.  The  upward 
and  outward  course  of  the  air  movement  in  the  cyclone  is  shown 
in  Fig.  607,  which  represents  a  vertical  section  of  a  cyclone,  and 
shows  that  the  outflow  above  is  chiefly  to  the  eastward,  the  direc- 
tion in  which  prevailing  winds  blow. 

3.  Cloudiness,  precipitation,  etc.  On  the  weather  maps  the 
open  circle  on  the  shaft  of  an  arrow  indicates  clear  skies;  the 


624  PHYSIOGRAPHY 

half-blackened  circle  shows  that  the  sky  is  partly  cloudy;  while  the 
black  circle  (Texas,  Wyoming,  etc.)  indicates  general  cloudiness. 
Where  R  appears  on  the'  arrow,  it  means  that  rain  is  falling,  as, 
for  example,  in  Iowa  and  Alabama.  Where  S  appears  in  the 
same  position,  it  shows  that  snow  is  falling,  as  in  north-western 
Minnesota,  Virginia,  and  Maryland. 

This  weather  map  shows  that  more  or  less  precipitation  accom- 
panies this  cyclone,  and  the  examination  of  a  series  of  weather 
maps  will  show  that  cyclones  are  vary  often  attended  by  rain  or 
snow.  Whether  the  precipitation  takes  the  form  of  rain  or  snow 
depends  on  the  temperature. 

4.  Temperature.  The  dotted  lines  of  the  weather  map  are  iso- 
therms. The  isotherm  of  50°  F.  (Fig.  605)  crosses  the  Gulf  States. 
South  of  it  the  temperature  is  above  50°,  but  not  so  high  as  60°, 
within  the  area  of  this  map.  The  isotherm  of  40°  is  more  irregular. 
It  extends  from  Georgia  to  New  Mexico,  but  between  these  points 
it  turns  north  into  Nebraska.  All  points  between  this  isotherm 
and  that  of  50°  have  a  temperature  intermediate  between  40°  and 
50°. 

The  isotherm  of  30°  is  still  more  irregular.  Dubuque,  la., 
Chicago,  Cleveland,  Charlotte,  N.  C.,  and  Norfolk,  Va.,  have 
about  the  same  temperature.  An  isotherm  of  30°  also  extends  from 
Idaho  to  Naw  Mexico  by  a  crooked  course,  while  a  third  isotherm  of 
30°  appears  about  the  low.  Two  isotherms  of  30°  are  therefore 
next  each  other  on  the  map,  one  in  the  area  to  the  east,  and  the 
other  in  the  area  to  the  southwest  of  the  low. 

The  temperature  between  these  isotherms  is  to  be  interpreted  as 
follows:  As  the  low  is  approached  from  the  east,  say  from  New 
York,  the  temperature  rises.  In  the  middle  of  Lake  Superior  the 
temperature  is  20°,  and  at  Duluth  30°.  The  next  isotherm  to  the 
west,  instead  of  being  40°,  is  30°,  and  the  one  still  farther  west  is  20°' 
This  arrangement  of  isotherms  means  that  the  temperature  west  of 
the  isotherm  of  30°  passing  through  Duluth  is  warmer  than  30°> 
but  not  so  warm  as  40°;  while  farther  west  the  temperature  again 
becomes  cooler,  reaching  30°  in  the  eastern  part  of  North  Dakota. 

On  the  whole,  the  isotherms  show  two  pronounced  features: 
(1)  they  have  little  relation  to  parallels,  for  places  in  the  same  lati- 
tude may  have  very  different  temperatures,  and  places  far  apart  in 
latitude  may  have  the  same  temperature;  and  (2)  the  isotherms 
show  a  pronounced  disposition  to  bend  poleward  where  the  isobars 


WEATHER  MAPS 


625 


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WEATHER  MAPS  627 

indicate  low  pressure,  and  equatorward  where  the  pressure  is 
high. 

Fig.  608  shows,  by  graphs,  four  of  the  weather  elements  for  the 
year  at  Chicago.  The  figure  shows  that  the  winds  are  strongest  in 
cold  weather,  that  the  proportion  of  sunshine  is  highest  in  mid- 
summer, while  precipitation  is  greatest  in  the  early  summer.  The 
same  weather  elements  at  other  localities  would  give  somewhat 
different  graphs,  and  in  some  cases  they  would  be  very  different 
(see  Figs.  662  to  673). 

All  the  weather  maps  which  follow  show  some  relationship  be- 
tween isobars  and  isotherms.  In  general  the  isotherms  curve 
southward  (equatorward)  about  a  high,  and  northward  (poleward 
about  a  low.  To  this  general  rule  there  are  some  exceptions. 

The  temperature,  the  pressure,  the  winds,  the  cloudiness,  the 
rain,  etc.,  are  the  elements  of  the  weather.  All  these  things  being 
shown  on  the  above  map,  it  is  appropriately  called  a  weather  map. 

The  lows  and  highs  are  sometimes  much  more  pronounced  than 
those  shown  in  Fig.  605.  In  Fig.  609  the  low  is  more  pronounced, 
the  pressure  ranging  from  29.0  at  the  center,  to  30.1  in  the  east 
and  to  30.5  in  the  west.  So  great  a  range  of  pressure  as  shown  by 
this  map  is  not  of  common  occurrence.  The  isobars  are  closer  to- 
gether in  this  figure  than  in  the  preceding,  and  therefore  indicate 
stronger  winds.  The  approximate  velocity  of  the  wind  at  various 
points  may  be  calculated  from  the  map.  The  direction  of  the 
winds  about  the  low  is  the  same  as  in  Fig.  605.  Cloudy  skies 
prevail  in  the  southeastern  part  of  the  low,  and  snow  is  falling  at 
some  points  (Montreal,  Duluth).  The  map  also  shows  great  ranges 
of  temperature  in  areas  not  far  apart.  Thus  there  is  a  temperature 
of  30°  F.  at  Sault  Ste.  Marie,  and  a  temperature  of  - 10°  at  Winni- 
peg, but  little  farther  north;  while  Montreal  has  a  temperature 
above  that  of  Santa  Fe~.  As  in  preceding  illustrations,  the  low 
temperature  goes  with  high  pressure,  and  the  higher  temperature 
with  low  pressure. 

Fig.  610  shows  a  large  and  less  symmetrical  low.  The  winds 
blow  toward  it,  but  are  deflected  to  the  right  of  its  centre.  Cloudi- 
ness prevails  over  a  great  area  about  the  cyclone,  and  snow  and 
rain  are  falling  at  some  points. 

The  low  of  this  map  dominates  almost  the  whole  country. 
Measuring  from  the  30-inch  isobar  on  the  east  to  the  30-inch  isobar 
on  the  west,  the  cyclone  is  about  1800  miles  across.  The  isotherms 


628 


PHYSIOGRAPHY 


bend  northward  on  the  south  side  of  this  low,  while  they  curve 
southward  about  the  high  north  of  Montana.     Fig.  611  shows  an 


85°    80°    75r    70J    65' 


115° 110J 105' 100- 95° 90°       85 


FIG.  609. — Weather  map  for  January  16,  1901.     (U.  S.  Weather  Bureau.) 


125°          120=          115°       110°       105°       100°        95°        90°        85°         80°         75°         70°         65" 


115"  HO'  105"  10W  95"  90°  85°  80°  75°  70° 


FIG  610. — \Veather  map  showing  a  large  asymmetrical  low,  March  2,  1904. 
(U.  S.  Weather  Bureau.) 

elongate  cyclone,  one  diameter  of  which  is  very  long;  and  Fig.  612 
shows  its  transformation  the  succeeding  day.     The  isotherms  of  Fig. 


WEATHER  MAPS 


629 


612  show  few  peculiarities,  save  in  the  northwest  (Nebraska,  Wyo- 
ming,  Montana),  where  the  temperature  drops  from  20°  near  Rapid 


FIG.  611. — Weather  map  showing  a  large  elliptical  cyclone,  January  22, 1900 
(U.  S.  Weather  Bureau.) 


6°       100°        95"        90"        85°         80"         75°         70°         65° 


115°  110° 105°  100° 95° 90° 85° 80°  75° 70° 


FIG.  612. — Weather  map  for  January  23,  1906,  showing  great  changes  in  the 
cyclone  of  the  preceding  day.     (U.  S.  Weather  Bureau.) 

City,  South  Dakota,  to  —30°  at  Q'Appelle,  in  Alberta,  a  difference 
far  greater  than  can  be  accounted  for  by  the  difference  in  latitude. 


630 


Q'Appelle,  it  will  be  seen,  is  southeast  of  a  high  where  the  fall 
of  temperature  is  pronounced,  as  shown  by  the  crowding  of  the 
isotherms  in  South  Dakota.  The  rapid  change  of  temperature  is  in 
a  region  where  the  wind  is  strong,  and  from  the  northwest. 

Around  all  the  preceding  cyclones  some  precipitation  is  indi- 
cated, while  around  most  of  the  anticyclones  there  is  an  absence 
of  precipitation.  The  chief  reason  for  rainfall  or  snowfall  about 


FIG.  613.  —  Weather  map  for  December  9,  1898,  showing  a  high  of  great  area. 
(U.  S.  Weather  Bureau.) 

a  low  is  as  follows:  The  inflowing  air  produces  an  upward  spiral 
current,  and  the  rising  air  expands  and  is  cooled  (p.  537),  and  so 
gives  up  some  of  its  moisture.  In  the  southeast  quadrant  of  the 
cyclone,  additional  precipitation  results  from  the  fact  that  the 
air  entering  the  cyclone  is  passing  from  warmer  to  cooler  latitudes. 
This  is  perhaps  one  reason  why  the  precipitation  about  a  cyclone 
is  greatest  in  this  quadrant.  The  right-handed  movement  of  the 
air  in  the  northern  hemisphere  tends  to  shift  the  center  of  prin- 
cipal precipitation  somewhat  to  the  east  of  south  of  the  center 
of  the  cyclone. 

In  the  anticyclone  there  is  a  descending  spiral  movement  of 
The  descending  air  comes  from  an  altitude  which  is  colder 


ar. 


than  that  at  the  bottom  of  the  atmosphere,  and  hence  brings  a 
low  temperature.     Since  the  air  is  compressed  and  warmed  as  it 


WEATHER  MAPS 


631 


descends,  the  winds  from  anticyclones  generally  bring  clear  weather. 
The  downward-  and  outward-moving  air  may,  however,  so  mingle 


115°      110°      105°      100°      95°       90°       85°       80°       75°       70° 


FIG.  614. — Weather  map  for  September  24,  1903.     The  shaded  area  in  this 
and  succeeding  maps  represents  precipitation.     (U.  S.  Weather  Bureau.) 


125°          120°          115°       110°        105°       100°        95°         90"        85 


115°  110°  105°  100°  95°  90°  85°  80°  •      75'  70° 


FIG.  615.— Weather  map  for  September  25,  1903.  (U.  S.  Weather  Bureau.) 
with  the  warm  air  about  it  as  to  cause  some  of  the  moisture  of  the 
latter  to  condense,  giving  rise  to  clouds,  or  even  to  precipitation. 


632 


PHYSIOGRAPHY 


Highs  of  great  area,  as  well  as  lows  of  great  area,  sometimes 
occur.  Fig.  613  shows  a  high  or  anticyclone  some  2200  miles 
across,  with  a  great  range  of  pressure.  The  isotherms  of  this 
chart  stand  in  very  definite  relations  to  the  isobars,  low  tem- 
peratures going  with  high  pressures.  Denver,  in  the  high,  is 
about  30°  colder  than  the  southern  part  of  Maine,  3°  farther  north, 
in  a  low. 

Movements  of  Cyclones  and  Anticyclones.  The  highs  and 
lows  do  not  remain  in  the  same  place  from  day  to  day.  This  is 
shown  by  Figs.  614-620,  as  well  as  by  the  other  weather  maps 
which  follow,  showing  the  weather  of  successive  days. 

In  Fig.  614  there  is  (1)  a  low  over  t!:e  Gulf  of  St.  Lawrence; 
(2)  a  high  central  over  Iowa;  (3)  a  low  over  British  Columbia; 
(4)  a  high  in  Oregon. 

The  map  of  the  succeeding  day  (Fig.  615)  shows  (1)  that  the 
low  of  the  St.  Lawrence  Gulf  has  moved  to  the  east;  (2)  that  the 
high  of  the  interior  has  moved  to  West  Virginia;  (3)  that  the  low 


FIG.  616. — Weather  map  for  September  26,  1903.     (u.  o.  \Veau.er 

which  was  over  British  Columbia  has  moved  to  Dakota;  while  (4) 
the  high  of  the  Oregon  coast  remains  about  where  it  was. 

The  map  of  the  succeeding  day  (Fig.  616)  shows  (1)  that  the 
high  of  the  Virginias  has  moved  on,  but  not  so  far  as  on  the  pre- 


WEATHER  MAPS 


633 


I25J          120'          115"       110'       105'       100'        95'        90          85J         80  75J          70'         65' 


115°  110"  105°  100"  95°  90°  85 


FIG.  617. — Weather  map  for  September  27,  1903.  The  symbol  which  appears 
in  central  Arkansas  and  western  Tennessee  indicates  a  thunder-storm  at 
or  near  the  point  where  the  symbol  occurs,  during  the  twelve  hours  pre- 
ceding the  issue  of  the  weather  map.  (U.  S.  Weather  Bureau.) 


115°  110°  105°  100°  95°  90°  85°  80"  75°  70° 

FIG.  618.— Weather  map  for  September  28,  1903.     (U.  S.  Weather  Bureau.) 


634 


PHYSIOGRAPHY 


ceding  day;  (2)  that  the  low  which  was  over  JNorth  Dakota  is 
now  north  of  Lake  Superior;  (3)  that  the  high  of  Oregon  has 
moved  east  to  Idaho  and  Montana;  and  (4)  that  a  weak  low  has 
developed  in  Indian  Territory. 

The  map  of  the  27th  (Fig.  617)  shows  (1)  that  the  high  which 
was  over  the  Virginias  has  disappeared,  presumably  to  the  east; 
(2)  that  the  low  which  was  north  of  Lake  Superior  is  now  north  of 
Lake  Ontario;  (3)  that  the  high  of  Montana  has  moved  southeast 
to  Kansas;  (4)  that  the  weak  low  in  Oklahoma  and  Indian  Ter- 
ritory has  disappeared;  and  (5)  that  another  low  has  appeared 
in  southern  California.  The  succeeding  map  (Fig.  618)  shows  that 
all  the  highs  and  lows  of  the  preceding  map  have  advanced  in  a 
general  easterly  direction.  Fig.  619  shows  that  the  two  lows  of 


FIG.  619. — Weather  map  for  September  29,  1903.     (U.  S.  Weather  Bureau.) 

Fig.  618  near  the  Pacific  have  unitsd,  the  southerly  one  having 
moved  over  to  the  more  northerly — a  not  uncommon  occurrence. 
Fig.  620  shows  the  progress  of  this  low  as  well  as  of  other  highs 
and  lows,  and  a  great  rain  area  about  the  central  low. 

While  the  highs  and  lows  of  these  maps  have  all  moved  in  a 
general  easterly  direction,  the  highs  moved  rather  more  to  the 
south  of  east  than  the  lows.  The  direction  of  the  progress  of  the 
highs  and  lows  shown  by  these  maps  is  the  normal  one,  though 


WEATHER  MAPS 


635 


individual  cyclones  and  anticyclones  depart  notably  from  the  nor- 
mal. The  average  direction  of  the  cyclone  in  our  middle  latitudes 
is  about  N.  80°  E.,  or  10°  north  of  east.  The  anticyclones  have  a 
somewhat  more  southerly  course. 

From  the  study  of  these  maps  not  only  the  fact  of  movement, 
but  the  rate  of  movement  of  the  highs  and  lows,  may  be  cal- 
culated. Thus,  from  the  25th  to  the  26th  (Figs.  615  and  616), 
the  low  of  British  Columbia  moved  about  1200  miles.  From 
the  26th  to  the  27th,  and  again  from  the  27th  to  the  28th,  the  same 


115°       110'       105' 


FIG.  620.— Weather  map  for  September  30,  1903.     (U.  S.  Weather  Bureau.) 

storm  moved  between  600  and  700  miles,  while  from  the  28th  to 
the  29th  the  movement  was  about  800  miles.  The  average  veloc- 
ity of  cyclones  in  the  United  States  is  a  little  less  than  29  miles  per 
hour  (about  700  miles  per  day);  that  of  anticyclones  somewhat 
less. 

It  is  not  to  be  understood  that  the  rate  of  progress  of  the  storm 
is  the  same  as  the  velocity  of  the  wind.  The  velocity  of  the  wind 
depends  on  the  isobaric  gradients.  A  weak  cyclone,  that  is,  a  cy- 
clone in  which  differences  of  pressure  are  not  great  (Fig.  620), 
gives  rise  to  weak  winds,  even  though  the  center  of  the  storm 
moves  rapidly.  A  strong  cyclone,  that  is,  one  in  which  the  dif- 


636  PHYSIOGRAPHY 

ferences  of  pressure  are  great  (Fig.  609) .  gives  origin  to  strong  winds, 
even  though  the  cyclone  itself  moves  forward  slowly. 

Figs.  621  and  622  show  the  progress  of  lows  and  highs,  or 
cyclones  and  anticyclones,  from  December  24  to  December  25, 
1904.  The  course  of  the  low  central  over  Oregon  on  the  24th,  is 
indicated  by  the  arrows  on  the  map  of  the  25th.  Figs.  623-626 
show  the  movement  of  cyclones  and  anticyclones  for  four  consecu- 
tive days  in  February,  1903,  and  especially  the  course  of  a  low  from 
Arizona  (Fig.  623)  to  Maine  (Fig.  626).  Figs.  627  and  628  show 
similar  features  for  November  26  and  27,  1898.  The  progress 
of  highs  and  lows  shown  on  these  maps  (Figs.  614-628)  represents 
the  general  course  of  movement  of  most  similar  atmospheric  dis- 
turbances. 

The  mean  tracks  of  cyclones  and  anticyclones  for  the  United 
States  are  shown  in  Fig.  629.  The  heavier  lines  show  the  average 
paths  of  anticyclones,  and  the  lighter  the  tracks  of  cyclones. 
Some  anticyclones  enter  the  United  States  from  the  Pacific,  while 
others  originate  on  the  land  north  and  northwest  of  Montana. 
The  anticyclones  take  either  a  northerly  or  a  southerly  route  across 
the  continent.  The  former  extends  through  the  Great  Lakes  region 
to  southern  New  England,  while  the  latter  reaches  the  Atlantic  or 
the  South  Atlantic  coast.  Anticyclones  entering  from  the  Pacific 
may  take  either  of  these  courses,  and  those  originating  in  the  north- 
west may  do  the  same,  as  shown  by  the  figure. 

The  cyclones  originate,  or  first  appear,  in  various  places.  More 
of  them  originate  near  the  places  where  anticyclones  are  generated 
than  in  any  other  place;  but  not  a  few  originate  in  Colorado,  the 
Great  Basin,  in  Texas,  and  elsewhere.  Those  originating  in  the 
northwest  usually  pass  through  the  Great  Lakes  region  to  northern 
New  England.  Those  originating  farther  south  may  follow  a 
southerly  course  to  the  Atlantic,  or  may  pass  to  the  northward. 
Tropical  cyclones,  to  be  mentioned  later,  sometimes  reach  the 
Gulf  of  Mexico  from  lower  latitudes,  and  follow  the  coast  thence 
to  the  northeast. 

Still  another  set  of  lines  in  Fig.  629,  marked  1  day,  2  days,  3 
days,  and  4  days,  show  the  average  rate  of  daily  progress  of  the 
storms  which  come  in  from  the  northwest  on  successive  days. 

Weather  maps  are  sometimes  more  complicated  than  those 
shown  in  the  preceding  figures.  Fig.  630  is  a  weather  map  on 
which  four  highs  and  four  lows,  some  of  them  feeble,  appear. 


WEATHER  MAPS 


637 


638 


PHYSIOGRAPHY 


WEATHER  MAPS 


639 


640 


PHYSIOGRAPHY 


WEATHER  MAPS 


641 


642 


PHYSIOGRAPHY 


WEATHER  MAPS 


643 


644 


PHYSIOGRAPHY 


WEATHER  MAPS 


645 


The  map  also  gives  some  idea  of  the  way  lows  and  highs  follow 
each  other.  The  relations  of  isotherms  and  isobars  are  also  in- 
structive. 

It  will  be  readily  seen  that  the  passage  of  a  cyclone  involves 
a  change  in  the  direction  of  the  wind.  Thus  in  Fig.  623  the  wind 
at  Buffalo  is  easterly,  though  in  the  zone  of  westerly  winds.  The 
next  day.  after  the  storm  centre  has  moved  forward  beyond  Buf- 
falo (Fig.  624),  the  wind  is  westerly.  The  easterly  wind  of  an 
approaching  cyclone  is  generally  taken  as  a  sign  of  rain  through- 
out much  of  the  eastern  part  of  the  United  States. 


FIG.  629. — The  heavier  lines  show  the  tracks  of  anticyclones,  and  the  lighter 
lines  the  paths  of  cyclones.  Off  the  South  Atlantic  coast  anticyclones 
are  likely  to  turn  northward.  (U.  S.  Weather  Bureau.) 

Cyclones  do  not  affect  the  air  to  great  heights.  Even  when 
the  great  whirl  or  eddy  is  2000  miles  across,  as  is  sometimes  the 
case,  its  height  (depth)  is  rarely  more  than  4  or  5  miles. 

Weather  conditions  of  cyclones  and  anticyclones.  During  the 
passage  of  a  cyclone  some  air  is  drawn  from  lower  to  higher,  and 
therefore  from  warmer  to  cooler  latitudes.  In  midsummer  this 
often  gives  rise  to  the  "hot  wave"  (Fig.  631),  though  "hot  waves" 
are  not  always  closely  associated  with  cyclones.  Similar  winds 
are  known  as  the  sirocco  in  the  western  Mediterranean  region,  and 
they  go  by  other  names  elsewhere. 

"  Cold  waves"  often  attend  the  anticyclones.  These  winds  are 


646 


PHYSIOGRAPHY 


known  as  northers  in  the  southern  part  of  the  United  States  and 
sometimes  as  blizzards  in  the  northern  part,  though  this  name 


'.29! ...  .1  £   ...'19:.  .  .'05°       1 00°          5° 90°        85*         80°         75°          70' 65' 


110°  105°  100°  95°  90°  85'  80°  75' 


FIG.  630. — Weather  map  for  December  8,  1900.     (U.  S.  Weather  Bureau.) 


HOT  W ATE 

KA7.I1TUV  TEMPERATURES 
JULY  22,  1901. 


FIG.  631.     (U.  S.  Weather  Bureau.) 


usually  implies  heavy  snowfall    and  high  wind,  as  well  as  low 
temperature.      Fig.    632    shows    a    map  for    January    3,    1896, 


WEATHER  MAPS 


647 


115' 110" 105° 100' 95° 90"  85* 


FIG.  632. — Map  showing  the  minimum  temperatures  for  January  3,  1896. 
(U.  S.  Weather  Bureau.) 


125°          120°          115"       110°        105°       100°        95°         90°        85°         80°         75"          70°         65° 


115°  110°  105"  IOC?  95°  90°  65° 80° 19"  70° 


FIG  633. — Map  showing  the  minimum  temperatures  for  January  4,  1836. 
This  figure  shows  the  progress  of  the  cold  wave  from  the  preceding  day. 
At  this  time  a  freezing  temperature  has  reached  the  orange  groves  of 
Florida.  (U.  S.  Weather  Bureau.) 


648  PHYSIOGRAPHY 

and  Fig.  633  a  map  for  the  following  day.  The  high  of  Mon- 
tana has  advanced  to  Arkansas  and  Mississippi,  and  a  free/ing 
temperature  has  been  carried  down  to  the  orange  groves  of  Florida. 

The  mistral  of  southern  Europe  belongs  to  the  same  class  as 
the  northers  of  our  country. 

Origin  of  the  cyclones  and  anticyclones  of  intermediate 
latitudes.  The  origin  of  cyclones  and  anticyclones  is  not  well  under- 
stood. Centres  of  low  pressure  might  be  brought  about  by  the 
excessive  heating  of  certain  areas;  but  this  can  hardly  be  the  origin 
of  most  cyclones  of  temperate  latitudes,  for  they  are  more  common, 
stronger,  and  move  faster  in  winter  than  in  summer.  In  the  winter 
season  they  often  originate  in  areas  covered  with  snow,  where 
excessive  heating  is  impossible.  Similarly,  anticyclones  might  be 
conceived  to  result  from  the  unusual  cooling  of  certain  areas;  but 
that  this  is  not  their  course  seems  clear  from  the  fact  that  they 
sometimes  originate  in  warm  regions,  and  from  the  further  fact 
that  they  are  not  notably  more  abundant  in  cold  weather  than 
in  warm  weather. 

The  origin  of  both  sorts  of  disturbances  is  probably  to  be  referred 
to  atmospheric  movements  rather  than  to  atmospheric  temperatures 
directly.  The  cyclones  are  frequently  regarded  as  eddies  in  the 
descending  air  which  started  poleward  from  the  equator.  While 
this  may  be  true,  it  does  not  appear  to  be  a  satisfactory  statement 
concerning  the  origin  of  these  common  air-whirls. 

Tropical  cyclones.  Cyclones  sometimes  originate  in  tropical 
regions,  and  follow  courses  very  different  from  those  of  the  cyclones 
in  temperate  latitudes.  The  cyclones  of  this  class  affecting  North 
America  usually  originate  in  the  West  Indies,  and  are  most  com- 
mon in  the  late  summer  and  early  autumn.  They  follow  a  north- 
westerly course  until  the  latitude  of  Florida  is  reached.  Here  they 
commonly  turn  to  the  northward,  and  later  to  the  northeastward, 
and  have  a  tendency  to  follow  the  Atlantic  coast.  Figs.  634-637 
show  the  course  of  one  of  these  storms  in  August  (27-30),  1893, 
and  Fig.  638  shows  the  average  path  of  the  tropical  cyclones  for  the 
months  of  August,  September,  and  October,  for  the  years  1878  to 
1900.  Storms  of  this  sort  are  sometimes  called  hurricanes. 

The  tropical  cyclones  are  usually  more  pronounced  than  those 
of  temperate  latitudes ;  that  is,  the  gradient  is  higher  and  the  winds 
therefore  stronger.  They  often  do  great  damage  along  the  coast,  both 
to  shipping  and  to  the  low  lands  near  the  water.  The  storm  which 
worked  such  devastation  to  Galveston  in  September,  1900,  is  shown 


WEATHER  MAPS 


649 


650 


PHYSIOGRAPHY 


WEATHER  MAPS 


651 


652 


PHYSIOGRAPHY 


WEATHER  MAPS 


653 


in  Fig.  639,  which  also  shows  the  course  of  the  storm  both  before 
and  after  September  8.  The  strength  of  the  storm  was  exceptional, 
and  its  course  unusual^  as  will  be  seen  by  comparing  Fig.  639  with 
Fig.  638.  The  unusual  course  was  probably  due  to  the  combined 
influence  of  (1)  the  anticyclone  central  over  New  York,  which 
tended  to  keep  the  tropical  cyclone  from  advancing  in  that  direc- 
tion, and  (2)  the  cyclone  of  the  northwest,  which  favored  the 
movement  of  the  storm  in  that  direction  (Fig.  640).  Fig.  639 


FIG.  638. — Course  of  West  Indian  storms  for  August,  September,  and  Octo- 
ber, 1878-1900.  The  lighter  lines  show  the  tracks  of  individual  storms, 
the  heavy  lines  the  mean  course.  (U.  S.  Weather  Bureau.) 

shows  that  the  rate  of  progress  of  the  storm  was  very  unequal. 
Thus  northwest  of  Cuba  its  progress  was  much  slower  than  it  had 
been  to  the  southeast.  Just  south  of  Florida  it  traveled  only  one- 
fourth  as  far  in  twelve  hours  as  it  traveled  in  one  hour  southeast 
of  Cuba.  Figs.  640-643  show  the  position  and  strength  of  the 
storm  at  four  stages  of  its  progress. 

Tropical  cyclones  do  not  occur  in  the  South  Atlantic,  and  their 
point  of  origin  is  several  degrees  north  of  the  equator,  usually  between 
10°  and  20°,  in  the  North  Atlantic.  Tn  the  Pacific  they  occur  on 
both  sides  of  the  equator.  They  come  in  the  later  part  of  the  hot 
season  of  the  latitudes  where  they  occur,  and  are  thought  to  be 


654 


PHYSIOGRAPHY 


WEATHER  MAPS 


655 


caused  by  strong  convection  currents.  Their  apparently  anomalous 
courses  are  probably  to  be  explained  by  the  courses  of  the  prevail- 
ing winds.  The  lower  part  of  the  cyclone  is  in  the  horizon  of  the 
trades,  but  the  upper  part  of  the  great  eddy  is  probably  above  the 
trade-wind  horizon  and  under  the  influence  of  northerly  currents. 
The  effect  of  these  two  controls  appears  to  be  to  carry  the  storm 
somewhat  to  the  north  of  west  (in  the  northern  hemisphere)  until 
it  escapes  the  control  of  the  trades  altogether,  after  which  it  is  influ- 


FIG.  640. — An  early  stage  of  the  Galveston  storm  when  it  was  central  over 
western  Cuba.     (Nat.  Geog.  Mag.) 

enced  primarily  by  the  southwest  winds.  The  course  of  the  storm 
after  it  escapes  the  trades,  following  a  more  northerly  course  than 
the  prevailing  winds,  is  probably  influenced  by  the  temperature  of 
land  and  sea. 

Storms  similar  to  the  West  Indian  hurricanes  occur  in  the  North 
Pacific,  originating  in  the  vicinity  of  the  Philippines,  and  sweeping 
the  coast  of  China.  These  storms  are  called  typhoons.  The  courses 
of  typhoons  are  shown  in  Fig.  644.  The  Society  Islands  and  the 
low  coral  islands  of  the  neighboring  Low  Archipelago  were  swept 


656 


PHYSIOGRAPHY 


by  a  destructive  storm  of  this  sort  on  February  7  and  8,  1906.  The 
Hongkong  typhoon  of  Sept.  18,  1906,  was  estimated  to  have  de- 
stroyed 5000  lives,  and  property  to  the  value  of  $20,000,000. 

Weather  predictions.  Weather  predictions  are  based  on  the 
phenomena  illustrated  by  the  weather  maps.  Take,  for  example, 
the  map  of  the  25th  of  September,  1903  (Fig.  615).  Rain  accom- 
panies the  cyclone  which  is  central  over  Dakota.  Since  this  storm 


FIG.  641. — A  later  stage  of  the  storm  after  its  center  had  reached  Galveston. 

(Nat.  Geog.  Mag.) 

has,  for  the  last  twenty-four  hours,  been  moving  a  little  south  of 
east  at  the  rate  of  about  40  miles  an  hour,  it  is  fair  to  presume  that 
it  will  move  in  this  same  general  direction  at  a  similar  rate  for  the 
next  twenty-four  hours.  If,  in  this  time,  it  advances  to  the  Lake 
Superior  region,  it  will  probably  bring  with  it  weather  similar  to 
that  which  it  is  now  giving  to  the  region  where  it  occurs.  Hence 
on  the  25th,  the  day  when  the  weather  conditions  are  shown  in 
Fig.  615,  the  prediction  might  be  made  that  rainfall  is  to  be  expected 
in  about  twenty-four  hours  in  the  region  about  the  head  of  Lake 
Superior. 


WEATHER  MAPS 


657 


The  map  of  the  26th  (Fig.  616)  shows  that  the  course  of  the  storm 
has  changed  a  little,  being  slightly  to  the  north  of  east,  the  common 
path  of  cyclones.  That  is,  after  descending  a  little  to  the  south 
of  east  from  British  Columbia,  cyclones  are  likely  to  turn  to  the 
east,  or  even  a  little  to  the  north  of  east,  in  the  middle  longitudes 
of  the  United  States  (Fig.  629).  On  the  26th  the  prediction  might 
be  made  that  the  low  which  is  central  north  of  Lake  Superior  (Fig.  616) 


FIG.  642. — The  same  storm  after  it  had  become  central  about  Dubuque,and. 
much  weaker.     (Nat.  Geog.  Mag.) 

will  move  on  to  the  Gulf  of  St.  Lawrence  by  the  succeeding  day, 
and  that  rain  will  accompany  it.  Rain  for  the  region  about  Lake 
Huron  and  the  area  east  of  it  may,  therefore,  be  predicted.  The 
map  for  the  27th  (Fig.  617)  shows  that  the  area  of  precipitation 
extends  far  to  the  south.  The  preceding  map  had  shown  some 
cloudiness  in  this  region,  but  had  afforded  no  warrant  for  the  pre- 
diction of  such  an  area  of  cloudiness.  Thunder-storms  are  shown 
in  the  southern  part  of  the  area  of  cloudiness. 

Temperature   changes  as  well  as  changes  in  precipitation  may 
be  predicted.      Thus  in  Fig.  614  the  isotherm  of  40°  bends  south- 


658 


ward  notably  in  the  high  central  over  Iowa.  As  the  high  moves 
east,  it  will  probably  carry  the  low  temperature  with  it.  Hence 
it  is  safe  to  predict  that  the  temperature  will  fall  in  the  area  into 
which  the  anticyclone  is  to  move.  The  map  of  the  succeeding  day 
(Fig.  615)  shows  that  the  temperature  of  western  Virginia  has  fallen 
from  about  60°  to  about  40°  along  the  path  of  the  high,  while  areas 
much  farther  north  are  warmer. 

The  same  map  (Fig.  615)  shows  that  North  Dakota  and  Alberta 
have  a  temperature  of  50°,  that  is,  a  temperature  10°  warmer  than 


FIG.  643. — A  still  later  stage  after  the  center  of  the  storm  had  reached  New 
England.     (Nat.  Geog.  Mag.) 

that  of  western  Virginia.  It  will  be  noted,  too,  that  the  relatively  high 
temperature  of  Dakota,  Montana,  and  Alberta  goes  with  a  low.  As 
the  low  moves  eastward,  the  presumption  is  that  the  temperature 
along  its  path  will  become  somewhat  higher.  This  is  shown  by  the 
succeeding  map  (Fig.  616),  which  shows  a  temperature  of  about  50° 
north  of  Lake  Superior.  The  same  map  shows  how  the  isotherm  of  40° 
bends  to  the  southward  in  front  of  the  high  which  is  central  over 
western  Montana.  On  this  day  Winnipeg  has  about  the  same  temper- 


WEATHER  MAPS 


659 


ature  as  Cheyenne,  several  hundred  miles  farther  south.  As  the  high 
of  Montana  moves  eastward,  it  will  be  likely  to  carry  cold  tempera- 
ture with  it.  From  this  map,  therefore,  it  may  be  predicted  that 
the  temperature  in  Nebraska,  Kansas,  Iowa,  and  Missouri  will  fall. 
The  next  map  (Fig.  617)  shows  that  the  temperature  at  Omaha 
has  fallen  from  50°  to  40°,  while  that  of  eastern  Kansas  has  fallen 
from  70°  to  40°. 

The  time  at  which  the  precipitation  which  a  given  storm  may 
bring  to  any  given  place  will  fall  is  calculated  from  the  rate  at  which 


FIG.  644. — Typhoon  tracks.     (After  Herbertson.) 

the  storm  is  progressing.  Similarly,  the  time  of  arrival  of  a  cold 
wave  which  an  anticyclone  is  likely  to  bring  is  predicted  on 
the  basis  of  the  rate  of  progress  which  the  anticyclone  is  making. 
These  rates  are  known  in  advance  by  telegraphic  reports.  Pre- 
dictions concerning  the  weather  may  be  made  more  readily  for 
the  central  and  eastern  parts  of  the  United  States  than  for  the 
western  part,  for  the  storms  have  been  under  observation  longer 
before  they  reach  the  central  and  eastern  parts. 

Predictions  may  also  be  made  as  to  the  strength  and  direction 


660  PHYSIOGRAPHY 

of  wind.  The  principles  involved  will  be  readily  understood,  and 
the  data  on  which  the  predictions  are  based  are  received  by  tore- 
casters  the  same  as  data  concerning  temperature  and  rainfall. 

Failure  of  weather  predictions.  Weather  predictions  often  fail. 
The  reasons  are  many.  Among  them  may  be  mentioned  the  follow- 
ing: 

1.  The  cyclones  and  anticyclones  sometimes  depart  widely  from 
the  courses  they  are  expected  to  take.     They  may  veer  so  widely 
from  their  normal  courses  as  to  avoid  altogether  the  places  they 
were  predicted  to  reach.     Thus  a  storm  may  be  in  line  for  St.  Paul, 
to  Avhich  it  is  expected  to  bring  rain  and  a  rising  temperature;  but 
instead  of  keeping  its  normal  course,  it  may  turn  off  to  the  north- 
ward, and  the  rain  which  was  predicted  for  St.  Paul  falls  farther 
north. 

2.  Storms  often  change  their  rate  of  advance,  so  that  they  arrive 
earlier  or  later  than  predicted.     Thus,  if  a  storm  which  has  been 
advancing   at   the  rate  of  600  miles  in  a  day  suddenly  stops  or 
advances  but  little,  it  does  not  bring  the  changes  predicted  to  the 
areas  into  which  it  was  expected  to  advance. 

3.  A  third  cause  of  the  failure  of  predictions  is  found  in  the 
fact  that  storms  sometimes  appear  and  disappear  without  warning. 
Fig.  616  shows  a  low  of  which  there  had  been  no  indication  on  the 
25th,  central  over  Oklahoma  and  Indian  Territory;  Fig.  617  shows 
that  this  low  has  disappeared.     It  occasionally  happens  that  much 
more  pronounced  storms,  promising  great  changes  of  weather,  dis- 
appear.    In  such  cases  the  predicted  weather  does  not  arrive,  and 
the  failure  is  charged  to  the  forecaster. 

4.  Predictions  are  sometimes  based  on  insufficient  data.     It  will 
be  noticed  that  on  some  weather  maps  the  letter  M  appears  in 
various  circles.     This  means  that  reports  from  the  station  where  the 
M  appears  are  missing.     If  many  reports  are  missing,  the  map  is 
correspondingly  imperfect,  but  the  forecaster  must  use  such  data  as 
he  has,  as  well  as  he  may,  and  issue  a  map. 

5.  Storms  sometimes  change  their  characters.     Thus  from  the 
map  of  January  20,  1895  (Fig.  645),  it  could  not  be  foreseen  that  the 
cyclone  central  in  Colorado  would  develop. the  pronounced  charac- 
teristics which  appear  on  the  map  of  the  following  day  (Fig.  646). 

6.  In  some  situations  storms  are  subject  to  many  freaks.     This 
is  the  case,  for  example,  at  Chicago.     The  frequently  erratic  be- 
havior of  storms  here  is  probably  due  to  the  influence  of  the  lake, 


WEATHER  MAPS 


661 


662 


PHYSIOGRAPHY 


WEATHER  MAPS  663 

which  modifies  temperature  and  air  currents.  No  other  of  our 
Great  Lakes  has  so  great  extension  in  a  north-south  direction,  and 
no  other  therefore  presents  so  broad  a  front  to  the  prevailing 
winds. 

Forecasters,  like  other  men,  are  fallible,  but  when  they  have  tc 
work  with  so  many  indeterminate  elements,  it  is  not  strange  that 
they  sometimes  make  mistakes,  and  one  mistake  is  likely  to  be 
remembered  longer  than  many  correct  prognostications. 

Property  saved  by  predictions  of  storms,  frosts,  floods,  etc.  In 
spite  of  all  shortcomings,  the  warnings  of  storms,  floods,  cold 
waves,  etc.,  sent  out  by  the  Weather  Bureau,  have  resulted  in  great 
benefit  to  various  interests.  The  value  of  this  service  of  the  Weather 
Bureau  is  not  always  duly  appreciated,  and  much  less  is  heard  of  it 
than  would  have  been  heard  of  the  losses  which  would  have  been 
incurred  in  the  absence  of  the  warnings.  Unfortunately,  it  is  not 
always  possible  to  devise  protection  against  the  evils  of  which  the 
Weather  Bureau  gives  warnings. 

It  has  been  estimated  that  property  valued  at  §15,000,000  was 
saved  in  1897  by  warnings  of  impending  floods.  While  this  was 
exceptional,  considerable  sums  are  saved  each  year  in  this  way.  In 
1903-4  the  estimated  value  of  the  saving  was  $1,000,000. 

Shipping  interests  are  served  by  storm  warnings.  Thus,  in  Sep- 
tember, 1903,  vessels  valued  at  $585,000  were  temporarily  held  in 
ports  along  the  coast  of  Florida  by  storm  warnings. 

Agricultural  interests  are  also  served  by  warnings  of  storms  and 
of  "cold  waves,"  and  especially  of  frosts.  Warnings  led  to  the  pro- 
tection of  $1,000,000  worth  of  fruit  about  Jacksonville,  Fla.,  in 
1901,  with  an  estimated  saving  of  half  this  amount.  Other  warn- 
ings of  cold  in  1901  are  estimated  to  have  been  the  means  of  saving 
$3,400,000  worth  of  property.  Fruit-  and  truck-farming  are  the 
phases  of  agricultural  work  most  effectively  served  in  this  way. 

Special  Types  of  Storms 

Thunder-storms.  Thunder-storms  are  of  common  occurrence 
in  the  United  States.  They  are  most  common  in  warm  regions— 
that  is,  either  in  low  latitudes,  or  in  the  summer  season  of  middle 
latitudes.  Not  only  this,  but  they  are  most  common  on  days  which 
are  unusually  warm,  and  during  the  warmer  parts  of  these  days. 
They  are,  however,  not  confined  to  the  summer  or  to  the  warm 
part  of  the  day,  for  there  are  occasional  thunder-storms  in  the  winter 


664 


PHYSIOGRAPHY 


in  middle  latitudes,  and  even  in  high  latitudes,  and  there  are  thunder- 
storms at  night.  Peary  reports  a  thunder-storm  in  North  Greenland 
in  midwinter. 

The  first  indication  of  a  thunder-storm  is  usually  a  large  cumulus 
cloud  (Fig.  647)  which,  in  the  zone  of  the  westerly  winds,  generally 


FIG.    647. — Ascending   currents   and    FIG.   648. — Air-Currents  in  thunder- 
cumulus     clouds    preparatory    to  storm.     (After  Ferrel.) 
thunder-storm.     (After  Ferrel.) 

appears  in  the  west.  The  cumulus  cloud  (or  thunder-head)  which 
yields  the  rain,  like  all  cumulus  clouds,  is  generated  by  an  ascending 
current  of  moist  air.  It  moves  eastward,  and  seems  to  rise  as  it 
approaches  the  observer,  but  the  rise  is  apparent  only.  As  the 
cloud  reaches  the  place  of  the  observer,  there  is  usually  a  sharp 
breeze,  or  "thunder-squall,"  rushing  out  before  it.  Shortly  after 
the  squall  the  rain  begins  to  fall.  The  rainfall  is  often  heavy  and 
the  drops  large;  but  the  downpour  does  not  usually  last  more  than 
an  hour,  and  often  much  less.  Sometimes,  however,  a  second 
thunder-storm  follows  close  upon  the  first  (Fig.  648),  thus  prolong- 
ing the  period  of  rainfall.  When  a  thunder-storm  has  moved  on  to 
the  east,  the  air  is  usually  cooler  and  fresher,  and  the  barometer 
distinctly  higher. 

As  water  vapor  condenses  in  the  air,  the  water  particles 
become  charged  with  electricity.  The  charge  of  the  individual 
droplets  increases  as  they  increase  in  size,  and  the  lightning  is  due 
to  the  discharge  of  the  electricity  from  one  part  of  a  cloud  to  an- 
other, or  from  one  cloud  to  another  cloud,  or  from  cloud  to  ground. 
Rain  and  snow  bring  down  much  electricity  to  the  land  and  water. 


WEATHER  MAPS  665 

The  flash  of  lightning  is  followed  by  thunder,  the  noise  being 
due  to  the  vibrations  in  the  air  resulting  from  the  disturbance 
caused  by  the  electrical  discharge.  The  thunder  has  been  com- 
pared to  the  noise  which  follows  any  other  violent  disturbance  in 
the  air,  such  as  the  explosion  of  a  rocket  or  the  cracking  of  a  whip 
(Davis).  Rolling  thunder  may  follow  a  prolonged  flash  of  lightning, 
or  it  may  be  due  to  a  succession  of  flashes  but  slightly  separated 
from  one  another,  or  sometimes  to  the  echoing  of  the  thunder  from 
hills  and  mountains. 

In  temperate  latitudes  the  thunder-storms  usually  occur  during 
the  passage  of  cyclones,  though  they  do  not  accompany  all  cyclones. 
They  are  more  common  on  the  south  sides  of  cyclones  than  else- 
where, and  they  often  occur  at  a  considerable  distance  from  the 
centre  of  the  storm.  In  middle  latitudes,  thunder-storms,  like 
cyclones,  move  in  a  general  way  from  west  to  east;  while  in  the  zone 
of  trade-winds  they  move  from  east  to  west.  In  both  cases  they 
move  with  the  prevailing  winds. 

The  forward  movement  of  a  thunder-storm  is  commonly  20  to 
50  miles  an  hour.  They  often  spread,  and  become  weaker  as  they 
move  forward  (Fig.  649) ,  and  do  not  usually  run  long  courses  before 


FIG.  649. — Vertical  section  of  a  thunder-storm  which  is  moving  toward  the 
right.     (After  Koppen.) 

disappearing.     The  period  of  a  thunder-storm  is  usually  much 
shorter  than  that  of  the  cyclone  which  it  accompanies. 

It  sometimes  happens  that  lightning  at  a  great  distance  illu- 
minates the  clouds  over  a  region  where  the  lightning  itself  cannot 
be  seen.  Where  the  clouds  seen  from  a  given  point  are  thus  illu- 
minated by  lightning  which  is  itself  invisible,  the  lightning  of  the 
clouds  is  called  heat  lightning.  The  heat  lightning  is  simply  a  re- 
flection of  lightning.  It  is  more  likely  to  occur  in  hot  weather 


666 


PHYSIOGRAPHY 


than  at  other  times,  because  lightning  is  more  common  at  such 
times. 

Rainbows    sometimes    accompany   or    follow    thunder-storms. 
They  are  always  seen  opposite  the  sun,  and  hence  are  seen  in  the 

west  in  the  morning,  and  in  the 
east  in  the  afternoon  or  evening. 
They  are  usually  seen  just  after 
the  passage  of  a  thunder-storm, 
while  a  little  rain  is  still  falling, 
but  after  the  sun  has  appeared. 
They  are  seen  on  looking  in  the 
direction  opposite  the  sun;  that 
is  they  are  in  the  east  in  the  even- 
ing, and  in  the  west  in  the  morn- 
ing. The  rainbow  is  due  to  the  effects  of  the  drops  of  water  in  the 
atmosphere  on  the  sun's  rays.  A  bow  is  also  seen  through  water 
spray,  such  as  that  at  a  great  fall,  even  when  no  rain  is  falling. 
Whirlwinds.  Distinct  ascending  whirls  of  air  are  often  seen 
on  hot  days.  They  are  especially  well  seen  in  dusty  regions,  for 


FIG.  650. — Shape  of  thunder-storm 
in  ground-plan, illustrating  growth 
and  change  as  it  progresses. 
(After  Waldo.) 


\ 


FIG 


.  651.  —  Graph  showing  the  relative  frequency  of  thunder-storms  in  Chicago 
in  different  months.     (Cox,  U.  S.  Weather  Bureau.) 


there  the  dust  is  swept  up,'  making  the  wrhirl  distinctly  visible. 
They  are  often  seen  in  dusty  roads,  plowed  fields,  etc.,  but  are 
seen  at  their  best  in  deserts.  From  a  given  point  in  the  Mojave 
Desert  of  California,  as  many  as  eight  or  ten  of  these  whirls,  some 
of  them  rather  conspicuous  and  imposing,  may  sometimes  be  seen  at 
one  time  from  a  single  point  on  a  hot  summer  day.  The  whirl- 


WEATHER  MAPS  667 

winds  are  probably  caused  by  the  excessive  heating  of  the  air  at 
some  point,  and  this  excessive  heating  gives  rise  to  a  sharp  con- 
vection current.  It  moves  on  for  a  time  with  the  prevailing  wind, 
but  soon  plays  out. 

In  humid  regions  the  whirlwinds  do  not  usually  appear  to  ex- 
tend up  to  any  considerable  height,  but  in  desert  regions  they 
often  reach  heights  of  1000  feet  or  more,  as  shown  by  the  whirling 
columns  of  dust.  The  rise  is  sometimes  so  great  that  the  air  is 
expanded  and  cooled  enough  to  cause  condensation  of  even  the 
small  amount  of  moisture  contained  in  the  desert  air.  Sharp 
showers  may  then  occur.  Showers  of  this  sort  are  likely  to  be  of 
short  duration,  but  the  rainfall  is  sometimes  very  heavy.  If  ex- 
ceptionally heavy,  such  rains  are  known  as  cloudbursts.  In  such 
a  storm  in  the  summer  of  1898,  rain  enough  fell  in  a  few  minutes, 
in  the  vicinity  of  Bagdad,  in  the  Mojave  Desert  of  California,  to 
occasion  serious  washouts  along  the  railroad  for  several  miles. 
A  cloudburst  at  Clifton,  S.  C.,  June  6,  1903,  caused  the  loss  of 
more  than  fifty  lives,  and  property  damage  to  the  estimated 
extent  of  $3,500,000. 

Tornadoes.  When  a  convection  current  is  very  strong,  but 
has  very  small  diameter,  the  whirl  sometimes  becomes  so  intense 
as  to  cause  great  destruction.  A  whirling  storm  of  this  sort  is 
known  as  a  tornado.  Tornadoes,  like  thunder-storms  and  whirl- 
winds, are  phenomena  of  hot  weather.  They  occur  in  the  United 
States  in  the  warm  season,  appearing  earlier  in  the  South  and 
later  in  the  North. 

The  tornado  may  be  looked  upon  as  a  concentrated  cyclone 
or  an  intensified  whirlwind.  The  pressure  in  the  center  of  the 
tornado  is  usually  much  lower  than  in  the  center  of  a  cyclone.  In 
a  strong  tornado  the  pressure  at  the  center  may  be  a  fourth  less 
than  that  of  its  surroundings.  Herein  lies  the  explanation  of  one 
phase  of  the  destructive  action  of  a  tornado.  During  the  passage 
of  a  tornado  the  pressure  may  be  reduced  from  the  normal  amount, 
14.7  Ibs.  per  square  inch,  or  2117  Ibs.  per  square  foot,  to  three- 
fourths  of  this,  or  to  11  Ibs.  per  square  inch  or  1584  Ibs.  per  square 
foot.  If  such  a  tornado  passes  over  a  closed  building  in  which  the 
air  pressure  is  normal  (2117  Ibs.  per  square  foot),  the  pressure  on 
the  outside  becomes  1584  Ibs.  The  walls  are  therefore  pushed  out 
with  a  force  of  533  Ibs.  per  square  foot,  and  unless  they  are  very 
strong,  they  will  collapse  outward,  as  if  the  building  had  ex- 


668 


PHYSIOGRAPHY 


ploded.  Often  it  is  only  the  weakest  part,  such  as  a  window, 
which  yields. 

Not  only  is  the  pressure  at  the  center  low,  but  the  area  of  low 
pressure  is  very  small.  While  a  cyclone  may  be  1000  miles  or  more 
across,  a  tornado  may  be  no  more  than  one-eighth  of  a  mile  across, 
or  even  less.  The  result  is  that  the  pressure  gradient  in  a  tornado 
is  very  much  higher  than  in  a  cyclone,  and  the  winds  are  violent. 
Their  velocities,  estimated  by  the  size  and  weight  of  the  objects 
moved,  have  been  thought  to  reach  400  or  500  miles  per  hour. 
With  this  velocity,  or  even  a  velocity  which  is  much  less,  the  de- 
struction is  great.  Trees  are  overturned,  buildings  unroofed  or 
even  blown  down,  and  bridges  hurled  from  their  foundations. 

A  tornado  is  often  heralded  by  a  funnel-shaped  cloud  (Fig. 
652),  the  point  of  which  may  be  far  above  the  ground.  As  the 
funnel  moves  forward,  its  lower  end  may  rise  or  fall.  The  tornado 


FIG.  652. — Funnel-shaped  cloud  of  a  tornado.     Solomon,  Kan. 
(U.  S.  Weather  Bureau.) 

becomes  especially  destructive  where  the  funnel  sinks  so  as  to  ap- 
proach or  touch  the  ground.  The  cloud  is  due  primarily  to  the 
condensation  of  the  moisture  in  the  sharp  convection  current,  and 
the  funnel  shape  is  due  to  the  expanding  and  spreading  of  the  air 
as  it  rises. 

The  tornado  is,  of  all  storms,  the  most  destructive,  but  it  usu- 


WEATHER  MAPS 


669 


ally  has  a  very  narrow  track,  and  does  not  commonly  work  destruc- 
tion for  a  very  great  distance.  After  a  short  course  it  generally 
plays  out,  or  rises  so  high  as  to  cease  to  be  destructive. 

One  of  the  most  destructive,  though  not  one  of  the  most  violent, 
tornadoes  of  recent  times  was  that  at  St.  Louis,  May  27.  1896. 
It  was  an  incident  of  a  thunder-storm  in  the  southeastern  part  of 
a  cyclonic  area  central  some  distance  northwest  of  St.  Louis. 

The  humidity  at  St.  Louis  was  exceptionally  high,  about  94. 
At  noon  the  barometer  at  St.  Louis  stood  at  29.87,  the  tempera- 
ture was  80°  F.,  and  the  velocity  of  the  wind  12  miles  per  hour. 
By  1.45  the  temperature  had  risen  to  86°.  At  2  o'clock  the  barom- 


j  f  1  f#     »2  46810X112   46   j  10  • 


FIG.  653. — Thermograph  (at  left)  and  barograph  (at  right);  traces  at  St.  Louis 
during  the  tornado  of  May  27,  1896.     (U.  S.  Weather  Bureau.) 

eter  began  to  fall  rapidly,  and  by  6  P.M.  it  had  dropped  to  29.59. 
Meanwhile  the  wind  had  become  shifting,  and  shortly  before  6 
o'clock  had  attained  a  velocity  of  45  miles  per  hour,  and  by  6 
o'clock  the  temperature  had  fallen  to  77°. 

During  the  earlier  part  of  the  afternoon,  cumulus  clouds  had 
been  abundant,  but  by  4.30  they  had  settled  into  a  stratus  cloud. 
Soon  after  5.00,  thunder  and  lightning  occurred,  and  rain  began  to 
fall  at  5.43. 

At  6.04  there  was  a  marked  increase  in  the  violence  of  the 
wind,  which  shifted  its  direction  rapidly.  The  barometer  rose  to 
29.67,  but  fell  almost  instantly  to  29.57,  then  rose  to  29.67  in  less 


670 


PHYSIOGRAPHY 


than  five  minutes,  falling  again  .31  inch  to  29.36  in  fifteen  minutes, 
and  then  rose  almost  instantly  to  29.76.  Sharp  oscillations  of 
barometric  pressure  occurred  until  10  P.M.  The  wind  probably 
attained  a  maximum  velocity  of  120  miles  per  hour  at  6.18,  with 
numerous  and  rapid  changes  in  velocity  and  direction.  The  rain- 
fall accompanying  the  storm  was  extremely  severe,  more  than  2\ 
inches  falling.  The  electrical  display  was  brilliant. 

The  destruction  began  at  about  6.10  P.M.  and  lasted  for  several 
minutes.  The  forward  motion  of  the  storm  was  at  the  rate  of 
about  36  miles  per  hour.  The  width  of  the  belt  of  destruction  was 
about  l\  miles  where  it  entered  the  city,  but  it  was  constricted  to 
less  than  a  mile  farther  on. 

One  of  the  extraordinary  features  of  the  storm  was  the  fact 
that  its  base  was  about  30  feet  above  the  surface.  Trees  were 


FIG.  654.- 


-  Weather  map  for  the  morning  of  the  day  (March  27,  1890)  of  the 
Louisville  tornado.     (U.  S.  Weather  Bureau.) 


twisted  off  at  this  level,  and  the  principal  destruction  of  houses 
was  above  the  first  floor.  Evidences  of  great  heat  wrere  visible 
after  the  storm,  as  shown  in  scarred  branches  and  twigs,  a  phe- 
nomenon which  has  been  noted  in  some  other  tornadoes. 

As  in  other  tornadoes,  the  wind  played  many  curious  freaks. 
Single  stones  and  bricks  were  picked  out  of  walls,  w7hile  the  walls 


WEATHER  MAPS 


671 


remained  standing.  In  one  case  a  span  of  horses  attached  to  a 
loaded  wagon  were  taken  away,  though  the  wagon  was  not  over- 
turned. The  most  extraordinary  recorded  instance  of  violence 
was  in  East  St.  Louis,  where,  at  the  approach  to  the  bridge,  a  plank 
2"X8"  "was  driven  into  ...  a  steel  girder  with  such  velocity 
that  it  punched  a  hole  in  the  webbing  and  remained  sticking  in  the 


FIG.  655. — Weather  map  for  the  evening  of  March  27,  1890,  at  the  time  of 
the  Louisville  tornado.      (U.  S.  Weather  Bureau.) 

girder."  The  destruction  of  property  in  and  about  St.  Louis  was 
estimated  at  about  $13,000,000. 

The  interpretation  put  upon  the  storm  was  that  "tornadic 
action  was  developed  successively  at  different  points  in  the  track 
of  the  general  storm,"  which  was  a  thunder-storm  belonging  to  the 
class  of  thunder-storms  "which  move  broadside  in  a  southeasterly 
direction." 

A  more  violent  tornado  was  that  at  Louisville  on  the  27th  of 
March,  1890,  just  before  nine  o'clock  in  the  evening.  Its  rate  of 
advance  was  nearly  40  miles  per  hour,  but  its  diameter  was  so 
slight,  about  300  yards,  that  it  took  but  about  three-fourths  of  a 
minute  for  the  storm  to  pass  a  point.  It  was  accompanied  by 
"a  most  terrific  electric  display."  Many  weak  buildings  were 
wrecked,  the  walls  falling  toward  the  center  of  the  storm.  Sev- 


672 


PHYSIOGRAPHY 


enty-six  persons  were  killed  and  about  200  injured  in  Louisville 
alone,  and  the  loss  of  property  was  estimated  at  about  $2,500,000. 


FIG.  656. — Track  of  tornado  in  the  outskirts  of  Chicago,  May  25,  1896. 
(Cox,  U.  S.  Weather  Bureau.) 


FIG.  657. — General  view  of  the  wreckage  caused  by  the  tornado  at  Rochester, 
Minnesota,  August  21,  1883. 

The  path  of  the  storm  was  traced  for  75  miles,  and  throughout 
this  distance  its  width  was  nearly  uniform.  At  least  five  tornadoes 
occurred  in  Kentucky  the  same  night. 


WEATHER  MAPS 


673 


Fig.  656  shows  the  track  of  a  tornado  in  the  suburbs  of  Chicago 
on  May  25,  1896. 


Fia.  658. — Wreckage  of  the  Union  Station  Power-house  at  St.  Louis,  May 
27,  1896.     (U.  S.  Weather  Bureau.) 


FIG.  659.— Trees  twisted  off  by  tor-  FIG.  660.— Straws  driven  into  dry 
nadic  winds.  (U.  S.  Weather  wood  by  tornadic  winds.  (U.  b. 
Bureau.)  Weather  Bureau.) 

Waterspouts.  Waterspouts  are  virtually  tornadoes  at  sea. 
When  the  base  of  the  upward  spiral  movement  is  as  low  down  as 
the  surface  of  the  water,  sea-water  may  be  drawn  up  to  some  slight 


674 


PHYSIOGRAPHY 


extent  by  the  ascending  current.  The  lesser  atmospheric  pres- 
sure in  the  centre  of  the  whirl  will  occasion  the  rise  of  the  water  to 
some  extent  at  that  point,  and  the  upward  current  of  air  may  catch 
it  and  carry  it  upward.  The  larger  part  of  the  water  in  a  water- 
spout is,  however,  due  to  the  condensation  of  the  water  vapor  in 
the  air,  and  not  to  the  uplift  of  water  from  the  sea. 

Foehn  winds,  Chinook  winds,  etc.  When  warm,  moist  air  is 
forced  up  over  mountains,  it  precipitates  some  of  its  moisture.  The 
precipitation  sets  free  heat,  so  that  the  air  is  cooled  much  less  than 
it  would  be  otherwise.  Beyond  the  crest  of  the  mountains  it  de- 


FIG.  661. — Distribution  of  tornadoes  in  the  United  States,  1794-1881. 

scends,  and  is  warmed  in  the  process.  It  is  warmed  much  more 
(often  twice  as  much)  in  the  descent  than  it  was  cooled  in  the 
ascent,  because  moisture  is  not  condensed  during  the  descent  (p. 
572).  It  may  therefore  descend  as  a  hot  wind.  Such  winds  are 
known  as  Foehn  winds  in  Switzerland  and  as  Chinook  winds  in  the 
United  States,  especially  just  east  of  the  Rockies. 

These  winds  may  be  beneficial  or  harmful.  Thus  the  Chinook 
winds  temper  the  rigorous  winters  of  certain  parts  of  the  Northwest- 
ern States  and  the  Canadian  provinces  east  of  the  mountains. 
They  frequently  evaporate  a  foot  or  more  of  snow  in  a  very  few 
hours.  Such  winds  are  sometimes  called  snow-eaters.  These 
winds  make  winter  grazing  possible  over  large  areas.  In  Alberta 


WEATHER  MAPS  675 

the  Chinook  has  been  declared  to  be  "the  grand  characteristic  of  the 
climate  as  a  whole,  that  on  which  the  weather  hinges."  These 
winds  sometimes  develop  with  great  suddenness.  At  Fort  Assini- 
boine,  Montana,  on  January  19,  1892,  the  temperature  rose  43°  F., 
from  —5.5°  to  37.5°,  in  fifteen  minutes,  under  the  influence  of  the 
Chinook  wind.  In  other  cases  the  temperature  has  been  known  to 
rise  80°  F.  in  six  or  eight  hours.  The  Chinook  winds  of  summer  are 
sometimes  so  hot  and  drying  as  to  wither  vegetation,  and  some- 
times to  destroy  crops  completely. 


CHAPTER  XIX 
CLIMATE 

IN  the  preceding  discussions  of  temperature,  rainfall,  winds,  and 
weather,  much  has  been  said  or  implied  concerning  climate.  The 
principal  points  involved  may  be  here  summarized  and  applied  to 
the  principal  zones  of  the  earth. 

/  >      Definition.    Climate  is  the  average  succession  of  weather  condi- 
\  )  tions  for  a  considerable  period  of  time.     The  summer  climate  of  a 
place  is  shown  by  the  weather  of  many  summers,  not  by  the  weather 
of  one.     So  with  the  climate  of  autumn  or  winter  or  spring.     The 
average  weather  conditions  for  10  years  would  give  some  approxi- 
mation to  the  true  climate,  those  for  25  years  would  give  a  closer 
approximation,  and  those  for  50  or  100  years  would  be  still  better. 
The  distinction  between  climate  and  weather  is  correctly  recognized 
by  such  expressions  as  these :  The  winter  climate  of  Chicago  is  cold 
and  windy,  but  the  winter  weather  of  Chicago  in  1905-6  was  mild. 
//       Climate  is  otherwise  defined  as  "the  sum  total  of  meteorological 
"  conditions  in  so  far  as  they  affect  animal  or  vegetable  life."    Ac- 
cording to  this  conception  of  climate,  those  meteorological  elements 
which  have  most  influence  on  life  are  most  important  in  climate 
(Hann) . 

The  principal  elements  of  climate  are  (1)  temperature  and  (2) 
humidity,  which  includes  (a)  relative  humidity  (p.  570),  (b)  absolute 
humidity,  (c)  degree  of  cloudiness,  and  (d)  precipitation.  A  climate 
may  be  described  as  warm  or  cold,  dry  or  moist.  In  common 
speech,  other  elements  of  climate  are  often  neglected,  but  there  are 
others  of  importance,  especially  (3)  wind. 

Of  these  elements,  temperature  is,  on  the  whole,  the  most  impor- 
tant, but  from  some  points  of  view,  relative  humidity  and  precipita- 
tion are  hardly  less  important. 

In  characterizing  the  climate  of  a  region,  account  is  taken  not 
only  of  the  average  temperature  of  the  year  and  of  the  several 

676 


CLIMATE  677 

seasons,  but  also  of  the  temperature  of  exceptional  seasons  and  of 
the  extremes  of  temperature  during  the  season.  These  extremes  are 
considered  not  merely  for  their  effects  on  averages,  but  also  on 
their  own  account.  Sensible  temperature,  as  distinct  from  absolute 
temperature,  is  also  to  be  taken  into  account.  Moist  air  of  a  given 
degree  of  heat  seems  much  warmer  than  dry  air  of  the  same  temper- 
ature when  the  temperature  is  high,  and  much  colder  when  the  tem- 
perature is  low.  Sunstroke  is  much  more  common  wtjere  the  rela- 
tive humidity  is  high  than  where  it  is  low.  Sunstrokes  are  rare,  for 
example,  in  the  arid  West,  even  with  temperatures  considerably 
above  those  of  Chicago  or  New  York.  Sudden  changes  of  tempera- 
ture are  also  less  injurious  where  the  relative  humidity  is  low  than 
where  it  is  high.  Air  of  a  given  temperature  seems  much  cooler 
when  in  motion  than  when  quiet. 

Sim'.larly,  climate  takes  account  not  only  of  the  average  amount 
of  yearly  precipitation,  but  of  the  variations  of  precipitation  from 
year  to  year  and  from  season  to  season,  of  its  average  distribution 
throughout  the  year  and  of  departures  from  this  average,  and  of  the 
proportions  \vhirh  fall  as  rain  and  snow  respectively. 

The  other  elements  of  climate  are  considered  in  the  same  way, 
their  variations  and  extremes  as  well  as  averages  being  taken  into 
account. 

Uniformity  and  variability.  If  the  range  of  temperature  is 
small,  the  distribution  of  precipitation  somewhat  equal,  and  the 
winds  reasonably  constant  in  direction  and  strength,  the  climate  is 
uniform.  If,  on  the  other  hand,  the  variations  of  these  climatic  ele- 
ments are  great,  either  in  a  year  or  in  successive  years,  the  climate 
is  variable.  The  climate  of  the  middle  and  northern  latitudes  of  the 
United  States,  for  example,  is  variable,  (1)  because  the  annual 
range  of  temperature  is  great,  (2)  because  the  range  varies  from 
year  to  year,  (3)  because  corresponding  seasons  have  very  different 
temperatures,  (4)  because  changes  of  temperature  may  be  very 
sudden,  and  (5)  because  the  amount  and  distribution  of  rainfall  vary 
notably  and  irregularly  from  year  to  year,  and  from  season  to  season. 
It  is  the  variability  of  the  weather  which  makes  weather  predictions 
important,  and  variable  weather  makes  a  variable  climate. 

Figs.  662-670  represent  certain  elements  of  variability.  Fig. 
662  shows  the  annual  range  of  temperature  in  eight  places:  Duluth, 
Chicago,  Memphis,  and  New  Orleans  in  the  upper  part;  and  Denver, 
Chicago,  New  York,  and  San  Francisco  in  the  lower  part  It  will  be 


678 


PHYSIOGRAPHY 


seen  that  the  range  is  greater  in  the  higher  latitudes.  It  is 
120°  at  Duluth,  108°  at  Chicago,  87°  at  Memphis,  and  70°  at  New 
Orleans.  Denver,  Chicago,  and  New  York  do  not  differ  widely  in 
annual  range  of  temperature,  but  the  range  at  San  Francisco,  where 
the  prevailing  wind  is  from  the  sea,  is  notably  less.  Though  near 


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FIG.  662. — The  upper  part  of  the  figure,  reading  from  the  top  down,  shows 
the  annual  range  of  temperature  in  degrees  Fahrenheit  at  Duluth  (aver- 
age, 120°),  Chicago  (average  108°),  Memphis  (average  87°),  and  New 
Orleans  (average  70°),  four  places  in  about  the  same  longitude,  but  in 
different  latitudes.  The  lower  part  of  the  figure,  reading  from  the  top 
down,  shows  the  an  ual  ranges  of  temperature  in  Denver  (average  113°), 
Chicago,  New  York  (average  94°),  and  San  Francisco  (average  53°),' 
four  places  in  similar  latitudes  but  in  different  positions  with  reference 
to  the  sea.  (Cox,  U.  S.  Weather  Bureau.) 

the  sea,  New  York  has  a  much  greater  range  of  temperature  than 
San  Francisco,  because  the  prevailing  winds  are  from  the  land. 

Fig.  663  shows  the  average  winter  temperature  by  years  for 
Chicago.  It  will  be  seen  that  the  average  temperature  of  some 
winters  is  7°  higher  than  that  of  other  years.  Fig.  664  shows  the 
temperature  of  Chicago  during  two  Januaries,  when  the  weather  was 


CLIMATE 


679 


abnormally  warm.  The  average  temperature  for  1880  was  40°,  and 
that  for  1906,  33°.  Fig.  665  shows  that  the  average  temperature 
for  January  at  the  same  place  is  about  23°.  It  also  shows  the 


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Fio.  663. — Average  winter  temperatures  at  Chicago  in  degrees  Fahrenheit 
1885-1905.     (Cox,  U.  S.  Weather  Bureau.) 

mean  monthly  temperatures  for  several  cities,  the  range  being 
greater  with  increasing  latitude,  and  with  increasing  distance  from 
the  sea  in  the  direction  whence  the  wind  blows.  Fig.  666  shows  the 


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FIG.  664. — Mean  daily  temperatures  during  two  warm  Januaries  at  Chicago. 
The  dotted  line  represents  January,  1880,  and  the  full  line  January,  1906. 
The  numbers  at  the  left  are  degrees  Fahrenheit.  The  average  tempera- 
ture for  the  former  month  was  40°  F.,  and  for  the  latter  33°.  The  aver- 
age temperature  for  January  in  Chicago  is  23°.  (Cox,  TJ.  S.  Weather 
Bureau.) 

variations  in  daily  range  which  are  sometimes  possible  in  middle 
latitudes  far  from  the  sea.  Fig.  667  shows  the  variation  in  snowfall 
in  one  locality  for  a  period  of  twenty  years,  while  Figs.  668  and 


680 


PHYSIOGRAPHY 


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_____   San  Francisco,   Cal.    55.7° 

„  ______  .._   Denver,  Colo.      49.4° 

_  ____    Boston,  L'ass.      48.9° 


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Chicago,  111.    46.5° 

Harquette,  Xich.  40.6° 

Uenphis,  Tenn.  61.1° 

Kev;  Orleans,  La.  68.8° 


FIG.  665.  —  Mean  monthly  temperatures  in  degrees  Fahrenheit  at  Chicago, 
San  Francisco,  Denver,  and  Boston,  four  places  in  similar  latitudes;  and 
for  Chicago,  Marquette,  Memphis,  and  New  Orleans,  places  in  different 
latitudes  in  about  the  same  longitude.  Averages  for  the  year  are  shown 
below  the  graphs.  (Cox,  U.  S.  Weather  Bureau.) 


FIG.  666. 


55  ;              ;              ; 

:     ;                           :  :  68 

1  iiii  INI  TUMI  iii  i;  40 

o  •HBBBBBBBI 

•     0 

FIG.  667. 


FIG.  666. — The  range  of  temperature  at  Chicago  for  February  9,  1900  when 
the  range  was  53°  F.  (from  9°  to  62°),  and  for  March  24,  1891,  when  the 
temperature  did  not  vary,  being  32°  throughout  the  day.  The  curves 
are  illustrative  of  the  great  variation  in  daily  range  in  "temperate  " 
latitudes.  (Cox,  U.  S.  Weather  Bureau.) 

FIG.  667. — Total  snowfall  at  Chicago,  in  inches,  by  winters,  1885-1905, 
showing  the  great  variations.     (Cox,  U.  S.  Weather  Bureau.) 


CLIMATE 


681 


669,  and  Figs.  670  and  671,  show  the  great  differences  in  the 
amount  of  snow  at  corresponding  dates  in  successive  years. 


DEPTH  OF  SNOW  OH  GROUND 
JANUARY  31,  1905. 


FIG.  668. — The  numbers  at  the  ends  of  the  lines  indicate  the  depth  of  snow 
in  inches.     (U.  S.  Weather  Bureau.) 


FIG.  669.     (U.  S.  Weather  Bureau.) 

It  will  be  seen  that  a  variable  climate  varies  in  different  ways. 
A  climate  which  is  regularly  dry  during  one  season  of  the  year  and 


682 


PHYSIOGRAPHY 


wet  during  another,  is  variable  within  the  year  with  reference  to  pre- 
cipitation, even  though  the  range  of  temperature  is  not  great;   the 


FIG.  670.     (U.  S.  Weather  Bureau.) 


FIG.  671.     (U.  S.  Weather  Bureau.) 

climate  of  such  a  region  may,  however,  be  very  constant  from  year 
to  year.     Such  a  climate  is  found  on  the  borders  of  the  equatorial 


CLIMATE  683 

calms,  which  shift  a  little  north  and  south  with  the  apparent  shift- 
ing of  the  sun.  A  narrow  belt  on  each  border  of  the  calm  zone  is 
therefore  alternately  in  the  calms  and  in  the  zone  of  the  trades. 
At  the  former  times  it  has  plentiful  rain,  while  at  the  latter  it  is 
generally  dry. 

A  region  which  is  hot  at  one  time  of  the  year  and  cold  at  another 
is  variable  within  the  year  with  respect  to  temperature.  In  such 
regions,  too,  one  winter  or  summer  may  be  much  cooler  or  warmer 
than  the  next,  giving  a  variation  from  year  to  year  rather  than  from 
season  to  season.  A  climate  which  is  variable  with  respect  to 
temperature  is  not  necessarily  variable  with  respect  to  moisture. 
Commonly,  however,  variations  in  temperature  and  moisture  go 
together. 

In  some  regions  the  winds  shift  regularly  from  season  to  season, 
as  where  monsoons  blow  (Figs.  595  and  597),  and  again  along  the 
poleward  borders  of  the  trades.  The  climates  of  such  places  are 
variable  within  the  year  with  respect  to  winds,  and  this  makes  them 
variable  also  with  respect  to  some  of  the  other  elements  of  climate. 
Considered  from  year  to  year,  the  climates  of  such  regions  may  be 
uniform. 

These  illustrations  are  sufficient  to  show  that  the  meaning  of 
"variable  climate  "  is  itself  variable. 

Classification  of  Climates 

As  in  the  case  of  many  other  topics,  climates  may  be  classified 
in  various  ways,  and  each  classification  helps  to  emphasize  some  im- 
portant point.  One  classification  has  already  been  suggested, 
namely,  uniform  and  variable.  Another  classification  has  reference 
primarily  to  the  amount  of  heat  received  from  the  sun.  On  this 
basis  the  earth  is  subdivided  into  climatic  zones,  the  borders  of 
which  are  parallels.  The  climatic  zones  based  on  insolation  rep- 
resent solar  climate.  Solar  climate  is  so  much  modified  by  various 
factors  other  than  insolation,  that  climatic  zones  bounded  by  lines 
other  than  parallels  have  been  suggested.  The  effect  of  land  and 
water  on  temperature  has  already  been  noted.  So  important  is 
this  effect  that  climates  are  also  classified  into  oceanic  and  conti- 
nental, and  continental  climates,  in  turn,  are  subdivided  on  the 
basis  of  (1)  distance  from  the  sea,  (2)  altitude,  and  (3)  topographic 
relations.  The  controlling  element  in  most  of  these  classifications 
is  temperature. 


684 


PHYSIOGRAPHY 


Climatic  Zones 

On  the  basis  of  climate,  the  earth  is  divided  into  several  zones- 
Those  commonly  recognized  are  (1)  the  torrid  zone,  which  is  cen- 
tered about  the  equator,  (2)  the  temperate  zones,  which  occupy 
the  extra-tropical  latitudes,  and  (3)  the  frigid  zones,  which  lie 
about  the  poles.  Better  names  for  these  zones  are  the  tropical, 


jj 

Portland 
Ore. 

Boston 
Cairo,  111. 

Winne- 
mucca, 
Nev. 

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-  5 

f 
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->- 

100 
90 
60 
70 
60 
60 
40 

30         N.W. 
...       Ceylon 
20       Siberia 
10          S-  W. 
Ceylon 
0     Malabar 

T 

V 

~-*- 
^ 

A 

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11 

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f- 

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-t 

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I 

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f 

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\ 

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x 

5 

IOC 
90 
60 
70 
60 
50 
40 
30 
20 
10 
0 

Havre, 
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San 
Dieeo 
£1  Paso 

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100         Coaf.t 

90 
60 
70j 
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50  NorthSea 
40  Germany 
S.  Medit- 
30    erranean 
Central 
20       France 
Steppes 
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0 

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Fio.|672. — Graphs  showing  percentage 
of' rainy  days  in  each  month  for  eight 


eight 

stations  in  the  United  States.     (Cox, 
U.  S.  Weather  Bureau.) 


FIG.  673.— Graphs  showing  percent- 
age of  rainy  days  in  each  month 
for  several  stations  in  foreign 
lands. 


the  intermediate,  and  the  polar  zones l  respectively,  and  these 
terms  will  be  used  hereafter.  The  limits  of  these  zones  have 
been  variously  denned.  One  classification  defines  them  by  lati- 
tude, a  second  by  the  direction  of  the  winds,  and  a  third  by  tem- 
perature. 

»Ward,  Bull.  Am.  Geog.  Soc.,  Vol.  XXXVII,  1905. 


CLIMATE 


685 


Zones  defined  by  latitude.  Defined  by  latitude,  the  tropical 
zone  is  limited  poleward  by  the  tropics,  and  the  polar  zones  are 
limited,  equatorward,  by  the  Arctic  and  Antarctic  circles  respec- 
tively, while  the  intermediate  zones  lie  between  the  tropical  zone 
and  the  polar  zones  on  either  hand. 

Stated  in  other  terms,  the  tropical  zone,  according  to  this  classifi- 
cation, is  the  zone  (1)  where  the  sun  is  vertical  at  some  time  during 
the  year  (except  at  the  tropics,  twice  a  year);  (2)  where  varia- 
tions in  the  length  of  day  and  night,  are  least;  (3)  where  the  annual 


FIG.  674. — Comparative  data,  average  monthly  precipitation.  The  letters 
are  the  initial  letters  of  the  months.  The  places  in  the  upper  row,  com- 
mencing at  the  upper  lett  corner  and  reading  to  the  right,  are:  Port- 
land, Ore.;  Havre,  Mont.;  Moorhead,  Minn.;  Marquette,  Mich.;  and 
Boston.  In  the  second  row:  San  Francisco;  Denver;  Omaha;  Chicago; 
and  Norfolk,  Va.  In  the  third  row:  Yuma,  Ariz.;  El  Paso.  Tex.;  Gal- 
veston;  New  Orleans;  and  Jacksonville,  Fla.  (U.  S.  Weather  Bureau.) 

insolation  is  greatest  and  the  average  temperature  highest;  (4) 
where  the  range  of  annual  insolation  is  least,  and  consequently  (5) 
where  the  annual  range  of  temperature  is  least. 

The  greater  insolation  of  this  zone  is  explained  by  the  lesser 
obliquity  of  the  sun's  rays.  At  the  equator,  at  noon,  they  depart- 
nearly  23^°  from  vertically  at  a  maximum;  at  the  tropics,  they 
depart  nearly  47°  from  vertically  at  a  maximum;  and  at  inter- 
mediate latitudes  by  an  intermediate  amount.  The  average  in- 


686  PHYSIOGRAPHY 

clination  of  the  sun's  rays  for  this  zone  at  noon  is  between  17° 
and  18°,  while  the  average  inclination  for  the  intermediate  zones  is 
about  45°.  The  sun's  rays,  therefore,  depart  much  less  from  ver- 
ticality  in  the  tropical  zone  than  in  the  intermediate  zones.  The 
slight  range  of  temperature  in  this  zone  is  explained  by  two  facts, 
namely,  (a)  within  the  tropical  zone  the  days  and  nights  are  never 
very  unequal  (Fig.  536),  and  (6)  the  change  in  angle  of  the  sun's 
rays  during  the  year  is  less  than  elsewhere. 

The  intermediate  (temperate)  zones  are  the  zones  (1)  where  the 
sun's  rays  are  never  vertical;  (2)  where  the  days  and  nights  are 
very  unequal,  each  ranging  from  10J  hours  at  the  equatorward 
limit  of  the  zone  to  nearly  24  hours  at  the  poleward  limit,  in 
the  course  of  the  year,  but  where  the  sun  never  appears  above  the 
horizon  for  twenty-four  hours  consecutively;  and  (3)  where  the 
amount  of  annual  insolation  is  less,  and  (4)  its  annual  range  greater, 
than  below  the  tropics. 

The  polar  zones  are  the  zones  (1)  where  the  days  and  nights  are 
sometimes  more  than  twenty-four  hours  long.  They  are  the  zones 
(2)  of  least  annual  insolation,  and  (3)  of  greatest  range  of  insola- 
tion. 

According  to  this  definition  of  the  zones,  the  tropical  zone  is 
about  47°  wide,  each  of  the  temperate  zones  about  43°,  and  each 
of  the  polar  zones  about  232°. 

This  classification  has  the  merit  of  simplicity,  and  it  has  a 
definite  astronomical  basis;  but  the  limits  of  the  zones  thus  de- 
fined do  not  always  separate  one  sort  of  climate  from  another. 
As  applied  to  actual  climate,  and  to  the  things  which  climate  affects, 
the  subdivision  seems  arbitrary.  Thus,  the  climate  in  that  part  of 
the  intermediate  zones  near  the  tropics  is  essentially  like  that  of 
the  tropical  zone,  while  the  climate  of  that  part  of  the  intermediate 
zones  next  to  the  polar  zones  is  not  very  different  from  that  of  the 
polar  zones.  On  this  basis  there  is  far  more  difference  between  the 
climate  of  the  lowest  and  the  highest  latitudes  of  an  intermediate 
zone,  than  between  the  climate  of  the  lowest  latitudes  of  an  inter- 
mediate zone  and  that  of  the  highest  latitudes  of  the  tropical  zone, 
or  between  the  climate  of  the  highest  latitudes  of  the  intermediate 
zone  and  that  of  the  lowest  latitudes  of  the  polar  zones. 

Zones   defined  by   winds.1    If  climatic  zones  be  defined  by 

1  Davis'  Elementary  Meteorology. 


CLIMATE  687 

the  direction  of  prevailing  winds,  the  tropical  (or  trade-wind) 
zone  is  the  zone  where  the  trade-winds  blow.  It  extends  some- 
what beyond  the  tropics,  even  to  latitudes  of  30°  or  35°  on  the  east- 
ern sides  of  the  oceans.  The  intermediate  zones  lie  poleward  from 
the  trade-wind  zone,  and  are  characterized  by  prevailing  westerly 
winds  and  variable  climate,  but  they  have  no  definite  poleward 
boundaries.  If  definite  poleward  boundaries  must  be  assigned, 
they  might  be  placed  at  the  polar  circles,  though  the  westerlies 
prevail  beyond  them.  This  definition  has  the  merit  of  placing 
the  dividing-line  between  the  tropical  and  intermediate  zones 
where  the  simple  and  uniform  climate  of  the  trade-wind  zone  gives 
place,  poleward,  to  the  more  complex  and  variable  climate  of  the 
zone  where  the  westerlies  prevail.  This  classification  has  the 
merit,  therefore,  of  grouping  together  climates  which  are  really 
similar.  It  takes  account  of  climatic  elements  other  than  tem- 
perature. The  poleward  limits  of  the  intermediate  zones  has 
1  much  less  logical  definition. 

The  lack  of  simplicity  and  of  mathematical  precision  is  some- 
times considered  an  objection  to  the  definition  of  zones  by  the 
direction  of  the  winds.  Thus,  the  zone  of  the  trades  shifts  north 
and  south  with  the  seasons.  On  this  basis,  therefore,  places  near 
the  border  of  the  trade-wind  zone  are  sometimes  in  that  zone  and 
sometimes  in  the  zone  of  the  westerly  winds.  As  a  matter  of  fact, 
this  annual  shifting  of  a  place  from  on&  zone  to  another  corre- 
sponds with  actual  climatic  conditions,  even  if  it  is  not  simple. 
The  basis  of  this  definition  of  zones  is  therefore  quite  as  rational 
as  that  of  the  preceding,  and  if  it  leaves  the  zones  with  rather 
vague  boundaries,  it  is  because  nature  has  left  them  somewhat  ill- 
defined. 

Zones  defined  by  isotherms.  If  the  zones  be  defined  on  the 
basis  of  temperature,  the  dividing-lines  between  zones  are  iso- 
therms. One  proposed  division  makes  the  annual  isotherms  of 
68°F.  the  equatorward  limits  of  the  intermediate  zones,  while  their 
polar  limits  are  the  isotherms  of  50°  for  the  warmest  month  (Fig. 
675).  On  this  basis,  the  tropical  zone  is  narrowed  over  the  eastern 
sides  of  the  oceans  and  broadened  on  the  western  sides,  as  a  result 
of  the  influence  of  the  lands. 

On  the  whole,  this  seems  a  fairly  satisfactory  basis  for  the 
definition  of  climatic  zones,  though  it  lacks  the  mathematical 


688 


PHYSIOGRAPHY 


simplicity  and  precision  of  the  first,  and  it  fails  to  take  account  of 
some  diverse  elements  of  climate  recognized  by  the  second.1 

Each  climatic  zone  has  at  least  two  principal  subdivisions,  a 
continental  and  an  oceanic.  The  oceanic  climate  of  any  zone 
prevails  where  there  are  extensive  areas  of  water,  and  the  con- 
tinental climate  prevails  elsewhere. 

Oceanic  climates.  Oceanic  climates  are  less  variable  than 
continental  climates  in  the  matter  of  temperature.  Between  the 


FIG.  675. — Temperature  zones.     Degrees  F.     (After  Supan.) 

latitudes  of  0°  and  40°  the  diurnal  range  of  temperature  is  only  2° 
to  3°  over  the  sea.  It  is  far  more  on  land.  The  annual  range  of 
temperature  over  the  sea  is  also  much  less  than  that  on  land. 
This  is  illustrated  by  Fig.  676,  which  shows  the  annual  variation 
on  the  island  of  Madeira  (curve  M)  and  at  Bagdad  (curve  Bd) 
in  Asia  Minor.  The  former  represents  a  marine  climate,  the  latter 
a  continental  climate.  In  higher  latitudes  the  differences  are  still 
greater,  as  shown  by  the  curves  V  and  N.  The  former  repre- 
sents the  marine  climate  of  Valentia  on  the  southwest  coast  of 
Ireland,  and  the  latter  the  continental  climate  of  eastern  Siberia. 
The  sea  retards  the  annual  march  of  temperature  more  than 

1  The    classification    of  climates  on  the  bases  here  considered  is  well 
discussed  by  Ward,  Bull.  Geog.  Soc.  of  Am.,  1906,  p.  4Q1, 


CLIMATE 


689 


the  land  does  (Fig.  677).  The  springs  are  therefore  relatively 
colder  and  the  autumns  warmer  than  on  land.  The  humidity  of 
the  oceanic  climate  is  greater  than  that  of  continental  climates. 
This  results  in  more  cloudiness  and  often  in  more  rainfall,  especially 
in  winter.  The  winds  of  the  sea  are,  on  the  whole,  stronger  than 
those  of  the  land.  The  leeward  shores  of  the  oceans  (the  shores 
to  which  winds  blow)  have  climates  which  are  essentially  oceanic. 
The  more  equable  temperatures  and  the  greater  amount  of  mois- 

J.  P.  M.  A.  M.  j.  j.  A.  s.  o.  N.  o.  j. 


F. 

A£O 

Bd 

C. 

7 

\ 

^ 

30* 
20» 

00 
-100 
-20» 

-300 

68° 

500 

320 

ko 

/ 

\ 

/ 

N 

M 

-~^ 

\ 

M 

x^ 

1 

\ 

M 

& 

J 

/ 

/ 

V 

v 

N 

^. 

Bd 

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/ 

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/ 

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N 

* 

N 

J.  F.  M.  A.  M.  J.  J.  A.  S.  0.  "N.  D.  J. 

FIG.  676. — Graphs  to  illustrate  oceanic  and  continental  climates  in  different 
latitudes.  Af=  Madeira,  Bd=  Bagdad,  F  =  Valentia,  and  JV=Eastern 
Siberia.  (After  Angot.) 

ture  on  such  coasts  have  important  effects  on  vegetation  and  on 
animal  life.  These  effects  go  beyond  the  mere  facts  of  life  and 
death,  and  even  beyond  degrees  of  thrift.  For  example,  wheat 
has  less  protein  in  a  marine  climate  than  in  a  continental.  Again, 
starch  decreases  and  gluten  increases  with  increase  of  temperature.1 
Potatoes  grown  in  the  arid  West,  where  the  necessary  (but  no  un- 
necessary) water  is  supplied  by  irrigation,  produce  more  nutritious 

1  Hann's  Handbook  of  Climatology. 


690 


PHYSIOGRAPHY 


matter  than  those  grown  in  moister  climates.    These  are  but  illus- 
trations of  general  facts. 

Continental  climates.     In  contrast  with  marine  climates,  con- 
tinental climates  have  greater  annual   and  daily  ranges  of  tem- 


C_T^ L- 

°"S 


FIG.  677. — Annual  march  of  temperature  (in  degrees  Fahr.)  in  continental 
and  oceanic  climates.  The  horizontal  line  represents  the  annual  average. 
(After  Hann.) 

perature,  and  the  seasons  lag  less  than  over  the  sea.  In  high 
latitudes  the  skies  are  clearer  and  the  winters  colder;  in  low 
latitudes  the  winters  are  warmer  than  over  the  sea.  The  hu- 
midity and  the  rainfall  are  less,  and  the  rain  less  frequent  in  the 
interiors  of  the  continents  than  over  the  sea;  but  its  amount  and 
distribution  are  largely  influenced  by  topography,  winds,  etc. 
The  air  over  continents  is  also  dustier  than  that  over  the  sea. 

The  differences  between  oceanic  and  continental  climates,  so 
far  as  temperature  is  concerned,  are  indicated  by  the  following 
tables: 


January. 

July. 

Year. 

Range. 

Mean  temperature  of  oceans  
Mean  temperature  of  continents  

17.  9°  C. 

7  3° 

10.  2°  C. 
22  9° 

18.  3°  C. 
15  0° 

7.6°C. 
15  6° 

Latitude. 

0°. 

10°. 

20°. 

30°. 

40°. 

50°. 

Mean      temperature      of      land 
hemisphere  

44.8° 

42.5° 

36  4° 

26  0° 

15.7° 

3.6° 

Mean     temperature     of     water 
hemisphere  

22.2° 

21.2° 

19.6° 

17.4° 

12.7° 

7  6° 

Difference.  .        

22  .  6° 

21.3° 

16.8° 

8  6° 

3  0° 

-4  0° 

Desert  climates  are  an  extreme  phase  of  continental  climates. 
Here  daily  ranges  of  temperature  are  great.     As  a  result,  winds  are 


CLIMATE 


691 


high  by  day,  and  the  air  often  so  dusty  as  to  make  travel 
difficult.  The  nights  are  calmer  and  cooler.  As  a  result  of  the 
great  daily  ranges  of  temperature  and  the  high  winds,  rock  break- 
ing, due  to  changes  of  temperature  (p.  73),  and  dust  and  sand 
transportation  by  the  wind  (p.  55),  are  at  a  maximum.  The 
dryness  is  hostile  to  plants,  and  therefore  to  animals. 

Since  the  littoral  (coastal)  climate  on  the  windward  side  of  the 
continent  is  very  like  the  oceanic  climate  of  the  same  latitude, 


15* 110*          105°          100°          9*  90°          85°  80"  75 


FIG.  678. — Figure  showing  sunshine  in  the  United  States  in  hours  per  year. 
The  numbers  on  the  lines  show  the  hours  per  year.      (After  van  Bebber.) 

the  west  coasts  of  the  continents,  in  the  zones  of  westerly  winds, 
have  oceanic  climates,  and  east  coasts  have  continental  climates. 
In  the  zone  of  trade-winds  the  reverse  is  the  case. 

The  climate  of  the  littoral  zones  is  sometimes  controlled  largely 
by  monsoon  winds.  So  important  are  the  monsoon  winds  that 
it  is  proper  to  speak  of  a  monsoon  climate.  Monsoons  are  gen- 
erally on  shore  in  summer,  and  so  give  summer  rains;  but  locally 
the  monsoons  give  precipitation  in  winter. 

Mountain  and  plateau  climates  differ  from  other  continental 
climates,  because  of  (1)  the  increased  insolation  and  radiation  which 
go  with  increase  of  altitude,  (2)  the  lesser  absolute  humidity,  (3) 
the  lower  temperature,  (4)  the  greater  range  of  solar  temperature, 


692 


and  (5)  the  greater  frequency  of  precipitation,  up  to  certain  alti- 
tudes. The  difference  between  soil  temperature  and  air  tempera- 
ture is  also  greater  than  at  lower  levels. 

Mountains,  like  oceans,  have  relatively  pure  air  and  high  winds. 
They  modify  general  winds  and  give  rise  to  local  winds,  such  as 
mountain  and  valley  breezes.  They  interfere  with  free  horizontal 


62 


67 


72    77   82   87   92   97  102  107  112  117  122 


35 


FIG.  679. — Figure  showing  sunshine  in  Europe  in  hours  per  year. 
(After  Konig.) 

interchange  of  air,  so  that  pressure  and  moisture  conditions  may 
be  quite  different  on  opposite  sides  of  a  mountain  range. 

Climatic  effect  of  forests.  Forests  also  exert  a  modifying  in- 
fluence on  continental  climates.  They  lower  the  summer  tempera- 
ture by  increasing  the  radiating  and  evaporating  surfaces,  and  by 
increasing  the  cloudiness.  They  increase  the  relative  humidity 
of  the  air;  but  it  seems  to  be  uncertain  whether  they  have  much 
effect  on  precipitation.  The  data  for  some  regions  seem  to  point 
to  an  affirmative  answer,  and  for  others  to  a  negative  one.  In 
any  case,  they  tend  to  hold  back  the  water  after  it  falls  and  to 


CLIMATE  693 

retard  the  melting  of  snow,  so  that  their  general  effect  on  the 
moisture  of  the  region  is  much  the  same  as  it  would  be  if  the  precipi. 
tation  were  increased.  Forests  also  afford  protection  against  winds. 
With  these  distinctions  in  mind,  we  may  study  briefly  the  cli- 
mates of  the  several  zones. 


THE  CLIMATES  OF  THE  SEVERAL  ZONES 
The  Tropical  Zone 

The  leading  characteristic  of  the  climate  of  this  zone  is  its 
relatively  high  temperature.  Uniformity  of  winds,  temperature, 
and  humidity  are  especially  characteristic  of  the  oceanic  climate  of 
this  zone.  On  land  the  variation  of  all  the  principal  elements  of 
climate  is  greater. 

The  prevailing  winds  of  the  tropical  zone  are  easterly, — north- 
easterly in  the  northern  part  of  the  zone  and  southeasterly  in  the 
southern  part, — with  a  zone  of  calms  (the  doldrums)  between 
(p.  603).  So  long  as  these  winds  blow  over  the  sea  or  over  low 
lands,  they  are,  in  general,  dry  winds  (p.  616).  Many  lands  in 
their  path,  notably  Sahara  and  parts  of  Australia,  are  desert; 
but,  where  they  blow  over  mountains  or  plateaus,  they  yield  mois- 
ture to  them,  especially  to  their  windward  sides  (p.  616).  The 
abundant  rainfall  on  the  east  slope  of  the  Andes,  on  the  table- 
land of  Brazil,  and  on  the  higher  parts  of  the  Hawaiian  Islands, 
are  illustrations.  Even  in  the  Sahara  there  are  mountains  which 
occasion  precipitation  enough  to  support  forests,  but  the  descend- 
ing streams  soon  disappear  in  the  surrounding  desert.  For  reasons 
which  have  already  been  pointed  out  (p.  617),  the  leeward  sides 
of  mountains  in  the  zone  of  the  trades  get  little  moisture  from  the 
trade-winds.  In  many  cases  the  water  which  falls  on  the  moun- 
tains, and  flows  thence  to  the  plains  below,  may  be  utilized  there 
for  irrigating  the  lands  not  favored  by  precipitation. 

Monsoon  winds  are  often  pronounced  in  the  tropical  zone. 
They  sometimes  displace  or  greatly  modify  the  trades,  and  become 
the  controlling  element  in  the  precipitation,  often  giving  rain  to 
regions  which  would  otherwise  be  dry.  It  is  from  the  southwest 
monsoon  that  India  and  Farther  India  receive  their  heavy  rains, 
concentrated  in  a  wet  season  when  the  monsoon  blows  from  sea 
to  land.  Since  this  monsoon  blows  during  the  warm  season,  rain 


694  PHYSIOGRAPHY 

must  fall  at  that  time  (July  to  September).  Like  other  winds, 
monsoons  which  blow  from  the  sea  over  low  lands  yield  much  less 
rain  than  those  which  blow  over  high  lands.  Monsoon  winds  often 
relieve  the  dryness  which  would  otherwise  exist  on  the  west  sides  of 
mountains  in  the  trade-wind  zone. 

The  tropical  zone  is  not  entirely  dependent  on  winds  for  its 
rainfall.  Rainfall  and  cloudiness  increase  toward  the  center  of 
the  tropical  zone,  while  the  strength  of  the  winds  decreases.  In 
the  doldrums  (p.  617)  convection  currents  give  daily  (afternoon) 
rains,  thus  interrupting  the  generally  arid  low  lands  of  the  tropical 
zone  by  a  middle  or  sub-equatorial  belt  of  more  abundant  rain- 
fall. In  this  belt  flourish  the  forests  of  the  Amazon  and  of  middle 
Africa.  Since  the  belt  of  calms  shifts  a  little  with  the  sun,  the 
zone  of  daily  rains  also  shifts.  A  place  which  is  in  the  belt  of  daily 
rains  at  one  time  of  the  year  may  be  in  the  path  of  the  trades  at 
another,  and  may  therefore  have  alternating  wet  and  dry  seasons. 

Near  the  poleward  margin  of  the  tropical  zone  also  there  is 
likely  to  be  variation  in  precipitation,  for  places  in  such  positions 
find  themselves  in  the  zone  of  dry  trades  in  the  summer  and  in 
the  zone  of  variable  winds  in  the  winter.  In  neither  case,  how- 
ever, are  they  favorably  situated,  so  far  as  winds  are  concerned, 
for  heavy  rainfall,  and  have  abundant  moisture  only  where  topog- 
raphy or  some  other  special  factor  favors.  The  most  pronounced 
desert  of  the  whole  earth,  the  Sahara,  lies  near  the  poleward  border 
of  the  trade-wind  zone. 

Along  the  coasts  of  tropical  lands  the  temperature  is  modified 
by  the  daily  sea-breezes  as  well  as  by  the  monsoons.  Because  of 
the  direction  of  the  prevailing  winds,  the  climates  of  the  eastern 
coasts  of  continents  in  tropical  latitudes  are  affected  by  the  sea 
more  than  those  of  western  coasts. 

The  range  of  temperature  in  the  tropical  deserts  is  very  great. 
The  annual  temperature  of  Sahara  is  about  80°  F.  The  temperature 
of  the  warmest  month  averages  about  90°  F.  and  that  of  the  cold- 
est about  70°  F.  The  average  annual  range  is  therefore  relatively 
slight;  but  the  yearly  extremes  are  far  greater,  for  it  sometimes 
reaches  120°  F.  (Fig.  540),  and  sometimes  drops  to  50°.  Great  as 
this  range  is,  it  is  far  less  than  that  of  most  inland  places  in  the 
intermediate  zones,  where  extreme  ranges  of  120°  are  not  uncom- 
mon. The  daily  range  of  temperature  in  tropical  deserts  is  great. 


CLIMATE  695 

Climate  of  Intermediate  Zones 

The  average  temperature  of  the  intermediate  zones  is  lower 
than  that  of  the  tropical  zone,  its  annual  range  greater,  and  its 
daily  range,  on  the  average,  less. 

The  lower  average  temperature  of  the  intermediate  zones 
results  from  the  fact  that  they  receive  less  heat  per  square  mile 
than  lower  latitudes,  where  the  rays  are  more  nearly  vertical. 
The  range  of  temperature  from  season  to  season  is  greater,  because 
of  (1)  the  greater  inequality  of  day  and  night,  and  (2)  the  greater 
range  in  the  angle  of  the  sun's  rays.  In  latitude  45°  there  are,  at 
the  maximum  (summer  solstice),  about  15^  hours  of  sunshine  (and 
heating),  and  8J  hours  of  night  (cooling  with  no  insolation),  while 
at  a  minimum  (winter  solstice)  there  are  but  8£  hours  of  sunshine 
with  15£  hours  of  night.  Not  only  this,  but  when  the  days  are 
longest  the  sun's  rays  are  most  nearly  vertical,  so  that  the  heating 
power  per  hour  is  greatest  when  the  days  are  longest,  and  least 
when  they  are  shortest  (Fig.  536).  The  result  is  that  the  summers, 
even  in  the  latitude  of  45°,  may  be  very  hot,  while  the  winters, 
except  near  windward  coasts,  are  very  cold.  The  annual  range 
includes  summer  heat  which,  at  its  maximum,  is  not  less  than  that 
of  the  tropical  zone,  and  winter  cold  which  is  often  frigid.  The 
annual  range  is  greater  in  the  higher  latitudes  of  this  zone  than 
in  the  lower.  These  great  extremes  of  annual  temperature,  as  well 
as  the  sudden  changes  of  temperature  and  of  humidity  which 
accompany  the  passage  of  cyclones  and  anticyclones,  make  the 
term  "temperate"  singularly  inappropriate  for  the  climate  of  the 
intermediate  zones.  These  last  statements  are  especially  true  of 
the  Intermediate  Zone  of  the  Northern  Hemisphere.  They  have 
less  application  to  the  Southern  Hemisphere,  where  the  area  of 
land  in  middle  latitudes  is  not  so  great. 

Figs.  662  and  666  show  different  phases  of  the  variations  to 
which  the  temperature  of  intermediate  latitudes  is  subject. 

The  effects  of  the  cold  winters  and  the  hot  summers  of  tem- 
perate latitudes  make  themselves  felt  in  the  temperature  of  the 
springs  and  autumns  respectively,  as  already  pointed  out,  but  this 
point  is  of  so  much  importance,  climatically,  that  it  will  now  be 
stated  with  greater  fullness. 

During  the  winter  the  ground  is  cooled,  and  in  the  higher 
latitudes  of  the  intermediate  zones  the  water  in  the  ground  is 
frozen  to  the  depth  of  several  feet.  The  frozen  water  in  the  ground 


696  PHYSIOGRAPHY 

may  be  looked  on  as  "stored-up"  cold.  Even  where  the  rock  is 
dry,  it  becomes  cold  in  winter,  and  has  much  the  same  effect  as 
ice  on  the  air  of  the  succeeding  spring.  Snow  accumulated  on  the 
land  in  winter,  and  ice  formed  on  lakes  and  ponds,  have  the  same 
effect.  When  the  lengthening  days  of  spring  come,  with  their  less 
oblique  rays  of  the  sun,  the  snow  and  the  frozen  water  in  the 
ground  must  be  melted  and  the  soil  and  rock  warmed,  before 
the  air  in  contact  with  it  can  fully  respond  to  the  increased  insola- 
tion; for  so  long  as  the  ground  is  cold,  the  air  next  to  it  cannot 
become  very  warm.  In  intermediate  latitudes,  therefore,  the 
spring  is  retarded  because  of  the  snow,  the  ice,  and  the  cold  soil 
and  rock.  A  simple  analogy  may  be  suggested.  If  a  fire  were 
built  in  a  stove  covered  with  ice,  it  would  take  notably  longer  for 
the  stove  to  warm  the  room  than  if  the  fire  were  built  in  a  stove 
already  warm;  for  the  ice  must  be  melted  before  the  stove  can 
bring  the  air  about  it  to  a  high  temperature. 

In  the  early  autumn,  on  the  other  hand,  the  ground  is  warm 
from  the  heat  of  the  summer,  some  of  which  has  been  absorbed 
and  retained  by  the  soil,  the  rock,  etc.  Warmth  has  been  stored 
up,  and  the  warmed  ground  helps  to  warm  the  air,  so  that,  as  the 
days  shorten  and  the  rays  of  the  sun  become  more  oblique,  the 
temperature  does  not  fall  as  fast  as  it  would  were  the  ground  not 
warm.  Thus,  the  effects  of  the  summer's  heat  hold  over  into  the 
autumn,  as  the  effects  of  the  winter's  cold  hold  over  into  the  spring. 
The  result  is  that  September  is  much  warmer  than  March  in  middle 
latitudes,  though  insolation  is  essentially  the  same  in  the  one 
month  as  in  the  other.  Cold  is  more  effectively  stored  up  than 
heat,  so  that  spring  lags  more  than  autumn. 

The  lagging  effect  produced  by  the  storing  up  of  heat  and  cold 
is  shown  by  a  comparison  of  the  average  monthly  temperatures 
of  individual  places.  The  monthly  averages  for  Chicago  are  as 
follows: 

January,     23°  F.  July,  75°  F. 

February,  27°  F.  August,     .    72°  F. 

March,        35°  F.  September,   65°  F. 

April,         50°  F.  October,        53°  F. 

May,  61°  F.  November,    39°  F. 

June,          70°  F.  December,     30°  F. 

These  figures  show  that  the  average  temperature  of  Chicago 


CLIMATE  697 

for  March  is  less  than  that  for  November.  The  insolation  of  No- 
vember is  far  less  than  that  of  March — is,  indeed,  but  little  less 
than  that  of  January,  the  coldest  month  of  the  year.  This  notable 
lagging  of  the  temperature  behind  that  normal  to  the  insolation 
is  the  result  of  the  storing  up  of  heat  and  cold.  Again,  the  insola- 
tion of  May  is  but  little  less  than  that  of  July,  but  the  stored-up 
cold  from  the  preceding  winter  prevents  the  insolation  during  May 
from  having  as  much  effect  as  it  does  in  July,  after  the  cold  stored 
up  in  the  preceding  winter  has  been  more  largely  dissipated. 

The  lagging  of  the  seasons  is  greater  in  the  higher  latitudes  of 
the  intermediate  zones  than  in  the  lower,  and  the  effects  are  more 
pronounced  inland  than  near  coasts,  and  are  less  on  west  coasts 
than  on  east  ones.  These  and  other  significant  facts  are  shown 
by  the  tables  on  page  698. 

Westerly  winds  prevail  in  the  intermediate  latitudes  (p.  603), 
and  many  of  the  distinctive  features  of  the  climate  of  these  zones, 
both  as  regards  temperature  and  moisture,  are  determined  by 
these  winds.  As  they  blow  from  sea  to  land,  as  from  the  Pacific 
to  the  American  coasts,  they  are  nearly  saturated  with  moisture. 
Where  they  blow  over  land  which  is  warmer  than  the  sea  (low 
lands  in  summer),  they  become  dry  winds,  because  they  take  up 
moisture;  but  when  they  blow  over  land  which  has  a  temperature 
lower  than  their  own  (most  lands  in  winter  and  mountains  at 
most  times),  some  of  their  moisture  is  condensed  and  rain  (or 
snow)  falls.  The  windward  slopes  of  high  mountains  in  these  zones 
are  therefore  well  supplied  with  moisture,  while  plains  to  the  lee 
of  such  mountains  are  likely  to  be  dry.  This  is  the  explanation 
of  the  general  aridity  of  the  regions  east  of  the  Sierra  Nevada 
and  the  Rocky  Mountains.  Though  these  regions  have  little  rain- 
fall, that  which  falls  in  the  mountains  is  coming  to  be  utilized  to 
some  extent  in  irrigating  the  valley  lands  adjacent. 

Middle  latitudes  are  fortunately  not  dependent  entirely  on 
the  westerly  winds  for  their  rainfall.  Cyclonic  storms  often  fur- 
nish a  sufficient  supply  of  moisture  where  the  westerlies  are  dry 
(p.  618).  Thus  east  of  the  98th  meridian  in  the  United  States 
the  rainfall  is  generally  adequate  for  agriculture,  though  most  of 
it  is  not  supplied  by  the  westerly  winds  from  the  Pacific. 

The  cyclone  and  the  anticyclone  are  important  factors  in  the 
temperature  as  well  as  the  precipitation  of  the  intermediate  zones. 
They  give  us  our  greatest  annual  extremes  of  heat  (cyclones  in 


698 


MONTHLY     AVERAGE    TEMPERATURES    (FAHRENHEIT)    FOR   FOUR    PLACES 
IN  LATITUDE  33°  TO  35° 


Los  Angeles. 

Santa  F<5. 

Vicksburg. 

Charleston. 

January  

53° 

40° 

48° 

50° 

February  

55° 

46° 

51° 

52° 

March  

58° 

55° 

57° 

57° 

April  

60° 

64° 

66° 

65° 

May  

63° 

72° 

73° 

71° 

June  

67° 

84° 

81° 

77° 

July  

70° 

88° 

83° 

81° 

August  

73° 

85° 

82° 

80° 

September  

71° 

76° 

76° 

70° 

October  

64° 

66° 

67° 

66° 

November  

60° 

52° 

57° 

55° 

December  

53° 

44° 

50° 

50° 

Range  

20° 

48° 

35° 

31° 

MONTHLY  AVERAGE  TEMPERATURES    FOR  FOUR  PLACES  IN  LATITUDE  41° 

TO  43° 


Coast  of 
North  Carolina. 

Long.  100°. 

Chicago. 

New  York. 

January  

46° 

23° 

23° 

30° 

February  

46° 

28° 

27° 

31° 

March  

50° 

39° 

35° 

36° 

April  

51° 

55° 

50° 

47° 

May  

54° 

64° 

61° 

59° 

June  

55° 

75° 

70° 

70° 

July  

60° 

81° 

74° 

73° 

August  

60° 

77° 

71° 

71° 

September  

55° 

67° 

65° 

C5° 

October  

53° 

56° 

53° 

55° 

November  

50° 

41° 

39° 

42° 

December  

46° 

31° 

30° 

34° 

Range  

14° 

58° 

51° 

43° 

MONTHLY  AVERAGE  TEMPERATURES  IN  THREE  PLACES  IN  LATITUDE  48°  TO  50° 


Seattle. 

Winnipeg. 

Mouth  of  the 
St.  Lawrence. 

January.  ... 

35° 

-6° 

7° 

February  

37° 

-1° 

12° 

March  

44° 

14° 

22° 

April  

49° 

38° 

33° 

May  

56° 

50° 

44° 

June  

60° 

63° 

55° 

July  

64° 

67° 

60° 

August  

65° 

65° 

60° 

September  

59° 

53° 

50° 

October  

52° 

40° 

40° 

November     

45° 

20° 

30° 

December  

40° 

4° 

15° 

Range  

30° 

73° 

53° 

CLIMATE 


699 


summer)  and  cold  (anticyclones  in  winter).  They  also  occasion 
the  great  aperiodic  changes  of  temperature  which  recur  at  short 
intervals  ("spells"  of  heat  and  cold),  as  well  as  the  sudden  changes 
of  weather,  and  so  are  an  element  of  the  variable  climate  of  these 
zones.  Figs.  668  to  671  represent  one  phase  of  the  variations  to 
which  the  continental  climate  of  the  intermediate  zone  is  subject. 

The  climates  of  the  northern  and  southern  intermediate  zones 
are  very  unlike,  because  of  the  greater  expanse  of  land  in  the  former. 
The  climate  of  the  southern  zone  is  essentially  oceanic,  for  the 
lands  are  limited.  The  prevalence  of  water  reduces  the  extremes  of 
temperature.  Compared  with  the  corresponding  zone  of  the 
northern  hemisphere,  the  cool  summers  are  one  of  the  most  notable 
characteristics  of  this  zone.  Cloudiness  and  humidity  are  also 
prevalent,  except  in  the  lee  of  mountains.  These  characteristics 
make  the  climate  relatively  inhospitable,  and  the  lands  of  the 
southern  hemisphere  in  latitudes  corresponding  to  those  of  London 
and  New  York  are  usually  unproductive,  more  because  of  the  cold 
summers,  than  because  of  the  low  temperatures  of  winter.  A  com- 
parison of  the  annual  and  seasonal  temperatures  of  southerly  lati- 
tudes, say  in  latitudes  of  30°  to  40°,  with  those  of  the  correspond- 
ing latitudes  in  the  northern  hemisphere  is  instructive  (Figs. 
538,  539,  and  540). 

The  temperature  of  January  and  July  for  the  northern  and 
southern  hemispheres  (all  latitudes)  is  shown  in  the  following 
table : 


January. 

July. 

Difference. 

Mean. 

Northern  hemisphere  

8.0°C. 

22.5°  C. 

14.  .5°  C. 

15.  2°  C. 

Southern  hemisphere.  .  . 

15.5° 

12.4° 

5.1° 

14.9° 

Earth  as  a  whole  

12.7° 

17.4° 

4.7° 

15.0° 

The  oceanic  climate  of  the  north  intermediate  zone  is  com- 
parable to  that  of  the  southern  zone.  The  prevailing  westerly 
winds  tend  to  carry  the  oceanic  climate  over  onto  the  western 
borders  of  the  continents.  Hence  the  mild  climate  of  the  western 
coasts  of  both  North  America  and  Europe.  On  both  these  coasts 
the  range  of  temperature,  like  that  of  the  tropical  zone,  is  rela- 
tively low.  Blowing  over  the  cooler  land,  the  oceanic  winds  give 
abundant  moisture  and  often  much  cloudiness  and  fog,  especially 
in  the  higher  latitudes.  In  the  Americas,  the  tempering  and  moist- 


700  PHYSIOGRAPHY 

ening  effect  of  the  winds  is  limited  to  a  relatively  narrow  belt,  for 
in  crossing  the  high  mountains  the  air  loses  both  the  warmth  and 
the  moisture  it  brought  from  the  ocean.  Beyond  the  mountains 
therefore  the  direct  effect  of  the  ocean  is  little  felt.  The  mountains 
separate  climates  of  notably  different  types. 

In  western  Europe,  on  the  other  hand,  there  are  no  high  moun- 
tains facing  the  ocean  for  any  considerable  distance,  and  the 
moist  climate,  without  great  extremes  of  temperature,  which  char- 
acterizes the  coast,  passes  gradually  into  the  continental  climate 
of  the  interior,  with  its  drier  air  and  clearer  skies. 

The  continental  interiors  of  the  intermediate  zones  have  much 
greater  ranges  of  temperature  than  the  western  coasts,  and  the 
ranges  become  greater  with  increasing  distance  from  the  ocean,  and 
with  increasing  latitude  (Figs.  665  and  666).  In  Siberia,  for 
example,  in  high  latitudes  and  far  from  a  western  coast,  are  found 
the  greatest  annual  ranges  of  temperature  known  (Fig.  547). 

In  these  zones  the  prevailing  westerly  winds  are  interrupted 
by  storms  throughout  the  year,  and  the  winds  are  stronger  and 
moister  in  winter  than  in  summer. 

The  interiors  of  the  continents  in  this  zone  receive  their  pre- 
cipitation largely  from  cyclonic  winds,  and  the  climate  has  the 
variability  which  always  characterizes  the  lands  of  cyclones  and 
anticyclones. 

The  eastern  borders  of  the  continents  are  in  contrast  with  the 
western.  On  the  former,  continental  rather  than  oceanic  climates 
prevail.  The  differences  are  brought  into  effective  contrast  be- 
tween Vancouver  and  Labrador,  or  between  England  and  Kam- 
chatka, on  opposite  sides  of  a  continental  area;  or  between  Lab- 
rador and  England,  or  Kamchatka  and  Vancouver,  on  opposite 
sides  of  an  ocean  (see  Figs.  538  to  540).  The  contrast  is  greater 
on  the  opposite  sides  of  the  Atlantic,  than  on  opposite  sides  of 
North  America,  (1)  because  the  tempering  effect  of  the  Gulf  Stream 
on  western  Europe  is  greater  than  that  of  the  Japan  Current  on 
western  America,  and  (2)  because  the  Atlantic  opens  more  broad 'y 
to  the  cold  Arctic  Ocean,  allowing  more  ice-water  to  pass  down  the 
eastern  coast  of  North  America  than  along  the  corresponding  coast 
of  Asia. 

Climate  of  the  Polar  Zoties 

The  distribution  of  the  sun's  heat  is  more  unequal  in  the 
polar  zones  than  in  lower  latitudes  (p.  525).-  At  the  poles  there  is 


CLIMATE  701 

half  a  year  of  continuous  night  and  half  a  year  of  continuous  day. 
Between  the  poles  and  the  polar  circles,  the  inequality  of  heat 
distribution  is  somewhat  less  than  at  the  poles,  but  still  great. 
The  seasonal  range  of  insolation  is  greater  here  than  in  other  lati- 
tudes, but,  in  spite  of  this  fact,  the  annual  range  of  temperature 
is  less  than  in  some  other  latitudes.  This  is  because  much  of  the 
surface  is  covered  with  snow  or  ice  (or  ice-cold  water),  and  the  heat 
received  cannot  bring  the  temperature  of  the  surface  above  32°  F., 
so  long  as  snow  and  ice  remain.  Where  these  conditions  exist, 
the  summer  temperature  of  the  air  is  raised  but  little  above  the 
freezing-point.  Where  the  land  is  free  from  snow  during  the  warm 
season,  on  the  other  hand,  the  annual  range  of  temperature  is 
great.  The  diurnal  range  of  temperature  is,  on  the  average,  not 
so  great  as  in  lower  latitudes. 

Precipitation  in  the  polar  zones  is  not  usually  heavy,  and  much 
of  it  falls  as  snow.  Where  the  surface  is  continually  covered  with 
snow  or  ice,  the  precipitation  is  generally  heaviest  in  the  summer. 
The  winds  are  then  more  heavily  laden  with  moisture,  and  blow- 
ing over  the  surface  of  snow  and  ice,  the  air  is  chilled  to  the  point 
of  precipitation.  Because  of  the  low  temperature  of  winter,  the 
air  of  that  season  contains  but  little  water  vapor,  and  so  gives  but 
little  rain  or  snow. 

Even  in  very  high  latitudes,  such  as  that  of  North  Greenland, 
anomalous  conditions  of  weather  sometimes  occur,  and  show  how 
variable  the  climate  may  be.  In  the  winter  of  1894-5  a  thunder- 
storm with  rain  occurred  in  latitude  78°  45';  and  even  in  the 
midst  of  the  long  winter  night,  a  temperature  above  the  freezing- 
point  has  been  known  to  occur  in  the  same  region. 

These  extraordinary  phenomena  are  doubtless  the  result  of 
extraordinary  movements  of  air — in  ways  and  for  reasons  not 
well  understood. 

Rainfall  and  Agriculture,  etc. 

The  relation  of  rainfall  to  agriculture  has  already  been  men- 
tioned (p.  615).  It  should  be  added  that  the  beneficial  effects 
of  precipitation  depend  not  only  on  its  amount  and  distribution 
through  the  year,  but  also  on  its  rate  of  fall.  Besides  the  damage 
they  occasion  through  floods,  heavy  downpours  of  rain  are  much 
less  advantageous  to  crops  than  slow  rains.  Some  figures  on  this 
point  are  shown  in  the  following  table. 


702 


PHYSIOGRAPHY 


ANALYSIS  OF  RAINFALL,  AT  IOWA  CITY  IN  1889  AND  1890 


1889. 

1890. 

Total  rainfall                          

724  mm. 

687  mm. 

Washing  and  flooding  rains  
Insignificant  rains  

330      " 
36     " 

149     " 
29     " 

Total  utilizable  rains  

358     " 

509     " 

Much  may  be  done  toward  utilizing  semi-arid  lands  by  selec- 
tion of  the  crops  to  be  raised,  with  especial  reference  to  the  tem- 
perature and  the  moisture  of  the  region  to  be  tilled. 

Harm  calls  attention  to  the  fact  that  in  Jamaica  and  the  Barba- 
does  the  sugar  crop  may  be  calculated  with  approximate  accuracy 
from  the  amount  of  precipitation.  In  South  Australia,  land  which 
has  8  to  10  inches  of  precipitation  wTill  support  8  or  9  sheep  to  the 
square  mile.  In  New  South  Wales,  4  inches  more  of  rainfall  will 
allow  the  land  to  support  96  sheep  per  square  mile;  an  increase  of 
7  inches  more  (20  inches  in  all)  will  allow  an  equal  area  of  land  to 
support  640  sheep.  In  Argentina,  with  34  inches  of  precipitation, 
land  will  maintain  2630  sheep  per  square  mile.  These  figures 
do  not  take  account  of  possible  differences  of  soil. 

From  the  human  point  of  view,  wrinds  are  an  important  element 
of  climate.  Calms  are  enervating  and  winds  stimulating.  Hy- 
gienically,  winds  are  of  great  importance  where  population  is  dense. 

Climate  and  life.  The  distribution  of  life  is  controlled  very 
largely  by  climate.  The  dry  deserts  of  low  latitudes,  the  deserts 
in  the  lee  of  lofty  mountains,  and  the  snow  deserts  of  polar 
regions,  are  essentially  climatic.  Where  rainfall  is  adequate  and 
where  temperature  favors,  life  abounds  wherever  there  is  a  proper 
soil;  and  even  the  accumulation  of  a  proper  soil  is  largely  in- 
fluenced by  climate.  The  best  soil,  inherently,  is  worthless  where 
water  is  wanting,  or  where  the  temperature  is  too  low  for  plant 
life. 

Of  Australia  it  has  been  said  that  "Land  without  rain  is  worth 
nothing;  and  land  in  an  Australian  climate,  with  less  than  10  inches 
a  year,  is  worth  next  to  nothing.  Rain-water,  without  land,  if 
the  water  can  be  stored  in  a  reservoir  and  sent  along  a  canal,  is 
worth  a  great  deal."1 

1  Wills,  cited  by  Harm. 


CLIMATE  703 

Changes  of  Climate 

Within  historic  time.  There  appears  to  be  little  basis  for  the 
popular  notion,  especially  among  elderly  people,  that  the  climate 
is  changing.  There  seems  to  be  a  natural  disposition  to  exaggerate 
the  striking  features  of  notable  seasons.  Thus  winters  of  heavy 
snow  or  of  intense  cold  come,  in  time,  to  be  the  only  winters  which 
are  distinctly  remembered.  Exceptional  seasons  thus  come  to 
stand  for  the  normal  winters  of  the  past.  Another  reason  for  the 
notion  that  climate  is  changing  appears,  in  many  cases,  to  be 
that  those  who  entertain  this  view  have  changed  their  place 
of  residence,  so  that  the  comparison  is  unconsciously  made  be- 
tween the  climate  of  New  York,  for  example,  and  that  of  Iowa, 
climates  which  are  somewhat  different.  Actual  records  of  climate, 
covering  as  much  as  a  century  for  some  parts  of  our  country, 
do  not  afford  any  basis  for  the  conclusion  that  the  climate  is 
changing  materially. 

Fluctuations  of  rainfall,  temperature,  etc.,  do  occur  in  rela- 
tively short  cycles.  Thus  there  seems  to  be  a  faintly  marked 
weather  cycle  of  about  eleven  years,  corresponding  to  the  sun-spot 
cycle;  but  it  is  not  yet  clear  that  this  periodic  change  is  general  or 
persistent.  Hann  says  that  the  only  thing  which  can  be  con- 
sidered as  proved  is  that  there  are  traces  of  a  parallelism  in  the 
march  of  certain  meteorological  elements  and  that  of  the  sun-spot 
period.1 

A  longer  cycle  of  about  thirty-five  years  is  indicated  for  Europe, 
where  records  have  been  kept  longer  than  in  our  own  country.  This 
conclusion  is  based  on  the  weather  data  of  more  than  two  centuries. 
Within  the  cycle  of  this  duration  there  may  be  said  to  be  two 
focal  periods  of  a  few  years  each,  one  when  the  rainfall  is  above 
and  the  temperature  below  the  average,  and  the  other  when  the 
rainfall  is  below  and  the  temperature  above.  These  two  focal 
periods  are  not,  however,  symmetrically  placed  in  the  cycle.  Thus 
the  period  of  minimum  rainfall  may  occur  five  years  after  the 
period  of  maximum  rainfall,  or  it  may  occur  thirty  years  after  it. 
In  view  of  this  great  irregularity,  it  may  be  doubted  whether  the 
cycle  is  to  be  regaided  as  based  on  laws  which  are  universally 
applicable. 

1  Hann,  Handbook  of  Climatology. 


704  PHYSIOGRAPHY 

The  focal  periods,  as  set  forth  by  Bruckner,  are  as  follows: 


Wet  and 
cool. 

Interval 
between. 

Dry  and 
warm 

Interval 
between. 

167  1-1675  i 

1681-1685  N 

) 

25 

I 

45 

1696-1700 

1726-1730  j 

I 

45 

I 

30 

1741-1745  f 

1756-1760  { 

I 

25 

30 

1766-1770  f 

1786-1790  < 

I 

50 

( 

r 

37 

1816-1820  ( 

1820-1830 

I 

35 

I 

38 

1851-1855  J 

1861-1865  J 

The  reader  may  judge  for  himself  as  to  the  adequacy  of  the 
basis  of  the  thirty-five-year  period. 

Variations  of  climate  reflect  themselves  in  the  movements  of 
the  glaciers.  This  has  been  observed  especially  in  connection  with 
the  glaciers  of  the  Alps.  The  glaciers  advance  after  (commonly 
some  years  after)  the  periods  of  maximum  precipitation  and  mini- 
mum temperature,  and  retreat  after  the  opposite  conditions  are 
most  pronounced. 

Certain  historic  facts  have  been  interpreted  to  indicate  changes 
of  climate  in  some  regions  since  the  beginning  of  the  historic  period. 
Thus  regions  once  populous  are  now  too  arid  to  support  an  abun- 
dant population.  This  is  the  case  in  southwestern  Asia  and  north- 
ern Africa,  where  the  ruins  of  aqueducts  and  irrigating  canals 
exist  where  there  are  now  no  adequate  sources  of  water. 

In  geologic  time.  Going  still  further  back,  there  is  abundant 
evidence  of  profound  changes  of  climate  in  the  course  cf  the  earth's 
history.  There  have  been  at  least  three  periods  and  perhaps  more, 
widely  separated  in  time,  when  glaciation  took  place  where  glaciers 
do  not  now  exist.  During  one  of  these  periods  of  cold  there  were 
extensive  glaciers  in  low  latitudes,  even  in  regions  which  now  have 
tropical  and  sub-tropical  climates  (India,  Australia,  South  Africa). 
The  first  of  these  periods  of  exceptional  cold  made  its  appearance 
early  in  the  earth's  history  (at  the  beginning  of  the  Paleozoic  era, 
or  perhaps  even  before),  and  the  last  (the  late  glacial  period)  has 
but  just  passed. 

Warm  climates,  on  the  other  hand,  have  persisted  for  long 
periods  in  polar  regions,  even  down  to  relatively  recent  times. 


CLIMATE  705 

Thus  Greenland  enjoyed  a  warm  climate  not  long  (geologically) 
before  the  development  of  its  present  ice-sheet.  The  data  now 
known  seem  to  indicate  that  the  climate  of  the  present  time  is 
cooler  than  that  which  has  existed  throughout  the  larger  part  of 
the  earth's  history. 

Repeated  changes  in  humidity  seem  to  be  as  clearly  indicated 
as  changes  in  temperature.  Arid  climates  have  existed  at  various 
periods  of  the  earth's  history,  in  regions  which  have  moist  climates 
at  the  present  time  (e.g.,  New  York  and  Ohio),  and  moist  climates 
have  been  enjoyed  by  regions  which  are  now  essentially  desert 
(e.g.,  Arizona).  The  aridity  in  the  one  case  is  indicated  by  salt 
and  gypsum  deposits,  and  the  humidity  in  the  other  by  conclusive 
evidence  of  luxuriant  plant  life  in  regions  which  are  now  desert. 

In  some  cases  the  causes  of  these  changes  were  doubtless  local, 
and  due  to  changes  in  topography;  but  in  others  this  explanation 
is  not  applicable.  It  seems  clear,  therefore,  that  causes  have 
long  been  in  operation  which  bring  about  variations  both  in  tem- 
perature and  humidity.  These  causes  have  sometimes  been  thought 
to  be  (1)  geographic,  and  due  to  the  changes  in  the  relations  of  land 
and  water,  or  to  changes  in  the  topography  of  the  land;  (2)  as- 
tronomic, due  to  changes  in  the  shape  of  the  earth's  orbit,  the  pre- 
cession of  the  equinoxes,  etc.;  and  (3)  atmospheric,  due  to  changes 
in  the  constitution  of  the  atmosphere.  Still  other  causes  have  been 
conjectured.  As  the  facts  concerning  these  changes  accumulate, 
they  seem  to  be  pointing  to  the  third  of  these  lines  of  explanation 
as  the  most  plausible.  It  cannot  be  said,  however,  that  final  con- 
clusions have  been  reached.1 

1  On  this  point  see  Chamberlin  and  Salisbury's  Earth  History. 


PART    IV 
THE   OCEAN 

CHAPTER  XX 

GENERAL   CONCEPTIONS 

THE  ocean  occupies  the  great  depressions  in  the  earth's  surface 
(p.  5).  The  area  of  the  depressed  segments  is  about  twice  as  great 
as  that  of  the  elevated  segments;  but  since  the  water  more  than 
fills  them,  it  spreads  out  over  some  10,000,000  square  miles  of 
the  continental  platforms.  The  result  is  that  the  ocean  water 
covers  nearly  three-fourths  (about  TW)  of  the  earth's  surface. 
All  the  oceans  are  connected  at  the  surface,  and  are  therefore  in 
some  sense  one,  though  different  names  are  applied  to  different 
parts;  but  the  ocean  basins  proper  are  measurably  distinct. 

Although  the  depressions  in  which  most  of  the  ocean  water  lies 
are  called  basins,  they  have  little  resemblance  to  the  homely  vessel 
which  this  name  suggests.  This  is  readily  seen  by  the  construc- 
tion of  a  diagram.  An  arc  three  feet  long,  with  a  radius  of  about 
four  feet,  represents  approximately  the  eighth  of  a  circle.  If  such 
a  curve  be  drawn  on  the  blackboard,  it  may  be  taken  to  represent 
the  width  of  the  Atlantic  Ocean  between  the  United  States  and 
Europe.  If  the  top  of  the  chalk-line  be  taken  to  represent  the  sur- 
face of  the  ocean,  another  line  representing  the  bottom  of  the  ocean 
could  not  be  drawn  below  it  with  an  ordinary  crayon,  without  ex- 
aggerating the  depth  of  the  water. 

Fig.  680  may  help  to  give  us  some  conception  of  the  real  shape 
of  an  ocean  basin.  It  is,  in  general,  convex  upward,  but  locally, 
especially  where  it  joins  the  continental  platforms,  it  is  often  con- 
cave upward.  Fig.  681  shows  the  belts  where  the  bottom  is  con- 
cave upward.  These  belts  are  usually  100  to  300  miles  in  width. 

706 


GENERAL  CONCEPTIONS 


707 


The  sea-level.  The  surface  of  the  sea  is  in  sharp  contrast 
with  that  of  the  land,  in  that  the  former  seems 
to  be  level.  We  are  accustomed  to  speak  of  it  as 
if  it  were  free  from  all  unevennesses,  and  it  is  the 
datum  plane  from  which  all  elevations  on  land 
are  measured.  It  is,  therefore,  of  importance  to 
understand  what  the  sea-level  really  is. 

In  the  first  place,  it  is  a  curved  surface,  and 
its  curvature  is  approximately  that  of  an  oblate 
and  slightly  imperfect  spheroid  (p.  482).  But  its 
surface  corresponds  only  approximately  to  that  of 
a  spheroid,  for  the  land-masses  which  rise  above 
the  ocean  basins  and  which  culminate  in  moun- 
tains  attract  the  waters  of  the  sea  to  themselves, 
and  so  act  somewhat  against  the  principal  at- 
traction  of  gravitation,  which  tends  to  draw  all 
objects  toward  the  center  of  the  earth.  The 
Andes  Mountains,  for  example,  rise  far  above  the 
sea  close  at  hand,  and  the  water  adjacent  to 
them  is  pulled  up  somewhat  above  the  normal 
spheroid  level  by  their  attractive  force.  It  has 
been  estimated  that  the  sea-water  at  the  mouth 
of  the  Indus  on  the  coast  of  India  is  300  feet 
higher  (that  is,  300  feet  farther  from  the  center  of 
the  earth)  than  that  about  the  island  of  Ceylon 
at  the  southern  end  of  the  peninsula.  This  dis- 
tortion  of  sea-level  is  due  to  the  attraction  of  the 
Himalaya  Mountains  and  the  adjacent  high  lands, 
All  land-masses  act  in  the  same  way,  and  the 
distortion  is  greater  the  greater  the  mass  of  land 
above  sea-level  close  to  the  shore. 

The  sea-level,  therefore,  does  not  correspond 
exactly  with  the  curvature  of  a  spheroid.  Further- 
more, the  heights  and  masses  of  mountains  vary 
from  age  to  age,  so  that  their  distorting  effects 
vary  somewhat  in  long  periods  of  time.  If  it,  is 
desired  to  record  the  elevation  of  a  place,  for 
example  in  California,  in  a  way  which  will  be  per- 
manently accurate,  it  should  be  recorded  not  only  that  it  is,  say, 
500  feet  above  sea-level,  but  that,  for  example,  it  was  500  feet 


708 


PHYSIOGRAPHY 


GENERAL  CONCEPTIONS  709 

above  sea-level  in  latitude  40°  on  the  coast  of  California  on 
January  1,  1900. 

Apart  from  the  more  or  less  permanent  distortions  of  the  sur- 
face of  the  sea,  due  to  the  attraction  of  land-masses,  there  are  tem- 
porary and  slight  inequalities  of  level,  which  will  be  considered 
later. 

What  the  physical  geography  of  the  sea  includes.  The  physi- 
cal geography  of  the  sea  includes  many  things.  Among  them  are 
(1)  the  distribution  of  its  waters,  (2)  its  depth  at  all  points,  (3) 
the  topography  of  its  bottom,  (4)  the  composition  of  the  water, 
(5)  its  color,  (6)  its  temperature  at  all  points  at  the  surface  and 
beneath  it,  (7)  its  movements,  (8)  its  life,  and  (9)  the  material  of 
its  bottom. 

The  physical  geography  of  the  sea  has  become  known,  so  far  as 
it  is  now  known,  in  various  ways.  The  distribution  of  its  waters 
has  been  made  clear  by  outlining  the  areas  of  the  land.  The  char- 
acter of  its  waters  is  determined  by  chemical  analysis.  The  move- 
ments of  its  waters  are  studied  in  various  ways.  Some  of  them, 
such  as  waves,  may  be  studied  from  the  shore;  others,  such  as  the 
currents,  are  less  readily  observed,  but  have  become  known,  so 
far  as  they  are  known,  (1)  by  their  effects  in  changing  the  courses 
of  sailing-vessels,  (2)  by  observations  on  the  course  of  objects 
floating  in  the  water,  (3)  by  their  effects  on  temperature,  and  in 
various  other  ways. 

Most  of  our  knowledge  concerning  the  depth  of  the  ocean,  its 
temperature,  its  life,  the  material  and  the  topography  of  its  bot- 
tom, and  much  concerning  its  movements,  has  been  gained 
through  expeditions  which  have  been  sent  out  from  time  to  time 
to  study  these  especial  problems.  The  expeditions  which  have 
contributed  to  our  knowledge  of  the  ocean  have  been  fitted  out 
by  governments  in  some  cases,  by  societies  in  others,  and  by  in- 
dividuals or  combinations  of  individuals  in  still  others.  The 
expedition  which  was  carried  out  on  the  most  elaborate  scale  was 
that  of  the  Challenger,  1872-6,  under  the  auspices  of  the  British 
Government.  This  vessel  made  extended  explorations  in  the 
Atlantic,  the  Pacific,  and  the  Southern  oceans  (Fig.  682).  The 
results  of  the  observations  made  during  the  voyage  of  the  Chal- 
lenger, and  inferences  from  them,  have  been  published  in  a  great 
series  of  large  volumes  which  give  us  our  most  detailed  knowledge 
of  the  sea.  Numerous  other  lesser  expeditions  have  made  less 


710 


PHYSIOGRAPHY 


voluminous  but  still  valuable  contributions  to  our  knowledge  of 
the  ocean.  Here  may  be  mentioned  the  work  of  the  U.  S.  S. 
Mercury  (Barbadoes  to  Sierra  Leone,  1871),  H.  M.  S.  Lightning  and 
H.  M.  S.  Porcupine  (Faroe  Islands  to  the  Mediterranean,  1868-70), 
the  German  frigate  Gazelle  (1874-6; ,  and  the  U.  S.  Coast  and 
Geodetic  Survey  steamer  Blake  (Gulf  of  Mexico,  Caribbean  Sea, 
east  coast  of  the  United  States,  1877-80),  and  the  work  of  the 
Coast  and  Geodetic  Survey  on  the  Gulf  Stream  (1845-59.)  The 


120-     140'     IW     IW     IW     IW     1M-      IW 


100°      HO'     140° 


FIG.   682. — The   course  of  H.M.S.  Challenger,   shown  by  broken  line  on 

oceanic  areas. 


expedition  of  Nansen  into  the  Arctic  region  and  the  numerous 
expeditions  of  the  last  years  into  the  Antarctic  regions  have  given 
us  much  information  concerning  the  waters  of  high  latitudes. 
Some  indication  of  the  manner  in  which  these  expeditions  do  some 
of  their  work  will  appear  in  the  following  pages. 

Distribution  of  the  ocean  waters.  The  distribution  of  the 
ocean  waters  has  been  outlined  in  a  general  way  in  connection 
with  the  distribution  of  the  land  (Chap.  I.).  The  ocean  encircles 
the  earth  in  latitude  60°  S.,  and  from  this  encircling  sea  three 
great  bodies  of  water,  the  Atlantic,  the  Pacific,  and  the  Indian 
oceans  respectively,  extend  northward.  South  of  latitude  60°  S. 
lies  an  elevated  tract,  Antarctica.  It  will  be  recalled  that  in  the 
northern  hemisphere  the  land  makes  an  almost  complete  circuit 
in  latitude  60°  to  70°,  whence  it  radiates  southward  in  three  (or 


GENERAL  CONCEPTIONS  711 

two)  great  arms,  and  that  north  of  the  encircling  land  lies  the  Arctic 
Ocean.  The  waters  south  of  latitude  40°  S.  are  often  called  the 
Southern  Ocean.  The  Arctic  Ocean  is  about  one-thirty-seventh  of 
the  sea  area,  the  Indian  Ocean  about  one-eighth,  the  Southern  Ocean 
one-fourth,  the  Atlantic  one-fifth,  and  the  Pacific  three-eighths. 

The  unequal  distribution  of  land  and  water  in  the  northern 
and  southern  hemispheres  has  an  important  influence  on  their 
climates,  as  already  stated. 

Depth.  The  average  depth  of  the  ocean  is  estimated  to  be 
about  2J  miles,  or  between  12,000  and  13,000  feet.  The  average 
depth  of  the  Pacific  is  estimated  at  2f  miles;  that  of  the  Atlantic 
at  2^  miles;  and  that  of  the  Indian  and  Southern  oceans  at  2J 
miles.  The  average  depth  of  the  Arctic  Ocean  is  not  known,  but 
Nansen  found  a  depth  of  more  than  12,000  feet  off  the  continental 
shelf  of  Eurasia.  The  greatest  known  depth  of  ocean  water  is 
nearly  six  miles.  This  depth  slightly  exceeds  the  height  of  the 
highest  mountain  above  sea-level.  There  are  numerous  places 
where  the  depth  of  the  ocean  exceeds  four  miles,  and  the  area  of 
^ery  deep  water  is  very  much  greater  than  the  area  of  very  high 
land.  The  tracts  which  are  notably  below  the  average  depth  of 
the  ocean  are  often  known  as  deeps. 

The  greatest  known  depth  of  water,  31,614  feet,  is  in  the  Pacific, 
aear  the  Ladrone  Islands.  Another  area  of  almost  equal  depth 
(30,930  feet)  occurs  in  the  Aldrich  Deep  northeast  of  New  Zealand. 
A  depth  of  nearly  28,000  feet  is  found  in  the  Tuscarora  deep  east 
of  Japan,  and  a  depth  of  about  25,000  feet  (nearly  five  miles)  off 
the  coast  of  Chile,  in  latitude  24°  to  25°  S. 

None  of  these  great  depths  is  in  the  midst  of  the  Pacific.  Some 
of  them  are  close  to  continental  shores,  and  the  others  are  in  regions 
of  abundant  islands,  and  in  surroundings  where  the  water  is  not 
very  deep.  Most  of  them  are  in  the  western  part  of  the  ocean. 
In  all  cases  the  slopes  down  to  these  great  depths  are  steep,  as 
submarine  slopes  go,  and  the  deeps  have  a  pronounced  tendency  to 
elongation  parallel  to  the  nearest  coasts  or  to  adjacent  submarine 
ridges,  or  to  ridges  the  crests  of  which  rise  into  islands. 

The  only  area  in  the  Atlantic  where  comparable  depths  are 
known  is  north  of  Porto  Rico  in  the  Blake  Deep  (lat.  20°  N.,  long. 
65°  to  68°),  where  a  maximum  depth  of  27,366  feet  has  been 
sounded.  This  deep,  too,  is  elongate,  has  steep  slopes,  and  is 
parallel  to  the  great  ridge  of  which  Porto  Rico  is  a  part,  and  near 


712 


PHYSIOGRAPHY 


which  it  lies.     In  few  other  places  in  the  Atlantic  does  the  depth 
reach  20,000  feet. 

The  Indian  Ocean  is  not  known  to  have  depths  much  exceed- 
ing 20,000  feet,  and  the  deepest  known  place  in  the  Southern  Ocean 
is  still  less. 

The  depth  of  the  ocean  becomes  known  by  soundings.     Sound- 
ings are  made  from  ships,  by  reeling  out  a  heavy  metallic   ball 
attached  to  a  fine  steel  wire.     (Why  not  a  rope?)     The  ball  is  so 
fastened  to  the  line  as  to  be  detached  when  it  reaches  the  bottom 
(Fig.  683),  because  it  is  much  simpler  to  leave  it 
than  to  draw  it  up  again.     A  sounding  of  3000 
fathoms  may  be  made  in  about  an  hour. 

There  is  a  more  or  less  wide-spread  notion  that 
the  deeper  waters  of  the  sea  are  so  dense  that 
weights  will  not  sink  readily,  and  that  deep-sea 
sounding  is  attended  with  difficulty  on  this  ac- 
count. This  is  incorrect,  for  water  is  but  slightly 
compressible,  and  the  water  in  the  deepest  part 
of  the  ocean  is  but  little  heavier  (probably  not  a 
twentieth),  volume  for  volume,  than  that  at  the 
surface.  There  are  difficulties  connected  with  deep 
soundings,  but  their  cause  is  not  the  great  density 
of  the  deep  water. 

Volume.  The  average  depth  and  the  area  of 
the  oceans  being  known,  the  volume  of  water 
which  they  contain  may  be  calculated.  It  is  found 
to  be  nearly  fifteen  times  the  volume  of  land.  If 
all  the  material  of  the  land  were  carried  to  the 
sea  and  deposited  in  its  basin,  it  would  raise  the 
level  of  the  water  about  650  feet.  If  the  surface 
of  the  lithosphere  were  brought  to  a  common  level 
by  planing  down  all  continental  platforms  and 
building  up  the  deep  parts  of  the  ocean  basins, 
the  ocean  water  would  cover  the  whole  of  the  earth  to  a  depth  of 
about  9000  feet,  or  nearly  two  miles. 

Mass.  The  mass  (weight)  of  the  sea  is  only  about  five  times  the 
mass  of  the  land  above  the  sea,  since  water  is  much  lighter  than  an 
equal  volume  of  rock.  The  mass  of  the  sea  is  about  265  times 
the  mass  of  the  air  which  surrounds  it,  and  about  j^g-  of  the  mass 
of  the  solid  part  of  the  earth. 


FIG.  683.— The 
sounding-line. 
(Challen  ger 
Report.) 


GENERAL  CONCEPTIONS 


713 


Topography  of  the  bottom.  The  larger  part  of  the  sea  bottom 
is  so  nearly  flat  that  if  the  water  were  removed  the  eye  would 
scarcely  detect  its  departure  from  flatness.  Its  topography  is 
therefore  very  different  from  that  of  the  land.  As  already  indi- 
cated, the  agent  which  does  most  to  roughen  the  surface  of  the 
land  is  running  water,  and  rivers  do  not  flow  on  the  bottom  of 
the  sea.  The  most  notable  difference  between  the  topography 
of  the  sea  bottom  and  that  of  the  land  is  due  to  their  absence. 

In  spite  of  the  prevailing  flatness  of  the  sea  bottom,  its  relief 
is  not  less  than  that  of  the  land.  Its  irregularities  of  bottom  are 
of  several  types.  These  are  (1)  vokanic  cones,  often  built  up  from 
the  bottom  of  the  deep  sea  to  the  surface  of  the  water,  and  even 
far  above  it  (Chap.  VII);  (2)  relatively  steep  slopes  or  scarps,  such 
as  those  at  the  junction  of  the  continental  platforms  with  the  deep 
sea  basins,  and  such  as  occur  about  some  of  the  pronounced  deeps; 
(3)  valley-like  depressions,  found  especially  in  the  shallow  waters 
about  the  borders  of  the  continents;  (4)  pronounced  swells  which 
may  be  compared  to  the  mountain  ridges  of  the  land;  and  (5) 
broad,  plateau-like  areas  rising  notably  above  their  surroundings, 
over  which  the  water  is  relatively  shallow.  The  general  con- 
figuration of  the  bottom  of  the  Atlantic  is  indicated  by  Fig.  5. 

1.  Volcanic  cones  are  wide-spread,  but  are  more  numerous  in  the 
Pacific  Ocean  than  elsewhere,  and  more  numerous  in  its  deeper 
western  part  than  in  its  shal- 
lower eastern  part.      Though 

such  cones  seem  to  rise  ab- 
ruptly, their  slopes  are  really 
much  less  steep  than  they  seem. 
The  summits  of  volcanic  islands 
rarely  have  a  slope  of  more 
than  30°,  and  their  lower  parts 
rarely  more  than  6°  to  10°. 
Below  the  sea  the  slope  is  still 
gentler,  rarely  more  than  3°,  or 
1  mile  in  20.  Figs.  684,  685, 
and  686  show  slopes  corre- 
sponding to  1  mile  in  5,  1  in  10, 
and  1  in  20. 

2.  Though  the  slopes  of  the  bottom  at  the  edges  of  the  con- 
tinental shelves  and  about  the  deeps  are  steep,  as  slopes  in  the 


FIG.    684. —  Diagram    illustrating   a 
slope  corresponding  to  1:5. 


FIG.    685.  —  Diagram    illustrating    a 
slope  corresponding  to  1 : 10. 


FIG.    686.  —  Diagram    illustrating    a 
slope  corresponding  to  1 :  20. 


714  PHYSIOGRAPHY 

ocean  bottom  go,  they  are  much  less  steep  than  many  slopes  on 
the  land.  A  slope  of  1  mile  in  8  is  rare,  and  a  slope  of  1  mile  in 
20  (Fig.  686)  can  hardly  be  said  to  be  common.  The  last  would 
make  a  steep  railway  grade.  Slopes  of  1  in  60  are  higher  than  the 
average  steep  slope  at  the  edge  of  the  continental  shelves.  Even 
up  most  of  thece  "steep"  slopes,  therefore,  railway  trains  could  be 
run  without  change  of  grade. 

In  rare  instances,  slopes  which  would  be  regarded  as  very  steep, 
even  on  land,  are  found  on  the  sea  bottom.  "  Thus  in  the  Mediter- 
ranean Sea,  differences  of  1500  feet  are  said  to  have  been  found 
between  the  bow  and  stern  soundings.  Such  slopes  or  scarps 
are  doubtless  the  result  of  faulting  (p.  406). 

3.  Many  continental  shelves  are  affected  by  valleys  which  have 
the  general  characteristics  of  river  valleys.     Many  of  them  seem 
to  be  continuations  of  valleys  now  in  existence  on  the  land.     Thus, 
the  Hudson,  the  Delaware,  the  Susquehanna,  the  St.  Lawrence, 
the  Saguenay,  and  other  valleys    have  submerged  continuations 
beneath  the  sea.     The  valley  of  the  Hudson  is  continuous  out  to 
the  edge  of  the  continental  shelf,  where  for   20  miles  it  becomes 
pronounced,  with  a  maximum  depth  of  2400   feet  below  its  sur- 
roundings, and  2844  feet  below  sea-level.     Elsewhere  it  is  shallow. 
The  others  are  not  so  deep.     The  submerged  continuations  of  the 
Delaware  and  the  Susquehanna  on  the  continental  shelf  are  less 
than  100  feet  below  their  surroundings,  but  those  of  the  Saguenay 
and  St.  Lawrence,  both  of  which  extend  out  to  the  edge  of  the 
continental  shelf,  are  much  deeper. 

Other  submerged  valleys,  as  some  of  those  on  the  Pacific  coast 
of  the  United  States,  do  not  seem  to  be  the  continuations  of  exist- 
ing land  valleys.  Some  of  these  valleys  are  hundreds  of  miles 
long  and  a  thousand  feet  or  more  (maximum)  deep.  Such  val- 
leys are  commonly  believed  to  have  been  formed  by  rivers  when 
the  sea  did  not  cover  the  areas  where  they  exist  (p.  173). 

4.  Examples  of  mountain-like  swells  are  furnished  by  Cuba  and 
the  adjacent  islands,  which  are  really  the  crests  of  a  great  moun- 
tain system  rising  from  deep  water. 

5.  The  plateau  type  of  elevation  is  exemplified  by  the  dolphin 
ridge  of  the  Atlantic   (Fig.  5).     This  broad,  low  "ridge,"  over 
which  the  water  is  less  than   12,000  and   sometimes  as  little  as 
5000  feet  deep,  traverses  the  Atlantic  lengthwise,  as  far  south  as 
latitude  40°  S.,  and  divides  its  basin  into  two  troughs,  the  one 


GENERAL  CONCEPTIONS  715 

to  the  east  and  the  other  to  the  west,  where  the  water  is  some- 
what deeper.  In  the  southern  Pacific,  volcanic  islands  often  rise 
from  submerged  plateaus. 

From  the  foregoing  it  will  be  seen  that  great  irregularities  are 
found  on  the  sea  bottom  as  on  the  land,  but  that  the  many  minor 
unevennesses  of  the  land,  especially  those  developed  by  running 
water,  wind,  glaciers,  etc.,  find  no  analogies  on  the  ocean's  bed, 
except  in  shallow  water. 


CHAPTER  XXI 
COMPOSITION   OF  SEA-WATER 

THE  most  distinctive  characteristic  of  sea-water  is  its  saltness ; 
but  besides  common  salt,  it  contains  dissolved  mineral  matters  of 
many  other  sorts.  One  hundred  pounds  of  sea-water  contain 
nearly  3J  (3.44)  pounds  of  mineral  matter.  Of  this  mineral  mat- 
ter, common  salt  makes  up  more  than  three-fourths  (about  77.758%). 
The  other  important  minerals  are  magnesium  chloride  (10.878%), 
magnesium  sulphate  (4.737%),  calcium  sulphate  (3.600%),  potas- 
sium sulphate  (2.465%),  and  calcium  carbonate  (.345%).  Very 
many  others  occur  in  very  small  quantities.  These  mineral  matters 
in  the  sea-water  make  it  somewhat  heavier  than  fresh  water. 
If  the  weight  of  fresh  water  be  taken  as  1,  the  average  weight  of 
salt  water  is  1.026. 

A  cubic  mile  of  fresh  water  weighs  about  4,205,650,000  tons  of 
2240  pounds  each,  while  a  cubic  mile  of  normally  salt  water  weighs 
4,314,996,900  tons  The  mineral  matter  in  a  cubic  mile  of  sea-water 
weighs  about  151,025,000  tons.  This,  it  will  be  seen,  exceeds  the 
difference  between  the  weight  of  a  cubic  mile  of  fresh  water  and  a 
cubic  mile  of  sea-water.  It  follows,  therefore,  that  a  cubic  mile 
of  sea-water  does  not  weigh  the  same  as  a  cubic  mile  of  fresh  water 
plus  the  weight  of  the  salts  in  the  former.  The  reason  is  that 
when  mineral  matter  is  dissolved,  it  increases  the  volume  of  the 
water,  bat  not  by  an  amount  equal  to  the  volume  of  the  mineral 
matter  dissolved.  If  all  the  salts  were  taken  out  of  the  sea-water 
and  removed  from  the  ocean  basins,  the  level  of  the  sea  would  be 
drawn  down  more  than  100  feet.  If  all  the  salts  of  the  sea  were 
taken  out  of  solution  and  laid  down  as  a  layer  of  solid  mineral 
matter  on  the  ocean  bottom,  they  would  make  a  layer  about  175 
feet  thick,  and  would  raise  the  surface  of  the  water  (then  with- 
out the  salt)  about  75  feet.  If  all  the  mineral  matter  now  in  solu- 

716 


COMPOSITION  OF  SEA-WATER  717 

tion  in  the  sea  were  taken  out  of  it,  its  aggregate  volume  would 
be  equal  to  about  one-fifth  of  the  volume  of  all  lands  now  above 
the  sea-level. 

The  mineral  matter  in  solution.  ^Mineral  matter  is  being 
constantly  brought  to  the  sea  by  rivers.  The  rivers  are  largely 
fed  by  springs,  and  the  spring  water,  while  underground,  dissolves 
various  sorts  of  mineral  matter  from  the  rocks,  as  we  have  seen 
(p.  96).  The  rivers  are  probably  the  chief  source  of  the  mineral 
matter  in  the  sea,  but  the  sea-water  also  dissolves  mineral  matter 
from  the  rocks  beneath  it.  The  amount  of  mineral  matter  carried 
in  solution  to  the  sea  by  rivers  each  year  is  estimated  to  be  nearly 
half  a  cubic  mile. 

The  rivers  do  not  carry  mineral  matters  to  the  sea  in  the  pro- 
portions in  which  they  exist  in  the  sea-water.  Of  the  minerals 
dissolved  in  river  water,  calcium  carbonate  is  by  far  the  most 
important,  being  nearly  as  abundant  as  all  the  rest,  while  com- 
mon salt  is  one  of  the  minor  constituents,  so  small  in  amount  as 
not  to  be  detected  by  the  taste.  Yet  the  amount  of  the  latter  in 
the  sea-water  is  more  than  200  times  that  of  the  former.  This 
great  difference  is  one  of  the  things  to  be  explained. 

It  is  to  be  noted  that  the  mineral  matters  which  are  most 
abundant  in  the  sea  are  not  those  which  are  most  abundant  in 
the  rocks  of  the  land.  Those  minerals  of  land  rocks  which  are 
most  soluble,  such  as  calcium  carbonate,  get  into  river  water,  and 
thence  to  the  sea,  in  greater  quantity  than  those  which  are  less 
soluble.  Many  minerals  in  the  sea-water  do  not  exist  as  such  in 
the  common  rocks  of  the  land,  but  are  made  by  the  combination 
of  matter  in  the  rocks  with  gases  (especially  CO2)  in  the  air.  Thus 
many  volcanic  rocks  contain  calcium  in  complex  combinations. 
These  complex  compounds  are  broken  up,  and  the  calcium  unites 
with  the  carbonic-acid  gas  of  the  air  to  form  calcium  carbonate. 
This  is  one  of  the  prolific  sources  of  this  material  carried  by 
rivers  to  the  sea.  Again,  common  rocks  do  not  contain  salt,  but 
some  of  them,  such  as  granite,  contain  sodium,  one  of  the  ele- 
ments of  salt.  When  the  sodium  unites  with  chlorine,  the  result 
is  salt.  It  takes  much  rock  to  yield  a  little  salt.  The  great  quan- 
tities of  salt  in  the  sea,  therefore,  must  mean  the  decay  of  much 
greater  quantities  of  rock. 

Some  mineral  substances  in  the  sea,  on  the  other  hand,  are 
derived  directly  by  solution  from  the  rock.  This  is  true  of  much 


718  PHYSIOGRAPHY 

of  the  lime  carbonate,  which  is  the  dissolved  substance  of  lime- 
stone. 

Withdrawal  of  mineral  matter  from  the  sea.  Some  of  the 
substances  in  solution  in  the  sea-water  are  extracted  from  the 
water  by  animals  to  make  their  shells,  tests,  etc.  Most  shells  are 
made  of  calcium  carbonate,  but  animals  appear  to  be  able  to  use 
the  sulphate  of  calcium  also  in  the  making  of  their  shells,  trans- 
forming it  into  calcium  carbonate.  In  spite  of  the  abundant  supply 
of  calcium  carbonate,  therefore,  the  amount  of  this  substance  in 
the  sea  is  relatively  small,  because  animals,  and  some  sea-plants  as 
well,  take  it  out  to  make  shells  and  other  hard  parts  about  as  fast 
as  it  is  brought  in.  Silica  also,  though  found  in  sea-water  in  small 
quantities  only,  is  extracted  by  some  animals  and  plants,  as  calcium 
carbonate  is  by  others.  Salt,  on  the  other  hand,  is  not  used  by 
any  of  the  animals  or  plants  of  the  sea,  and  so  remains  in  solu- 
tion, and  most  of  all  that  has  ever  gone  to  the  sea  appears  to  b« 
there  still. 

A  suggestion  as  to  the  age  of  the  ocean.  At  the  rate  at 
which  salt  is  now  being  taken  from  the  land  to  the  sea  by  rivers, 
it  would  take  some  370,000,000  years  for  the  salt  of  the  sea  to 
have  accumulated.  It  is  by  no  means  certain,  however,  that  salt 
has  always  been  carried  in  at  the  present  rate,  and  it  is  certain 
that  some  of  the  salt  which  has  been  carried  to  the  sea  has  been 
taken  out  again  to  make  the  great  salt  beds  which  occur  in  various 
parts  of  the  earth.  While,  therefore,  370,000,000  years  is  not  to 
be  taken  as  the  age  of  the  ocean,  it  may  give  us  some  hint  of  the 
length  of  time  during  which  it  has  been  in  existence. 

Gases  in  sea-water.  Besides  the  solids  in  solution  in  sea- 
water,  there  are  numerous  gases.  The  most  abundant  are  those 
which  exist  in  the  air  in  abundance,  namely,  nitrogen,  oxygen,  and 
carbonic-acid  gas.  The  amounts  of  these  gases  in  solution  vary 
from  place  to  place  and  from  time  to  time,  but  the  averages  of 
many  analyses  show  that,  of  the  total  amount  of  gases  in  sea-water, 
nitrogen  makes  up  about  ^1\%,  oxygen  about  33J%,  and  car- 
bonic-acid gas  about  16J%.  In  the  aggregate,  the  amount  of  oxygen 
dissolved  in  the  ocean  water  is  rather  more  than  3-J¥  of  the  amount 
hi  the  air;  that  of  the  nitrogen  about  TTBTT;  while  that  of  the  car- 
bonic-acid gas  is  about  18  times  the  amount  in  the  air. 

The  gases  in  the  sea-water  are  dissolved  chiefly  from  the  atmos- 
phere, in  proportions  determined  by  the  pressure  of  each  gas,  by 


COMPOSITION  OF  SEA-WATER  719 

its  solubility,  and  by  the  temperature  of  the  water.  Gases  are  more 
soluble  in  cold  water  than  in  warm,  and  carbonic-acid  gas  is  more 
soluble  than  oxygen,  and  this  in  turn  is  more  soluble  than  nitrogen. 
Once  dissolved  at  the  surface,  these  gases  are  distributed  through 
the  ocean  water  by  the  movements  of  the  water  and  by  diffusion. 
Carbonic-acid  gas  is  also  furnished  to  the  sea-water  in  abundance 
by  the  animals  which  live  in  the  sea,  and  it  issues  from  submarine 
volcanic  vents. 

The  oxygen  of  the  water  is  being  constantly  consumed  by  the 
animals  which  live  in  the  sea,  and  its  supply  is  being  as  constantly 
renewed  by  solution  from  the  air.  The  amount  in  the  sea-water 
decreases  with  increasing  depth,  and  its  paucity  at  great  depths 
is  probably  one  of  the  reasons  why  animal  life  is  not  more  abun- 
dant there.  Though  constantly  diffused  downward,  diffusion  is  a 
very  slow  process.  The  nitrogen  of  the  water  is  little  used,  and 
the  same  nitrogen  probably  stays  in  solution  from  year  to  year 
and  from  age  to  age.  The  carbonic-acid  gas  of  the  sea  is  con- 
sumed by  some  of  the  plants  of  the  sea,  and  some  of  that  exhaled 
by  the  marine  animals  and  volcanic  vents  escapes  into  the  air.  This 
is  one  of  the  sources  of  the  carbonic-acid  gas  of  the  air. 

The  gases  dissolved  in  the  water  do  not  greatly  affect  its  volume, 
though  they  increase  it  slightly. 

Salinity,  density,  and  movement.  The  waters  of  different 
parts  of  the  earth  contain  slightly  different  amounts  of  salt  and 
other  mineral  matters.  The  variation  comes  about  in  different 
ways:  (1)  Evaporation  is  more  rapid  at  some  points  than  at 
others.  Since  the  salts  are  left  behind  when  sea-water  evaporates, 
the  water  becomes  more  saline  where  evaporation  is  great.  The 
greater  the  amount  of  mineral  matter  in  solution,  the  greater  the 
density  of  the  water.  (2)  Where  rainfall  is  great,  the  water  is 
freshened  and  so  made  lighter.  (3)  Where  rivers  enter  the  sea, 
they  bring  in  fresh  water,  which,  mingling  with  the  salt  water, 
makes  it  lighter. 

In  all  the  above  ways  the  salinity  of  the  sea-water  at  the  top 
of  the  ocean  is  being  continually  altered.  Every  alteration  in 
salinity  changes  the  density  of  the  water,  and  unequal  density 
causes  movement.  When  the  surface  water  becomes  more  dense 
than  that  beneath,  it  sinks,  and  the  lighter  water  comes  in  over  it 
from  all  sides.  When  the  surface  water  becomes  less  dense  than 
the  surrounding  water  at  the  same  level,  the  heavier  water  dis- 


720  PHYSIOGRAPHY 

places  the  lighter,  causing  it  to  spread  out  on  the  surface,  for  the 
same  reason  that  oil  spreads  on  water.  Since  variations  in  the 
salinity  of  water  are  being  constantly  produced,  motion  due  to 
inequalities  of  density  resulting  from  inequalities  of  salinity,  is 
also  constant.  Movements  brought  about  in  this  way  are  partly 
vertical  and  partly  horizontal.  They  are  usually  so  slow  as  to  be 
imperceptible,  and  may  appropriately  be  called  creep. 

Density  of  Water  under  Certain  Conditions 

Pure  water  at  39.6°  F 1 .00 

"      "  212°  F. 95 

Surface  sea-water  at  60°  F 1 .024  to  1.03 

Sea-water  five  miles  down 1  06 

Salinity  and  color.  The  water  of  the  sea  is  generally  blue  or 
green,  but  its  color  varies  from  point  to  point  and  from  time  to 
time.  It  seems  to  be  indicated  by  numerous  observations  that 
the  blue  color  of  sea-water  is  intensified  by  increase  of  salinity. 
The  Gulf  Stream  is  distinctly  bluer  than  the  less  salty  cold  current 
off  Labrador,  and  inland  seas,  such  as  the  Mediterranean,  which 
are  more  salty  than  the  open  ocean,  are  of  deeper  blue.  The  cold 
and  less  salty  waters  of  high  latitudes  are  often  distinctly  green. 
Many  of  the  variations  of  color  are  due  to  the  solid  matter  in  sus- 
pension in  the  water.  Microscopic  animals  and  plants,  and  the 
sediment  washed  or  blown  out  from  the  land  or  furnished  by  ex- 
plosive volcanoes  beneath  the  sea,  all  contribute  to  the  observed 
variations. 


CHAPTER  XXn 
THE   TEMPERATURE  OF  THE   SEA 

THE  temperature  of  the  sea  is  to  be  considered  both  horizontally 
and  vertically.  In  other  words,  account  must  be  taken  of  the 
temperature  both  at  the  surface  and  beneath  it. 

Temperature  of  the  surface.  In  general,  the  temperature  of 
the  surface  of  the  ocean  water  decreases  from  the  equator  toward 
the  poles,  the  same  as  the  temperature  of  the  land  (Fig.  538). 
It  varies  from  about  80°  F.  in  the  equatorial  regions,  to  about  28°  F. 
in  the  polar  regions.  When  the  temperature  sinks  below  the  latter 
figure,  the  sea-water  freezes,  and  the  temperature  of  the  surface 
of  the  ice  may  sink  as  low  as  the  temperature  of  the  air  above  it; 
but  the  temperature  or  the  water  immediately  beneath  the  ice  does 
not  sink  much  below  28°  F.  The  decrease  of  temperature  with 
increase  of  latitude  is  by  no  means  regular,  as  shown  by  the  isother- 
mal charts.  In  Figs.  539  and  540,  for  example,  the  isothermal 
lines  over  the  ocean  are  far  from  parallel  with  the  parallels  of  lati- 
tude. 

The  notable  departures  of  the  ocean  isotherms  from  the  parallels 
of  latitude  are  due  to  various  causes.  In  the  open  ocean  they  are 
due  chiefly  to  currents  in  the  ocean  water.  Some  of  these  cur- 
rents are  of  water  which  is  flowing  into  waters  warmer  than  them- 
selves, and  some  are  of  water  flowing  into  waters  cooler  than  them- 
selves. The  former  are  known  as  cold  currents,  and  the  latter  as 
warm  currents.  A  cold  current  deflects  an  isotherm  equatorward, 
and  a  warm  current  deflects  it  poleward.  Fig.  539  furnishes  a 
good  illustration  of  the  effect  of  a  warm  current  in  the  North 
Atlantic  on  the  position  of  the  isotherms. 

There  are  other  reasons  why  the  temperature  of  the  surface 
water  of  the  ocean  does  not  decrease  steadily  from  equator  to 
poles  Thus  rivers  entering  the  sea  are  sometimes  (especially  in 

721 


722  PHYSIOGRAPHY 

summer)  warmer  and  sometimes  (especially  in  winter)  colder  than 
the  sea-water  where  they  enter.  Rivers,  therefore,  help  to  pro- 
duce irregularities  of  surface  temperatures.  Enclosed  or  partially 
enclosed  arms  of  the  sea  in  low  latitudes  are  generally  warmer 
than  the  open  ocean  in  the  same  latitude,  and  in  such  situations 
the  highest  temperatures  of  the  sea  are  found.  Thus  the  surface 
temperature  of  the  Red  Sea  is  sometimes  90°  or  even  100°. 

Temperature  and  movement.  Since  water  expands  on  being 
warmed,  warm  water  is  lighter  than  cold,  if  both  are  equally  salt. 
Unequal  surface  temperatures  therefore  cause  movement  of  the 
surface  waters.  The  tendency  of  the  resulting  movement  is  to 
cause  the  colder  waters  of  the  higher  latitudes  to  displace  the 
warmer  waters  of  the  same  level  in  lower  latitudes,  while  the 
warmer  waters  of  the  latter  zone  spread  widely  over  the  sur- 
face. The  movement  is,  therefore,  circulatory.  The  movements 
due  to  this  cause  are  always  slow,  but  since  the  surface  temperature 
is  constantly  kept  unequal  by  unequal  heating,  by  the  inflow  of 
rivers,  and  by  melting  ice,  there  must  be  constant  movement  of 
the  surface  waters  as  a  result  of  the  constantly  renewed  inequality 
of  temperature. 

The  surface  of  the  sea  is  subject  to  both  seasonal  and  daily 
changes  of  temperature.  Both  are  much  less  than  the  corre- 
sponding changes  on  land  in  the  same  latitude  (p.  530). 

Temperature  beneath  the  surface.  Except  where  the  surface 
waters  of  the  sea  are  at  or  near  the  freezing-point,  the  temperature 
becomes  cooler  with  increasing  depth.  Even  where  the  surface 
water  is  warmest,  its  temperature  at  the  depth  of  a  few  hundred 
fathoms  (rarely  more  than  800,  and  generally  much  less)  is  below 
40°  F.,  and  that  at  the  bottom  still  colder.  The  following  table 
shows  the  average  temperature  of  the  sea  at  various  depths : 

Depth.  Temperature. 

600  feet  60.7° 

1,200   "  50.0° 

3,000   "  40.1° 

6,000   "  36.5° 

13,200   "  35.2° 

It  is  estimated  that  not  more  than  one-fifth  of  the  water  of  the 
ocean  has  a  temperature  as  high  as  40°  F.,  while  its  average  tem- 
perature is  probably  below  39°  F.  At  the  bottom  of  the  deep  sea 


THE  TEMPERATURE  OF  THE  SEA 


723 


I4'C 


\ 


5  '_ 


the  temperature  is  generally  below  35°  F.  The  only  parts  of  the 
ocean-bottom  where  the  temperature  is  as  high  as 
40°  F.  are  the  areas  of  shallow  water  and  the  en- 
closed seas  of  relatively  low  latitudes.  Such  areas 
do  not  constitute  more  than  8%  of  the  area  of 
the  sea.  Fig.  687  represents  a  temperature  curve 
for  the  South  Atlantic,  and  is  fairly  typical  for  the 
ocean  in  general.  Fig.  688  shows  a  similar  curve 
for  the  North  Atlantic,  and  Figs.  689  to  691  show 
the  temperature  curves  for  other  places. 
^v  The  tempera- 

ture   of    the    sea 
does     not    every- 
where  decrease 
IsooFms,   steadily  from  the 
FIG.  687. — A  temperature  curve  in  the  South  Atlan-  surface  down,  for 
tic;  latitude  35°  59'  S.,  longitude  1°  34'  E.     (Chal-  th  more  or 

lenger  Report.) 

less      well-defined 

currents  beneath 
the  surface,  some- 
times warmer  and 
sometimes  colder 
than  their  sur- 
roundings. Such 
currents  introduce 


lenger  Report.) 


D 


isooFms 


Fms. 


irregularities  into 
the  temperature 
curves  (Fig.  692). 
The  tempera- 


FIG.  689.— Temperature    curve    for   the    South  At-  f  f} 

lantic,  where  the  water  is  affected  by  the  Antarr-    ' 
tic  current;  latitude  42°  32'  S.,  longitude  56°  27' 
W.     (Challenger  Report.) 


parts   of  enclosed 
seas    in   low   lati- 
tudes   present    striking   contrasts   to    the   temperatures   of    the 


724 


PHYSIOGRAPHY 


deeper  parts  of  the  open  sea.  Thus  the  temperature  of  the  Red 
Sea  decreases  from  90°  F.  or  more  at  the  surface,  to  70°  F.  at  a 
depth  of  1200  feet,  and  then  remains  nearly  constant  to  the  bottom 
at  3600  feet.  In  the  Mediterranean,  the  temperature  falls  from 
about  75°  F.  at  the  surface  to  55°  F.  at  a  depth  of  750  feet,  and 
then  remains  essentially  constant  to  the  bottom,  13,000  feet,  while 
the  temperature  of  the  ocean  outside  falls  to  37°  F.  in  its  deeper 
parts.  The  high  temperature  of  the  deep  waters  of  these  enclosed 
seas  is  due  to  the  submerged  barriers  which  partially  shut  them 
off  from  the  ocean,  and  do  not  allow  the  colder  and  therefore  denser 
waters  outside  to  flow  in  and  displace  the  warmer  and  lighter 
waters  below  the  top  of  the  basin  (Fig.  693).  The  temperature 
of  the  bottom  of  enclosed  seas  is,  in  general,  approximately  the 

temperature  of  the  adjacent  open- 
sea  water  at  the  level  correspond- 
ing to  the  top  of  the  submerged 
barrier. 

The  phenomena  of  these  and 
other  similarly  situated  basins 
show  that  the  sun  shining  on  an 
enclosed  body  of  water  may,  in 
the  course  of  time,  warm  it  to  the 

K 


50  C 


25  li 


5  . 


25- 


20*_ 


K>*_ 


5'. 


\ 


Fms. 


G.  690. — -Temperature  curve  for  the  equatorial 
Atlantic;  latitude  0°  9' N.,  longitude  30°  18'  W. 
(Challenger  Report.) 

bottom,  even  where 
the  water  is   deep, 
"Soo  Fms.  in  low  latitudes  at 

FIG.  691. — Temperature    curve    for   the   equatorial  least     The  low  tern- 
Pacific:  latitude  0°  40'  N.,  longitude  148°  41'  E.  , 
(Challenger  Report.)                                                perature  of  the  body 

of  the  ocean  is  not, 
therefore,  due  to  the  inability  of  the  sun  to  heat  it. 

The  reasons  for  the  low  temperature  of  the  great  body  of  the 
sea-water  are  readily  understood.  1.  The  heat  of  the  sun  has 
little  direct  effect  below  some  such  depth  as  200  to  300  feet,  and 
none  at  all  below  600  feet.  Taken  alone,  this  does  not  account 


THE  TEMPERATURE  OP  THE  SEA 


725 


for  the  low  temperature  of  the  body  of  the  ocean,  as  the  phenomena 
of  the  enclosed  basins  show,  but  it  is  one  of  the  elements  of  the  prob- 


1000 


1500 


Fio.  692. — Temperatures  in  the  South  Atlantic  between  Falkland  Islands 
Rio  de  la  Plata,  Tristan  d'Acunha  Islands,  and  Cape  of  Good  Hope. 
(Challenger  Report.) 

lem.  The  sun  has  been  shining  long  enough  to  have  warmed  the 
ocean  to  its  bottom,  even  by  the  slow  (in  water)  process  of  con- 
duction. 2.  The  bottom  of  the  sea,  though  warmed  by  conduc- 
tion from  the  lithosphere  below,  is  warmed  with  extreme  slowness. 


Indian  Ocean 


Bed  Sea 


FIG.  693. -^Diagrammatic  section  of  Red  Sea  and  the  adjacent  part  of  the 
Indian  Ocean,  to  illustrate  the  effect  of  a  barrier  on  the  temperature  of 
the  waters.  The  temperature  is  expressed  in  degrees  Fahrenheit.  The 
numbers  at  the  left  show  depth  in  fathoms. 

As  its  temperature  rises,  the  expanded  water  is  crowded  up  by  the 
colder,  heavier  water  which  gets  beneath  it.  3.  The  cold  waters 
of  the  surface,  whether  produced  by  contact  with  the  cold  air  or 
by  the  melting  of  ice  and  snow,  tend  constantly  to  sink.  The  supply 
of  ice-water  from  the  polar  regions,  especially  from  the  Antarctic 
region,  is  very  great,  and  though  that  which  comes  from  the  land 
is  fresh,  and  therefore  lighter  than  sea-water  at  the  outset,  it  soon 
becomes  salt  by  diffusion  and  by  mixing  with  salt  water.  This 
enormous  supply  of  ice-water  is  the  great  cause  of  the  low  average 
temperature  of  the  sea.  Without  the  polar  ice-caps,  the  average 
temperature  of  the  ocean  would  in  time  be  raised  perceptibly. 


726  PHYSIOGRAPHY 

If  the  ice-caps  were  melted,  it  would  probably  go  far  toward  re- 
storing the  genial  climates  of  earlier  times  in  high  latitudes,  when 
temperate  and  even  subtropical  plants  and  animals  lived  in  Green- 
land and  in  Antarctica. 

The  temperature  below  the  surface  is  ascertained  by  a  ther- 
mometer constructed  for  this  especial  purpose.  Its  chief  peculiar- 
ity is  such  construction  as  will  enable  it  (1)  to  withstand  the 
great  pressure  of  deep  water  without  having  the  position  of  the 
mercury  in  the  tube  influenced  seriously  by  it,  and  (2)  to  register 
the  temperature  of  any  desired  depth.  Since  the  pressure  in  the 
sea  increases  a  ton  per  square  inch  for  a  little  less  than  a  mile  of 
descent,  it  will  be  seen  that  the  ordinary  thermometer  used  in  the 
atmosphere  would  not  be  serviceable.  A  satisfactory  thermometer 
was  not  devised  until  1869,  just  before  the  departure  of  the  Chal- 
lenger expedition. 

The  ice  of  the  sea  has  been  referred  to  in  other  connections 
(pp.  210  and  269). 

The  movement  of  floating  ice  is  controlled  partly  by  the  winds, 
and  partly  by  the  movements  of  the  water  in  which  the  ice  is 
floating. 


CHAPTER  XXIII 
THE  MOVEMENTS  OF  SEA-WATER 

Causes  of  Movement 

WE  have  seen  that  inequalities  of  density  in  sea-water  arise 
chiefly  from  (1)  unequal  salinity,  and  (2)  unequal  temperature,  and 
that  these  inequalities  taken  by  themselves  insure  a  constant, 
though  slow,  circulation  of  the  waters  of  the  sea.  There  are  other 
causes,  also,  which  produce  movement.  Chief  among  them  are 
(3)  inequalities  of  level,  (4)  the  wind,  and  (5)  the  differential  at- 
traction of  heavenly  bodies,  especially  the  moon  and  the  sun. 
There  are  also  (6)  occasional  causes,  such  as  earthquakes,  volcanic 
explosions  in  the  sea,  landslides  on  coasts,  etc.,  which  produce 
temporary  and  sometimes  disastrous  movements.  The  effect  of 

(1)  and  (2)  have  been  noticed  already  (pp.  719  and  722). 
Movements  due  to  inequalities  of  level.    The  inequalities  of 

level  which  produce  movement  are  brought  about  by  (1)  the  in- 
flow of  land  waters,  which  raise  the  surface  at  the  point  of  inflow; 

(2)  winds,  which  tend  to  pile  up  the  waters  along  the  shores  against 
which  they  blow;    (3)  unequal  rainfall,  which  tends  to  raise  the 
surface  where  it  falls;    (4)  unequal  evaporation,  which  tends  to 
lower  the  surface  where  it  is  excessive;   and  (5)  variations  in  at- 
mospheric pressure,  the  surface  of  the  water  being  slightly  de- 
pressed where  atmospheric  pressure  is  excessive. 

All  such  inequalities  of  level  in  the  surface  of  the  sea  cause 
movement.  The  movement  begins  as  soon  as  the  inequality  of 
surface  appears,  and  before  it  becomes  considerable.  The  result 
is  that  the  movements  due  to  differences  of  level  are  generally 
slow.  So  far  as  they  are  due  to  rainfall,  to  inequalities  of  evapo- 
ration and  to  atmospheric  pressure,  they  are  generally  impercep- 
tible. The  movement  occasioned  by*-  the  inflow  of  rivers  is  more 
noticeable,  and  is  often  distinctly '.felt  for  some  distance  off 

727 


728  PHYSIOGRAPHY 

shore.  When  waters  are  piled  up  against  a  shore  by  winds,  there 
is  sooner  or  later  a  return  movement  which  tends  to  make  the 
surface  level  again.  During  a  storm  on  the  coast  of  India  in 
1864  (Oct.  5),  the  water  was  raised  24  feet  at  Calcutta,  drowning 
i8,000  people.  The  raising  of  the  surface  of  the  water  was  the 
most  destructive  element  in  the  storm  at  Galveston,  already  re- 
ferred to  (p.  654). 

Since  the  several  causes  producing  inequalities  of  level  are  in 
constant  operation,  it  follows  that  movements  due  to  inequalities 
of  level  are  constant. 

It  will  be  recalled  that  there  are  other  inequalities  of  level, 
due  to  the  attraction  of  land  masses  (p.  708).  These  inequalities 
are  in  some  sense  permanent,  and  therefore  do  not  produce  cir- 
culation of  the  sea-water. 

Movements  due  to  the  wind.  Winds  not  only  produce  tem- 
porary inequalities  of  level,  as  indicated  above,  but  they  affect  the 
water  in  other  and  important  ways.  Their  most  familiar  effect 
is  in  the  generation  of  waves,  but  they  also  drag  the  water  along 
beneath  themselves.  If  a  floating  solid  is  dragged  rapidly  through 
the  water,  the  water  about  it  moves,  and  that  immediately  beneath 
the  object  moves  faster  than  that  farther  below.  If  the  object 
were  so  light  as  to  sink  but  little  into  the  water,  there  would  still 
be  movement  of  the  water  beneath  because  of  the  friction  at  the 
plane  of  contact.  The  air,  though  not  a  solid,  acts  in  the  same 
way.  When  it  moves  rapidly  over  water,  it  drags  the  surface 
water  along  with  it. 

Since  winds  are  always  blowing,  the  movements  to  which  they 
give  rise  are  always  taking  place.  When  winds  have  a  more  or 
less  constant  direction,  as  in  the  zone  of  trades,  there  must  be  a 
somewhat  constant  movement  of  the  surface  water  in  the  same 
direction.  A  constant  movement  in  one  direction  necessarily 
involves  a  return  movement,  that  is  a  circulation,  of  the  sea- 
water. 

Movements  due  to  differential  attraction  of  sun  and  moon. 
Another  cause  of  movement  in  sea-water  is  found  in  the  attrac- 
tion of  bodies  outside  the  earth,  especially  the  sun  and  moon. 
Bodies  attract  each  other  in  proportion  to  their  masses,  and  in- 
versely as  the  squares  of  their  distances;  that  is,  a  body  t\vice 
as  massive  as  another  has  twice  the  attractive  force  at  the  same 
distance,  and  if  one  of  two  bodies  of  a  given  mass  be  twice  as  far 


THE  MOVEMENTS  OF  SEA-WATER  729 

from  a  third  body  as  the  other  is,  their  attractive  forces  on  the 
third  are  to  each  other  as  J:  1  (£2: 1). 

The  side  of  the  earth  towards  the  moon  is  nearer  to  the  moon 
than  the  center  of  the  earth  is,  and  so  is  attracted  more  strongly 
than  the  center.  The  opposite  side  is  attracted  less  strongly. 
Because  of  these  inequalities  of  attraction,  the  mobile  waters  on 
the  earth's  surface  are  disturbed.  The  attraction  of  the  sun  pro- 
duces similar  but  less  pronounced  effects.  These  inequalities  of 
attraction  of  moon  and  sun  on  different  parts  of  the  earth  are  the 
cause  of  the  movements  known  as  tides. 

Movements  due  to  occasional  causes.  The  aperiodic  and 
more  or  less  accidental  causes  of  movement  which  belong  in  this 
class  often  produce  violent  wave  movements  which  last  but  a 
short  time.  Illustrations  of  their  nature  and  effects  have  already 
been  given  in  connection  with  earthquakes.  Landslides  along  the 
coast,  submarine  volcanic  eruptions,  etc.,  also  occasion  violent  but 
temporary  movements  of  the  water  of  the  sea. 

Types  of  Movement 

The  general  types  of  movement  resulting  from  these  various 
causes  are  (1)  waves,  with  their  accompanying  undertow,  and  shore 
or  littoral  currents,  (2)  ocean  currents,  (3)  drift  (slow  ill-defined 
currents),  (4)  tides,  and  (5)  creep.  The  first  two  and  the  fourth 
are  more  obvious  than  the  third  and  fifth,  and  the  importance  of 
the  less  obvious  is  often  overlooked. 

Waves 

The  nature  and  the  work  of  waves  has  already  been  outlined 
(p.  318).  Since  the  water  in  waves  does  not  commonly  move 
forward,  waves  do  not  involve  a  general  circulation  of  the  ocean 
water.  In  addition  to  what  has  been  said  concerning  the  work  of 
waves,  it  may  be  added  that  in  the  aggregate  the  sea  destroys 
more  land  by  erosion  than  it  makes  by  deposition,  so  that  if  noth- 
ing else  interposed,  the  sea,  by  its  continued  gnawing  at  the  shores, 
would  ultimately  destroy  all  land.  It  has  already  been  pointed 
out  that  wave  erosion  tends  to  do  away  with  large  irregularities  of 
coasts,  though  not  with  small  ones  (p.  320).  In  the  long  run,  too, 
deposition  along  coasts  tends  to  develop  regularity  of  outline,  but 
temporarily  the  coasts  are  often  made  very  irregular  (p.  328). 
On  the  whole,  the  final  effect  of  coastal  activities  is  to  make  the 


730  PHYSIOGRAPHY 

coasts  more  regular  if  they  have  any  considerable  irregularity  at 
the  outset. 

Currents 

Experience  and  observation  have  shown  that  there  are  more 
or  less  distinct  currents  in  various  parts  of  the  ocean.  This  be- 
came known  first  through  the  effect  of  the  moving  water  on  the 
courses  of  sailing  vessels,  and  it  was  later  confirmed  in  various  other 
ways,  as  by  following  the  course  of  floating  bottles. 

The  better-known  currents  are  at  the  surface,  extending  down 
to  depths  of  several  hundred  feet;  but  there  are  also  currents 
beneath  the  surface,  as  shown  by  variations  of  temperature  and 
by  some  other  phenomena.  Ocean  currents  are  much  less  well 
defined  than  currents  of  running  water  on  the  land,  because  the 
former  flow  through  a  liquid,  while  the  latter  flow  over  a  solid 
bed  between  solid  banks.  These  currents  of  the  ocean  are  the 
most  distinct  feature,  though  not  the  only  one,  of  the  oceanic 
circulation,  which  has  already  been  referred  to. 

Fig.  694  shows  the  general  course  of  movement  of  the  surface 
waters  of  the  seas.  The  figure  represents  a  large  part  of  the  sur- 
face water  as  involved  in  movement.  There  is  a  westward  move- 
ment of  surface  water  in  low  latitudes  in  both  the  Atlantic  and 
the  Pacific  oceans.  These  are  the  equatorial  currents  or  drifts,  as 
they  are  sometimes  called.  In  each  ocean  the  drift  is  double,  and 
a  narrow  counter-current  moves  eastward  between  them.  The 
equatorial  current  of  the  Atlantic  is  divided  as  South  America  is  ap- 
proached, one  part  being  deflected  to  the  southwest  and  the  other 
to  the  northwest.  A  part  of  the  latter  flows  through  the  Caribbean 
Sea  and  into  the  Gulf  of  Mexico.  From  this  Gulf  a  distinct  cur- 
rent issues  through  the  narrow  passageway  between  Cuba  and 
Florida.  This  is  the  Gulf  Stream.  It  is  fed  partly  by  the  water 
which  enters  the  Gulf  from  the  equatorial  drift,  and  partly  by  the 
large  amount  of  water  which  enters  the  Gulf  from  the  land,  thus 
tending  to  raise  the  level  of  the  water.  The  stream  issuing  from 
the  Gulf  has  a  velocity  of  more  than  four  miles  per  hour  where  it 
is  swiftest. 

Escaping  from  its  narrow  passageway  between  Florida  and 
Cuba,  the  Gulf  Stream  becomes  wider  and  deeper.  The  current 
tends  to  drag  along  the  mobile  water  beneath  and  beside  it,  and  as 
more  water  becomes  involved  in  the  movement,  the  rate  of  progress 


THE  MOVEMENTS  OF  SEA-WATER 


731 


732  PHYSIOGRAPHY 

becomes  slower,  and  in  the  open  ocean  the  rate  of  movement  is 
perhaps  no  more  than  10  to  15  miles  per  day.  As  the  current  be- 
comes slow,  its  boundaries  become  less  well  defined.  In  the  open 
ocean  it  is  detected  by  its  temperature,  its  color,  its  life,  etc.,  more 
readily  than  by  its  motion. 

After  leaving  the  Gulf,  the  Gulf  Stream  manifests  a  pronounced 
tendency  to  turn  to  the  eastward.  Following  this  tendency,  it 
crosses  the  Atlantic,  approaching  the  coast  of  Europe  in  a  latitude 
higher  than  that  where  it  leaves  America.  Here  it  divides  and 
spreads  widely.  Long  before  this  point  is  reached,  it  has  ceased 
to  be  a  definite  stream,  and  is  to  be  looked  upon  rather  as  a  general, 
wide-spread  drift  of  water. 

That  part  of  the  equatorial  drift  which  is  turned  southward 
on  the  coast  of  South  America  first  follows  the  coast  of  that  conti- 
nent, but  soon  shows  a  tendency  to  turn  to  the  left  (Fig.  694). 

The  equatorial  drifts  of  the  Pacific  follow  similar  courses.  The 
part  corresponding  to  the  Gulf  Stream  of  the  Atlantic  is  known 
as  the  Japan  Current.  The  Indian  Ocean  has  a  south  equatorial 
drift  only,  and  its  course  corresponds  to  that  of  the  southern  part 
of  the  corresponding  drifts  of  the  other  oceans. 

All  currents  or  drifts  moving  poleward  from  the  equatorial 
region  consist  of  warm  water  moving  into  cooler  water,  and  they 
are  known  as  warm  currents. 

The  poleward  movement  of  warm  currents  necessitates  a  re- 
turn equatorward  movement,  and  this  movement  is  strengthened 
by  the  inequalities  of  temperature  in  high  and  low  latitudes.  The 
cold  waters  moving  equatorward  are  deflected  to  the  right  in  the 
northern  hemisphere  and  to  the  left  in  the  southern,  and  the  tend- 
ency of  this  deflection  is  to  concentrate  them  on  the  eastern  coasts 
of  the  continents  in  both  hemispheres. 

The  equatorward  currents  start  from  latitudes  where  ice 
abounds.  They  are  cold,  but  not  so  salt  (in  summer)  as  normal 
sea-water.  By  virtue  of  their  temperature,  therefore,  they  would 
be  denser  than  average  sea-water ;  but  by  virtue  of  their  deficiency 
of  salt,  they  tend  to  be  less  dense  than  normal  sea-water.  As  they 
flow  equatorward,  they  become  warmer  and  more  salt,  and  finally 
attain  such  a  degree  of  salinity  that  they  sink  and  continue  their 
courses  toward  the  equator  as  cold  undercurrents.  On  the  other 
hand,  the  poleward  (warm)  currents  start  in  low  latitudes  as  sur- 
face currents,  kept  at  the  surface  by  their  high  temperature  in 


THE  MOVEMENTS  OF  SEA-WATER  733 

spite  of  their  slight  excess  of  salt.  But  in  their  poleward  jour- 
ney, they  may  sink  beneath  the  cooler  though  fresher  water, 
and  continue  as  warm  undercurrents.  Undercurrents  of  both 
the  types  cited  have  been  detected. 

Cause  of  ocean  currents.  The  equatorial  drifts  in  the  Atlantic 
and  Pacific  oceans  correspond  somewhat  closely,  both  in  position 
and  direction,  with  the  trade-winds.  It  is  now  generally  believed 
that  winds  which  are  constant  in  direction  will  cause  a  general 
movement  of  the  surface  waters  beneath  them.  It  therefore  seems 
rational  to  infer  that  the  equatorial  currents  or  drifts  are  generated 
by  the  trade-winds.  The  effect  of  the  westward-moving  equa- 
torial currents  is  to  bank  up  waters  on  the  east  coasts  of  the 
continents,  especially  South  America.  Some  of  this  water  moves 
eastward  between  the  main  west-bound  currents,  and  constitutes 
the  narrow  counter  current  of  the  equatorial  calms.  This  current 
of  warm  water  is  felt  on  the  coast  of  Africa. 

In  extra-tropical  latitudes  the  winds  are  less  constant,  and 
therefore  less  effective  in  generating  currents.  But  in  regions  of 
strong  monsoon  winds,  as  about  India,  the  drift  of  the  surface 
waters  changes  with  the  shifting  winds,  thus  showing  the  com- 
petency of  winds  to  generate  surface  movements. 

Were  the  ocean  universal,  the  westward  drift  of  the  equatorial 
waters  under  the  influence  of  the  trade-winds  would  doubtless 
correspond  with  the  trade-winds  themselves;  that  is,  they  would 
encircle  the  earth.  But  where  the  waters  of  this  equatorial  drift 
reach  a  continent,  as,  for  example,  South  America,  they  are  de- 
flected from  their  westerly  course. 

After  the  moving  waters  pass  out  of  the  control  of  the  trade- 
winds,  they  are  directed  (1)  by  the  continental  borders,  (2)  by  the 
configuration  of  the  ocean  bottom,  (3)  by  the  prevailing  winds  of  the 
latitudes  which  they  reach,  and  (4)  by  the  rotation  of  the  earth. 
Their  courses  are  therefore  determined  partly  by  the  causes  which 
generate  them,  and  partly  by  other  causes  which  direct  them. 

Another  factor  which  is  of  importance  in  the  development  of 
ocean  currents  is  inequalities  of  temperature.  This  alone  would 
not  give  rise  to  distinct  currents,  but  movements  thus  generated 
(p.  722)  may  be  concentrated  and  directed  so  as  to  emphasize  the 
currents  generated  by  the  winds. 

Climatic  effects  of  ocean  currents.  The  air  over  a  warm 
ocean  current  is  warmed  by  contact  with  the  warm  water.  In 


734  PHYSIOGRAPHY 

middle  latitudes  the  prevailing  westerly  winds  carry  the  warmed 
air  over  to  the  coasts  of  the  continents  to  leeward,  giving  them,  in 
winter,  temperatures  higher  than  they  would  otherwise  have,  and 
giving  them,  at  the  same  time,  an  abundant  supply  of  moisture. 
The  winter  temperature  of  the  west  coast  of  northern  Europe  is 
much  less  severe  than  it  would  be  but  for  the  Gulf  Stream. 

The  amount  of  heat  which  the  Gulf  Stream  carries  northward 
from  low  latitudes  has  been  estimated  by  Croll  to  be  "one-fourth 
of  all  the  heat  received  from  the  sun  by  the  North  Atlantic,  from 
the  tropic  of  Cancer  up  to  the  Arctic  Circle."  Its  benefit,  so  far 
as  the  land  is  concerned,  is  primarily  to  Europe. 

The  similar  warm  current  in  the  North  Pacific  lessens  the 
severity  of  the  winter  climate  of  the  northern  part  of  western 
North  America.  Similar  results  would  be  seen  in  the  southern 
hemisphere,  were  there  land  so  situated  as  to  feel  the  effects  of  the 
corresponding  currents  in  the  southern  oceans. 

One  other  atmospheric  effect  of  currents  should  perhaps  be 
mentioned.  When  the  wind  blows  over  a  warm  current,  such  as 
the  Gulf  Stream,  it  is  warmed  and  takes  up  a  goodly  supply  of 
moisture.  On  blowing  from  the  current  over  colder  water,  its  tem- 
perature is  lowered,  and  some  of  its  moisture  may  be  condensed. 
The  result  is  often  a  fog.  Fogs  are  common  along  the  leeward  side 
of  the  Gulf  Stream,  in  latitudes  where  the  adjacent  land  or  water 
is  much  cooler  than  the  current  itself.  Fogs  are  more  abundant 
in  the  latitude  of  Newfoundland  than  farther  south,  because  the 
difference  in  the  temperature  of  the  Gulf  Stream  and  its  surround- 
ings is  here  greater  than  farther  south.  Fogs  also  occur  about 
the  Gulf  Stream  when  there  is  no  wind.  This  appears  to  be  due 
to  the  chilling  of  the  warmer  air  by  proximity  to  cooler  air  above 
or  on  either  side. 

Fogs,  often  grading  into  mist  or  into  clouds  which  yield  rain, 
are  rather  common  in  the  northwestern  parts  of  North  America 
and  Europe,  especially  in  the  cold  season. 

Gradation al  effects  of  ocean  currents.  Currents  have  rela- 
tively little  effect  on  the  ocean  bottom,  and  almost  none  on  coasts, 
because  they  rarely  touch  either.  Where  the  water  is  shallow,  how- 
ever, as  between  Florida  and  Cuba,  the  Gulf  Stream  reaches  and 
scours  its  bottom  effectively,  somewhat  as  a  great  river  might. 
Since  ocean  currents  do  little  eroding,  except  locally,  they  carry  but 
little  debris.  They  do,  however,  transport  considerable  quantities 


THE  MOVEMENTS  OF  SEA-WATER  735 

of  matter  of  organic  origin.  The  waters,  especially  of  warm  cur- 
rents, teem  with  minute  organisms,  and  these  organisms,  or  their 
shells  after  the  organisms  are  dead,  are  often  carried  far,  and 
finally  scattered  over  the  bottom  of  the  ocean. 

Historical  suggestions.  The  currents  of  the  Atlantic  played 
an  important  part  in  the  early  history  of  America.  Once  Iceland 
was  colonized  by  the  Northmen,  the  currents  southwest  from  the 
Arctic  insured  the  early  discovery  of  North  America.  The  south 
equatorial  current  carried  the  Portuguese,  bound  for  India,  in 
1500,  to  the  shores  of  South  America. 

Tides 

The  level  of  the  ocean  water  rises  and  falls  twice  every  day, 
or,  more  exactly,  every  24  hours  and  52  minutes.  This  periodic 
rise  and  fall  of  the  water  constitutes  the  tides.  The  tide  rises 
(flood-tide)  for  about  six  hours,  when  it  is  high,  and  then  falls  (ebb- 
tide) for  about  six  hours,  when  it  is  low.1  The  tide  often  "comes 
in"  as  a  series  of  waves,  the  water  after  each  wave  failing  to  sink 
to  its  former  level.  In  other  cases  it  rises  quickly,  without  dis- 
tinct waves. 

Tides  are  not  perceptible  in  the  open  ocean,  for  there  is  nothing 
there  to  mark  the  slight  rise  of  water;  but  they  are  readily  seen 
wherever  there  is  an  island  on  the  shores  of  which  the  rise  and 
fall  may  be  measured.  The  rise  in  the  open  sea  has  been  esti- 
mated to  be  two  to  three  feet.  Along  coasts,  the  variation  in 
the  water  level  between  high  and  low  tides  is  generally  several  feet. 
In  bays  which  open  broadly  to  the  sea  but  narrow  toward  their 
heads,  the  range  is  sometimes  20  or  30  feet,  or  in  rare  cases  50  feet 
or  more,  as  in  the  Bay  of  Fundy.  Where  the  tide  runs  in  among 
islands  or  passes  through  narrow  straits,  it  often  gives  rise  to  dis- 
tinct currents  which  scour  the  channels  through  which  they  flow. 

The  tide  sometimes  runs  up  a  broad  open  river.  As  it  ad- 
vances up  the  channel,  its  progress  is  retarded  by  the  shallowness 
of  the  water,  and  its  front  may  become  a  steep  and  often  wall- 
like  wave.  Such  a  wave  is  called  a  bore.  The  bore  is  felt  in  the 
Severn  and  the  Wye  of  England,  in  the  Seine  of  France,  in  the 
Petit-Codiac  of  Canada,  in  the  Hugli  of  India,  and  the  Tsien-Tang- 
Kiang  of  China.  In  the  last-named  river  the  waves  are  some- 

1  The  ebb-tide  is  usually  somewhat  longer  than  the  flood-tide. 


736  PHYSIOGRAPHY 

times  25  feet  high,  and  are  disastrous  to  navigation.  On  one 
occasion,  Captain  Moore  estimated  that  1J  million  tons  of  water 
went  by  a  point  in  the  river  in  a  minute  in  the  bore  wave.  Trading 
ships  at  Calcutta  formerly  hastened  to  the  middle  of  the  stream 
for  safety  on  the  approach  of  the  bore. 

Bores  do  not  appear,  even  in  the  rivers  subject  to  them,  with 
every  high  tide.  Favoring  winds  seem  to  be  an  important  factor 
in  their  development,  and  they  are  stronger  in  spring  tides  (p.  744) 
than  at  other  times. 

High  tides  make  themselves  felt,  though  not  as  bores,  up  the 
Hudson  River  to  Troy,  where  the  range  of  the  tide  is  more  than 
two  feet,  and  up  the  Delaware  nearly  to  Trenton,  though  the  salt 
water  does  not  run  up  so  far.  The  sea-tide  raises  the  sea-level  at 
the  debouchures  of  these  streams,  and  so  dams  back  their  waters. 
The  tide  runs  70  miles  up  the  St.  Johns  River  in  New  Brunswick, 
and  is  felt  where  the  elevation  of  the  river  is  14  feet  above  mean 
sea-level.  The  tide  runs  up  the  estuary  of  the  St.  Lawrence  283 
miles  to  Three  Rivers,  near  Montreal. 

Tides  are  imperceptible  in  small  lakes  and  feeble  in  large  lakes 
and  enclosed  seas.  In  Lake  Michigan,  for  example,  there  is  a  tide 
of  about  two  inches.  Tides  are  feeble  in  all  bodies  of  water  con- 
nected with  the  open  sea  by  a  narrow  passageway.  Thus,  at 
Galveston  in  the  Gulf  of  Mexico,  the  range  of  the  tide  is  less  than 
one  foot. 

In  many  harbors,  especially  where  the  water  is  shallow,  the  rise 
and  fall  are  enough  to  have  an  important  effect  on  navigation. 
Vessels  arriving  at  such  harbors  at  low  tide  are  often  obliged  to 
wait  until  high  tide  before  entering.  Tidal  currents  or  races  are 
sometimes  so  strong  as  to  interfere  with  navigation.  The  race 
through  Hell  Gate  near  New  York  City  is  a  case  in  point. 

The  periodicity  and  the  cause  of  tides.  The  time  between 
successive  high  or  successive  low  tides  is  about  half  the  time  of  the 
apparent  revolution  of  the  moon  around  the  earth.  It  appears  to 
have  been  this  fact  which  suggested  a  connection  between  the  tides 
and  the  apparent  motions  of  the  moon,  a  connection  which  was 
known,  or  at  any  rate  suspected,  some  2000  years  ago,  though 
not  fully  understood  until  the  time  of  Newton,  about  200  years  ago. 

The  law  of  attraction  between  heavenly  bodies  has  already 
been  stated  (p.  729).  Without  attempting  to  give  a  detailed 
explanation  of  the  tides,  the  essential  principles  involved  may  be 


THE  MOVEMENTS  OF  SEA-WATER  737 

readily  understood.1     We  may  consider  first  the  tide  produced 
by  the  moon. 

If  a  weight  be  attached  to  a  string  and  whirled,  the  string  is 
put  under  tension.  The  weight  constantly  tends  to  move  forward 
in  a  straight  line,  but  it  is  prevented  from  doing  so  by  the  string. 
The  tendency  of  the  weight  to  depart  from  the  circle  in  which  the 
string  holds  it,  is  often  called  centrifugal  force,  though  it  is  only 
inertia.  The  pull  of  the  string  which  holds  the  weight  is  a  cen- 
tripetal force.  The  taut  string  therefore  is  affected  by  two  opposite 
and  equal  forces. 

The  motion  of  the  moon  about  the  earth  is  not  unlike  the 
motion  of  the  weight  at  the  end  of  the  string  in  the  above  illustra- 
tion. In  place  of  the  string  there  is  the  attraction  of  gravitation, 
and  the  moon  goes  about  the  earth  at  such  a  rate  that  her  centrif- 
ugal tendency  is  just  balanced  by  the  attraction  of  the  earth.  The 
center  about  which  the  moon  revolves  is  not,  however,  the  center 
of  the  earth  but  their  common  center  of  gravity.  Since  the  earth 
is  about  80  times  as  massive  as  the  moon,  the  center  of  gravity  of  the 
two  bodies  is  much  nearer  the  center  of  the  earth  than  the  center 
of  the  moon.  It  is,  in  fact,  1000  miles  below  the  surface  of  the 
earth,  and  3000  miles  from  its  center  (Fig.  695).  Both  the  moon 
and  the  earth  revolve  about  this  common  center,  as  they  travel 
together  about  the  sun.  The  earth's  center  describes  a  circle 
with  a  radius  of  3000  miles  about  the  common  center  of  gravity, 
while  the  moon  describes  a  circle  with  a  radius  of  about  237,000 
miles  about  the  same  point.  The  concepticn  may  be  made  more 
definite  by  conceiving  two  very  unequal  weights  at  the  opposite 
ends  of  a  stiff  but  extremely  light  rod.  These  weights  may  be 
such  that  the  two  will  be  balanced,  if  the  point  corresponding  to 
g,  Fig.  695,  is  supported.  If  now  the  couple  (E  and  M)  be  ro- 
tated, the  center  of  E  will  rotate  about  g  in  a  small  circle,  while 
the  center  of  M  will  rotate  about  it  in  a  much  larger  circle,  each 
in  about  28  days. 

The  earth  and  the  moon  attract  each  other  and  would  fall 
together,  but  for  the  centrifugal  tendency  developed  by  the  revo- 
lution. The  distance  of  the  two  bodies  from  each  other  is  de- 
termined by  the  balance  between  (1)  their  mutual  attractions 

1  For  further  accounts  of  the  tides,  see  the  astronomies  referred  to  on 
p.  505. 


738  PHYSIOGRAPHY 

on  the  one  hand,  and  (2)  their  centrifugal  tendencies  on  the  other. 
This  balance  is  perfect  at  the  center  of  the  earth  and  at  the  center 
of  the  moon.  But  on  the  side  of  the  earth  nearest  the  moon  the 
attraction  is  somewhat  stronger  than  at  the  center  of  the  earth, 
and  overbalances  the  centrifugal  tendency.  The  attraction  there- 
fore tends  to  make  the  earth  bulge  up  under  the  moon.  On  the  oppo- 
site side  of  the  earth  the  attraction  is  weaker  than  at  the  center, 


FIG.  695. — Diagram  showing  the  position  of  the  center  of  gravity,  g,  of  the 
earth-moon  system. 

and  is  overbalanced  by  the  centrifugal  tendency.  Here,  also,  there 
is  therefore  a  tendency  for  the  earth  to  bulge  out,  as  a  result  of  the 
differential  attraction  of  the  moon.  If  the  mass  of  the  earth  were 
fluid,  these  bulgings  or  tides  would  be  sensible.  But  the  solid  part 
of  the  earth  is  essentially  rigid,  and  there  is  not  time  for  its  parts 
to  yield  sensibly  to  the  strain  set  up  by  the  differential  attraction 
of  the  moon,  before  rotation  carries  them  forward  where  there  is 
no  tendency  to  bulging.  The  surface  waters,  however,  are  mobile 
and  respond  to  the  distorting  effect  of  the  moon's  differential  at- 
traction, with  the  result  that  the  water  is  bulged  up  on  opposite 
sides  of  the  earth,  so  as  to  produce  a  slight  elongation  of  the  earth's 
diameter  in  the  direction  of  the  moon.  These  bulges  of  water  are 
the  high  tides,  and  between  them  the  tides  are  low. 

The  fact  of  the  differential  attraction  of  the  moon  may  be 
stated  in  other  terms.  The  distance  of  the  center  of  the  moon 
from  the  center  of  the  earth  is  about  240,000  miles.  The  side  of 
the  earth  nearest  to  the  moon  is  therefore  about  236,000  miles 
from  the  center  of  the  moon,  while  the  side  farthest  away  is  about 
244,000  miles  distant. 

If  the  mass  of  the  moon  be  taken  as  1,  the  average  pull  of  the 

moon  on  the  earth  is  represented  by  the  fraction  The 

^ 


fraction  which  represents  the  moon's  pull  on  the  side  of  the  earth 


THE  MOVEMENTS  OF  SEA-WATER  739 

nearest  the  moon  is  2g        2,  and  the  fraction  which  represents 

the  pull  on  the  opposite  side  is  The  solid  part  of  the 

earth  acts  essentially  as  a  unit,  since  its  parts  are  not  free  to  move 
on  one  another.  The  effect  of  the  attractive  force  of  the  moon 
on  the  solid  part  of  the  earth  is  therefore  essentially  the  same  as 
it  would  be  if  it  were  all  exerted  on  its  center,  that  is,  the  same  as 

the  average  pull  of  the  moon  on  the  earth,  2. 

^40000 

Since  the  waters  on  the  surface  are  readily  mobile,  they  re- 
spond to  the  differential  attraction  of  the  moon.  The  waters  on 
the  side  nearest  the  moon,  being  pulled  with  a  force  stronger  than 
the  average  pull  on  the  solid  part  of  the  earth,  are  bulged  up  a 
little.  The  force  of  the  moon's  pull  here  is  represented  by  the 

fracti°n  236»>  and  236^-240^  ^presents  the  moon's 
tide-producing  force  on  the  side  of  the  earth  nearest  to  it.  The 
waters  on  the  opposite  side  of  the  earth  are  farther  from  the  moon 
than  the  center  of  the  earth  is,  and  so  are  pulled  less  strongly 
than  the  latter,  and  are  allowed  to  bulge  out.  Mathematically 
the  force  on  the  side  of  the  earth  most  distant  from  the  moon  is 

.,  and  the  tide-producing  force  there  is 


The  result  is  a  rise  of  water  on  opposite  sides  of  the  earth  at  the 
same  time.  These  are  the  high  tides.  Midway  between  the  places 
where  the  tides  are  high  the  water  is  correspondingly  lowered  and 
the  tides  are  low. 

It  will  be  seen  from  the  above  figures  that  the  tide-producing 
force  on  the  side  of  the  earth  away  from  the  moon  is  slightly  less 
than  that  on  the  side  nearest  the  moon: 

i         JL  <_!_       J_ 

2400002     2440002^2360002     2400002' 

The  explanation  of  the  tides  is  sometimes  so  troublesome 
that  another  statement  of  their  cause  is  added: 

"Let  E  (Fig.  696)  represent  the  center  of  the  earth,  and  M  the 
moon.  (The  distance  of  the  moon  is  greatly  minimized.)  Con- 
sider the  tendency  of  the  moon  to  displace  the  particle  P  on  the 
surface  of  the  earth.  Let  EB  represent  the  acceleration  of  A/'  on 


740  PHYSIOGRAPHY 

E  (the  solid  earth)  in  direction  and  amount.  In  the  same  units 
let  PA  represent  the  acceleration  of  M  on  P  in  direction  and  amount. 
Since  P  and  M  are  nearer  together  than  E  and  M,  it  follows  that 
PA  is  greater  than  EB. 

Let  the  acceleration  PA  be  resolved  into  two  components  so 
that  one  of  them  shall  be  equal  and  parallel  to  EB.  It  is  PK  in 
the  figure.  The  other  component  is  found  by  using  PA  as  a  di- 
agonal and  PK  as  a  side,  and  completing  the  parallelogram.  It  is 
PQ  in  the  figure.  By  the  law  of  the  parallelogram  of  forces  PA 
is  exactly  equivalent  to  PK  and  PQ,  and  conversely.  By  the 
preliminary  theorem,  EB  and  PK  being  parallel  and  equal  do  not 


FlG.  696. — Diagram  to  illustrate  the  cause  of  tides.  Explanation  in  text. 
(From  Moulton's  Introduction  to  Astronomy.  By  permission  of  The 
Macmillan  Company.) 

tend  to  change  the  relative  positions  of  E  and  P,  and  therefore 
cause  no  tide.  The  remaining  acceleration  PQ  cannot  be  paired 
with  any  other,  and  is  the  tide-raising  acceleration. 

The  part  of  the  figure  with  accents  is  drawn  from  precisely  the 
same  principles.  P'K'  is  parallel  and  equal  to  EB,  and  P'Q'  is 
the  tide-raising  acceleration. 

Suppose  figures  are  constructed  for  points  all  the  way  around 
the  earth.  The  lines  representing  the  tide-raising  accelerations 
will  be  as  given  in  Fig.  697.  The  method  of  drawing  them  is  the 


FIG.  697. — Diagram  to  illustrate  tides.      (From  Moulton's  Introduction  to 
Astronomy.    By  permission  of  The  Macmillan  Company.) 

geometrical  counterpart  of  the  rigorous  mathematical  treatment 
of  the  subject,  and  may  be  relied  upon  as  giving  the  full  explana- 
tion of  the  reason  for  the  tides."  * 

1  Moulton's  Introduction  to  Astronomy. 


THE  MOVEMENTS  OP  SEA-WATER  741 

Tides  if  the  ocean  were  universal.  If  the  earth  were  com- 
pletely covered  with  a  deep  ocean,  its  surface  would  have  two 
extensive  tidal  bulges  or  waves  at  the  same  time.  The  highest 
part  of  each  would  be  a  point,  the  one  directly  under  the 'moon, 
and  the  other  directly  opposite  it.  Each  wave  would  be  hemi- 
spherical, and  their  borders  would  meet  in  a  great  circle,  where  the 
tide  would  be  low.  This  circle  may  be  conceived  of  as  the  trough 
of  the  tidal  wave. 

The  period  of  the  earth's  rotation  is  shorter  than  that  of  the 
revolution  of  the  moon  about  the  earth.  The  result  is  that  rota- 
tion tends  to  carry  the  high  tides  on  beyond  the  position  which 
the  moon  would  give  them.  The  moon  tends  to  hold  them  back, 
and  so  they  seem  to  travel  about  the  surface  of  the  earth  in  a 
direction  opposed  to  its  rotation.  The  tides  are  therefore  said 
to  lag. 

Theoretically,  successive  high  tides  are  180°  (12  hours)  apart, 
and  rotation  of  the  earth  alone  considered,  high  tides  at  any 
place  should  recur  every  12  hours.  The  longer  period  (12  hrs. 
26  min.)  is  the  result  of  the  forward  movement  of  the  moon  in  its 
orbit  about  the  earth  (Fig.  698). 

There  are  two  points,  the  tidal  poles,  where  the  tide  does  not 
rise  and  fall.  When  the  moon  is  vertical  at  the  equator,  it  will  be 
seen  that  the  highest  point  of  the  high  tide  should  be  on  the  equator 
continuously,  and  that  the  great  circle  marking  the  position  of 
the  low  tide  will  pass  through  the  geographic  poles.  The  poles 
will  therefore  have  low  tide  continuously  so  long  as  the  moon  is 
vertical  at  the  equator.  Whatever  the  latitude  where  the  moon  is 
vertical,  there  will  be  a  point,  the  tidal  pole,  90°  from  the  latitude 
where  the  moon  is  vertical,  where  there  would  be  no  rise  and  fall 
of  the  tide.  Since  the  latitude  where  the  moon  is  vertical  varies 
from  time  to  time,  the  position  of  the  tidal  poles  varies. 

The  simplicity  of  the  tidal  movements  outlined  above  is  inter- 
fered with  by  many  things,  especially  by  (1)  the  continents,  which 
stop  the  tidal  wave,  and  (2)  the  shallowness  of  water  in  many 
places.  The  tidal  wave  travels  more  slowly  in  shallow  water  than 
in  deep,  for  the  same  reason  that  other  waves  do.  Since  tides 
are  retarded  most  in  this  way  near  continents  and  islands,  their 
advance  is  here  most  irregular.  Irregular  tidal  waves  often  inter- 
fere with  one  another. 

Solar  tides.    The  sun  also  attracts  the  earth  and  tends  to 


742 


PHYSIOGRAPHY 


cause  tides     If  there  were  no  moon  we  should   still  have  tides 
produced  by  the  sun. 


FIG.  698. — Diagram  illustrating  the  motion  of  the  moon  about  the  earth 
The  larger  circles  represent  the  earth,  and  the  smaller  the  moon  on  the 
line  which  represents  its  orbit. 

In  spite  of  its  great  distance  from  the  earth  (about  93,000,000 
miles),  the  sun,  because  of  its  great  size,  attracts  the  earth  much 


THE  MOVEMENTS  OF  SEA-WATER 


743 


more  strongly  than  the  moon  does.      If  the  moon's   attraction 

were  the  stronger,  the  earth  would  revolve  about  the  moon  instead 

of  the  sun.     But  in  spite  of  its  greater  attraction,  the  tide-producing 

force  of  the  sun  is  less  than  that  of  the  moon.     It  is  not  difficult 

to  calculate  their  relative  attractions.     If  the  moon's  mass  be  taken 

as  1,  the  mass  of  the  sun  is  26,648,000.     Mass 

alone  considered,  the  sun  should  attract  the 

earth  26,648,000  times  as  strongly  as  the 

moon.      The  sun  is  about  389  times  as  far 

from  the  earth  as  the  moon  is.     Distance 

alone  considered,  its  pull  should  therefore  be 

1/3892   (=1/151321)  of  that  of  the  moon. 

1/151321 X  26 ,648,000=  175     approximately. 

That  is,  the  sun  pulls  the  earth  with  175 

times  the  force  that  the  moon  does. 

It  has  been  seen  that  the  tide  produced 

by  the  moon  is  due  to  the  difference  between 
the  pull  of  the  moon  on  the  center  of  the  earth 
and  on  the  parts  nearest  to  and  farthest  from 
it.  Any  tide  which  the  sun  produces  must 
also  be  due  to  the  difference  between  its  pull 
on  the  center  of  the  earth  and  on  the  sides 
nearest  and  farthest  from  it. 

The  sides  of  the  earth  nearest  to  the  sun 
and  farthest  from  it  are  4000  miles  nearer 
to  and  farther  from  the  sun  than  the  center 
of  the  earth  is;  but  4000  miles  is  a  very 
much  smaller  part  of  93,000,000  miles  than 
it  is  of  240,000  miles.  Hence  the  difference 
between  the  attractive  force  of  the  sun  on  the  center  and  on  the 
side  of  the  earth  nearest  it  is  much  less  than  the  difference  in  the 
attractive  force  of  the  moon  on  the  same  points.  In  other  words, 
the  differential  pull  of  the  sun  is  less  than  the  differential  pull  of  the 
moon.  The  moon's  tides  are  therefore  higher  than  the  sun's. 
Their  ratio  is  0.0342:0.0151.  If  the  sun  were  as  near  the  earth  as 
the  moon  is,  its  tidal  effect  would  be  millions  of  times  greater  than 
now,  and  perhaps  sufficient  to  disrupt  the  earth. 

Some  of  the  tides  are  the  result  of  the  combined  influence  of 
the  moon  and  the  sun,  but  since  the  lunar  tides  are  the  stronger, 
the  solar  tides  serve  merely  to  modify  them.  The  solar  influence 


FIG.  699. —  Diagram 
to  illustrate  the  lag- 
ging of  the  tides. 
(After  Comstock.) 


744  PHYSIOGRAPHY 

strengthens  the  tides  when  sun  and  moon  work  together,  and  weak- 
ens the  tides  when  they  work  against  each  other. 

Spring  tides  and  neap  tides.  When  the  sun  and  the  moon 
stand  in  the  relation  to  each  other  and  to  the  earth  shown  in  Fig. 
700  (New  moon),  each  tends  to  make  high  tides  at  the  same  points. 
When  the  relations  are  those  shown  in  Fig.  701  (Full  moon),  the 
result  is  the  same.  At  these  times,  and  each  occurs  once  a  month, 


FIG.  700. — Diagram  to  illustrate  the  relative  positions  of  earth,  moon,  and 
sun  at  the  time  of  full  moon.     Spring  tide. 


FIG.  701. — Diagram  to  illustrate  the  relative  positions  of  earth,  moon,  and 
sun  at  the  time  of  full  moon.     Spring  tide. 

the  high  tides  are  higher,  and  the  low  tides  lower,  than  at  other 
times.  The  tides  of  such  times  are  called  Spring  Tides.  Spring 
tides  therefore  have  no  relation  to  the  spring  season. 

When  the  earth,  moon,  and  sun  sustain  the  relative  positions 
shown  in  Fig.  702,  and  this  occurs  twice  each  month,  the  tidal 
influences  of  the  sun  and  the  moon  are  opposed,  and  the  result  is 
that  the  high  tides  are  not  so  high,  or  the  low  tides  so  low,  as  under 
other  conditions.  The  tides  of  such  times  are  known  as  Neap 
Tides. 

Other  variations  in  the  height  of  high  tides.  There  are  sev- 
eral other  causes  of  variation  in  the  height  of  high  tides.  Two  of 
these  causes  show  themselves  daily,  tw;o  have  monthly  periods, 
and  one  an  annual  period. 

The  two  successive  high  tides  of  a  given  place  are  often  of  un- 
equal height.  One  daily  variation  in  the  height  of  high  tides  is  due 
to  the  fact  that  the  high  tide  on  the  side  of  the  earth  away  from  the 


THE  MOVEMENTS  OF  SEA-WATER 


745 


moon  is  slightly  lower  than  that  on  the  side  next  the  moon,  for 

1  _L  1  1 

2360002     2400002  >  2400002  ~  2440002'     The  difference  '*>  however, 


© 


M 
O 

FIG.  702. — Diagram  to  illustrate  the  relative  positions  of  sun,  moon,  and 
earth  at  the  time  of  neap  tides. 

slight,  and  in  the  presence  of  larger  variations  is  not  commonly 
noticed. 

Again,  if  the  high  tide  on  one  side  of  the  earth  is  highest  at  A 
(Fig.  703),  the  highest  point  in  the  high  tide  on  the  opposite  side 
would  be  at  B,  if  the  ocean  were  universal  and  of  uniform  depth. 
From  A  on  the  one  side,  and  from  B  on  the  other,  the  height  of 


.--M 


FIG.  703.— Diagram  showing  why  successive  high  tides  are  often  unequal. 

the  high  tide  diminishes  in -all  directions.  The  point  A'  has  high 
tide  at  the  same  time  that  A  'and  B  have,  but  the  tide  at  A' is  not 
so  high  as  that'  at  A.  Twelve?'  hours  (and  twenty-six  minutes) 


746 


PHYSIOGRAPHY 


later,  point  A  will  have  the  same  position  relative  to  the  moon  that 
A'  now  has,  because  of  the  rotation  of  the  earth  and  the  revolution 
of  the  moon.  The  high  tide  which  will  occur  at  A  when  this  point 
shall  have  reached  the  position  A'  will  not  be  so  high  as  that  which 
it  had  when  in  the  position  A.  Similarly  the  high  tide  which  the 
point  A'  will  have  when  it  reaches  the  position  A,  will  be  higher 
than  the  preceding  high  tide  at  the  same  place.  The  amount  of 
daily  variation  due  to  this  cause  is  often  considerable.  Locally  at 
least  it  is  several  feet.  It  is  to  be  noted  that  it  would  not  occur 
when  the  moon  is  vertical  at  the  equator,  for  then  all  points  on  the 
same  parallel  stand  in  the  same  relation  to  the  highest  part  of  the 
tidal  wave. 


Moon  Farthest 
North  of  Equator 


Moon  Over 
the  Equator 


Moon  Farthest 
South  of  Equator 


FIG.  704. — Diurnal  inequality  of  the  tides  at  San  Francisco.  The  space 
between  the  vertical  lines  represents  a  day.  The  several  crests  of 
the  curves  represent  high  tides  and  the  troughs  low  tides. 

The  monthly  variations  in  the  height  of  the  high  tide  are  less 
notable.  One  is  due  to  the  variation  in  the  distance  of  the  moon 
from  the  earth.  This  distance  decreases  from  its  maximum  for 
about  two  weeks,  and  then  increases  from  its  minimum  for  about 
the  same  length  of  time.  The  variation  in  the  distance  of  the 
moon  from  the  earth  makes  a  slight-  difference  in  the  height  of 
the  tides,  the  high  tides  being  highest  and  the  low  tides  lowest 
when  the  moon  is  nearest.  Another  monthly  variation  at  any 
given  place,  is  due  to  the  fact  that  the  moon  is  vertical  in  differ- 
ent latitudes  at  different  times.  In  this  particular,  its  monthly 
range  is  comparable  to  the  annual  range  of  the  sun. 

The  distance  of  the  earth  from  the  sun  also  varies  during  each 
year,  and  this  variation  has  its  appropriate  effect,  small  though 
it  is,  on  the  height  of  the  solar  tides,  and  so  on  the  height  of  the 
observed  tides.  Other  variations  in  the  distance  of  the  sun  from 
the  earth  occur  in  much  longer  periods  of  time,  but  they  need  not 


THE  MOVEMENTS  OF  SEA-WATER 


747 


>o 
g 


748  PHYSIOGRAPHY 

be  considered  here.  The  variations  for  any  given  place,  produced 
by  the  apparent  annual  motions  of  the  sun,  are  trivial. 

The  highest  high  tides  in  any  given  place  should  occur,  theo- 
retically, when  the  sun  and  the  moon  work  together  (spring  tides), 
at  that  time  of  day  when  the  moon  is  most  nearly  in  the  zenith, 
at  that  time  of  the  month  when  the  moon  is  nearest  to  the  earth, 
and  at  that  time  of  year  when  the  sun  is  nearest  to  the  earth. 

Cotidal  lines.  If  a  line  were  drawn  on  the  ocean  surface, 
connecting  all  points  which  have  the  crest  of  the  same  high  tide 
at  the  same  time,  it  would  be  a  cotidal  line.  Any  line  connecting 
points  having  the  trough  of  the  same  tidal  wave  at  the  same  time 
would  also  be  a  cotidal  line;  or  in  general,  any  line  connecting 
points  having  the  same  phase  of  the  same  high  tide  at  the  same 
time,  is  a  cotidal  line.  If  the  ocean  were  universal  and  equally 
deep,  the  cotidal  lines  would  be  the  halves  of  great  circles;  but  the 
continents  and  the  shallow  waters,  the  islands  and  the  straits,  cause 
many  irregularities  in  them.  The  tide  runs  ahead,  relatively, 
where  the  water  is  deep  and  lags  when  it  is  shallow. 

Rate  of  movement.  Theoretically  the  tide  should  move  for- 
ward, from  east  to  west,  so  as  to  complete  a  circuit  in  24  hours 
and  52  minutes.  This  would  give  it  great  velocity  in  low  latitudes, 
a  velocity  nearly  equal  to  that  of  the  rotation  of  the  earth.  This 
velocity  is  nearly  reached  in  the  deep  and  open  sea,  but  nowhere 
else. 

Effects  of  tides  on  shores.  Since  tides  commonly  rise  in  a 
series  of  waves,  they  affect  shores  much  as  wind  waves  do.  The 
erosion  effected  by  tidal  currents  among  islands,  and  through 
straits  has  been  referred  to.  Tidal  scour  often  keeps  thorough- 
fares open  through  tidal  marshes,  to  which  the  tide  has  access 
through  bays.  Illustrations  are  found  on  the  coast  of  New  Jersey. 
Tidal  scour  also  sometimes  maintains  deep  waterways  in  bays  to 
the  great  advantage  of  navigation. 


CHAPTER  XXIV 
THE  LIFE  OF  THE  SEA 

THE  sea  teems  with  plants  and  animals.  The  latter  abound  at 
and  near  the  surface  nearly  everywhere;  they  abound  at  the  bot- 
tom in  shallow  water,  and  they  occur,  though  far  less  abundantly, 
at  the  bottom  of  even  the  deep  sea.  In  the  great  body  of  water 
intermediate  between  the  uppermost  100  fathoms  and  the  bottom, 
life  is  nearly  absent.  It  has  been  estimated  that  the  amount  of  life 
in  the  sea  exceeds  that  of  the  land,  square  mile  for  square  mile; 
but  there  is  probably  no  one  level  in  the  sea  where  life  is  so  abun- 
dant as  on  the  surface  of  the  fertile  parts  of  the  land.  Murray  has 
estimated  that  the  weight  of  the  lime  carbonate  of  the  shells  of 
organisms  in  the  uppermost  100  fathoms  of  sea-water  is  some- 
thing like  16  tons  per  square  mile.  This  is  far  less  than  the  weight 
of  the  plant  and  animal  life  per  square  mile  on  land  in  fertile 
regions. 

The  abundance  of  life  in  the  sea-water  may  be  shown  in  an- 
other way.  If  a  bucket  of  water  be  dipped  up  from  the  surface  of 
the  ocean  almost  anywhere,  it  will  be  found  to  contain  hundreds 
or  even  thousands  of  minute  organisms,  though  most  of  them  are 
too  small  to  be  visible  to  the  unaided  eye. 

The  distribution  of  the  plant  life  of  the  sea  differs  somewhat 
from  that  of  the  animal  life.  Plant  life  is  plentiful  at  the  surface 
nearly  everywhere,  and  at  the  bottom,  down  to  the  depth  of  about 
50  fathoms.  Where  conditions  are  favorable,  it  occurs  somewhat 
sparingly  down  to  depths  of  nearly  200  fathoms  or  so;  but  below 
some  such  depth  it  is  absent,  probably  because  of  the  absence  of 
sunlight.  Animal  life  is  abundant  where  plant  life  is,  and  also  to 
considerably  greater  depths,  beside  being  found  to  some  extent 
over  the  whole  of  the  ocean's  bed. 

The  most  important  physical  factors  which  influence  the  dis- 

749 


750  PHYSIOGRAPHY 

tribution  of  the  various  types  of  sea  life  are  (1)  temperature,  and 
(2)  depth  of  water.  Other  less  important  factors  are  (3)  the 
clearness,  (4)  the  degree  of  saltness,  (5)  the  quietness  or  roughness, 
and  (6)  the  presence  or  absence  of  ice.  The  relations  of  various 
types  of  life  to  one  another  are  also  important.  Some  are  depend- 
ent on  others  for  food,  some  are  hostile  to  others,  and  some  are 
rivals  for  the  same  sorts  of  food. 

The  manner  in  which  most  of  these  factors  influence  the  dis- 
tribution of  life  will  be  readily  understood  from  analogy  with  the 
factors  which  control  the  distribution  of  land  life.  One  factor, 
however,  which  finds  no  analogy  in  connection  with  the  distribu- 
tion of  land  life,  is  the  depth  of  the  water.  Land  life  is  restricted 
practically  to  the  surface  of  the  land,  while  sea  life  has  a  wide  ver- 
tical range.  The  depth  of  the  water  affects  the  distribution  only 
of  those  plants  and  animals  which  rest  on  the  bottom;  it  has  little 
effect  on  the  range  of  those  which  float  or  swim  near  the  surface. 

The  most  important  influence  of  depth  appears  to  be  in  con- 
nection with  the  penetration  of  light  and  with  the  supply  of 
oxygen.  Light  is  so  rapidly  absorbed  by  the  water  that  vision 
is  virtually  cut  off  at  a  depth  of  some  50  fathoms,  though  a  little 
light  penetrates  to  somewhat  greater  depths.  But  in  the  great  body 
of  the  ocean  darkness  reigns.  No  form  of  plant  life  which  depends 
directly  on  sunlight  can  live  in  darkness.  This  includes  all  forms 
of  green  plants,  and  some  others.  At  the  bottom,  too,  the  water 
is  not  stirred,  and  any  oxygen  it  contains  must  pass  down  from 
the  surface  after  being  dissolved  there.  As  it  is  consumed  below 
by  the  animals,  the  supply  is  renewed  by  diffusion,  an  extremely 
slow  process. 

Since  the  several  factors  which  influence  the  distribution  of 
sea  life  vary  widely,  the  distribution  of  various  types  of  life  also 
varies  widely.  Some  animals,  such  as  coral  polyps,  are  restricted 
to  warm  regions  where  the  water  is  shallow,  clear,  and  normally 
salt,  while  others,  such  as  narwhales,  seals,  etc.,  are  found  only 
in  cold  waters.  Still  others  range  through  great  differences  of 
temperature. 

The  plant  life  of  the  sea  varies  less  with  latitude  than  the  plant 
life  of  the  land,  and  less  than  the  animal  life  of  the  sea. 

The  life  of  the  sea  is  in  strong  contrast  in  many  ways  with  that 
of  the  land.  Thus  most  plants  with  which  we  are  familiar  on 
land  are  fixed  in  position,  while  many  of  the  plants  of  the  sea 


THE  LIFE  OF  THE  SEA  751 

float.  Most  animals  on  the  land  are  free  to  move  about,  while  a 
very  considerable  proportion  of  sea  animals,  such  as  coral  polyps, 
barnacles,  crinoids,  etc.,  are  fixed  through  most  of  their  lives. 
Many  others,  though  not  fixed,  move  about  but  little,  either  lying 
on  the  bottom  or  burrowing  into  it.  Some,  on  the  other  hand, 
such  as  many  of  those  in  the  surface  waters  (pelagic  life),  appear 
to  be  always  in  motion. 

The  pressure  of  the  water  at  the  bottom  of  the  ocean  is  very 
great,  but  the  animals  living  there  withstand  it,  because  their 
tissues  are  full  of  liquids  under  equally  high  pressure,  and  these 
high  internal  pressures  counterbalance  the  external  pressure.  If 
an  animal  from  the  bottom  of  the  deep  sea  were  brought  suddenly 
to  the  surface  it  would  explode.  This  has  indeed  happened  in 
raising  animals  from  the  deep  sea,  even  when  the  raising  was  by 
no  means  instantaneous. 

The  deep-sea  animals  have  some  notable  peculiarities.  Some 
are  blind,  but  some  have  eyes,  implying  sight  and  therefore  light. 
It  has  been  conjectured  that  the  phosphorescence  of  the  animals 
themselves  supplied  the  light.  Some  of  the  deep-sea  animals  also 
are  ornamented,  a  fact  which  seems  to  have  no  rational  explanation, 
unless  the  ornamentation  is  seen. 

All  the  great  groups  of  animal  life  are  represented  in  the  sea- 
water.  Even  warm-blooded  mammals  (whales,  narwhales,  seals, 
walruses,  etc.)  abound  in  the  frigid  waters  among  icebergs  and 
ice-floes.  Some  of  these  animals,  like  the  seals  and  walruses,  do 
not  spend  all  their  time  in  .the  water,  but  frequently  crawl  up  on 
the  floes  of  ice  to  warm  themselves  and  sleep  in  the  sun.  From 
this  highest  class  of  animals  (mammals)  down  to  the  lowest,  every 
important  subdivision  of  the  animal  kingdom  is  represented,  though 
no  birds  spend  all  their  time  in  the  water.  The  variations  of  plant 
life  are  also  great,  though  the  higher  forms,  such  as  we  are  most 
familiar  with  on  land,  are  wanting. 

It  is  to  be  noted  not  only  that  the  range  of  marine  plants  and 
animals  is  great,  but  that  the  largest  living  animals,  the  whales, 
are  marine.  Many  of  the  marine  plants,  too,  are  of  great  size. 
Some  seaweeds  are  six  inches  in  diameter,  and  some  are  hundreds 
of  feet  long,  exceeding  in  length  the  hei~ht  of  the  tallest  trees. 
They  are,  however,  not  so  bulky  as  large  trees,  and  the  amount 
of  solid  matter  which  the  largest  seaweed  contains  is  far  less 
than  that  of  the  largest  tree. 


752  PHYSIOGRAPHY 

The  life  of  the  sea  is  important  in  many  ways.  Many  of  the 
animals,  fish,  oysters,  clams,  crabs,  lobsters,  etc.,  are  used  for  food. 
The  total  value  of  food  products  derived  from  the  sea  is  probably 
not  less  than  $500,000,000  per  year.  Other  animals  furnish  other 
articles  of  commerce;  for  example,  the  seal  furnishes  fur  and  oil; 
the  whale,  oil  and  whalebone;  the  walrus,  exceptionally  strong 
leather,  etc.  Corals  and  sponges,  the  products  of  animal  life,  are 
also  articles  of  commerce. 

Many  of  the  animals  of  the  sea  have  shells  or  other  hard  parts. 
These  hard  parts  accumulate  on  the  bottom  of  the  sea  when  the 
animals  are  through  with  them,  and  this  is  a  chief  source  of  the 
sediments  of  the  sea  bottom.  When  the  shells,  etc.,  accumulate 
with  little  admixture  of  other  material,  they  may  in  time  be  solidi- 
fied by  cementation  and  form  limestone.  Most  of  the  limestone 
now  found  on  land  was  formed  hi  this  way  beneath  the  sea,  when 
the  sea  covered  the  areas  where  it  now  occurs.  The  animals  which 
make  the  heavier  shells  or  other  secretions  of  calcium  carbonate 
live  chiefly  in  shallow  water,  and  the  seas  in  which  the  limestones 
of  the  land  were  formed  were  generally  shallow. 


CHAPTER  XXV 


MATERIALS  OF  THE  SEA  BOTTOM 

Dredging.  The  material  on  the  bottom  of  the  sea  has  been 
made  known  by  dredging.  Various  forms  of  apparatus  have  been 
used  to  bring  up  matter  from  the  bottom.  One,  known  as  the 
Cup  Lead,  is  shown  in  Fig.  706.  B  is  a  hollow  inverted  cone  on  a 
spike.  Above  the  cone  is  a  sliding  disc,  D,  somewhat  larger  than 
the  base  of  the  cone.  This  piece  of  apparatus  is  let  down  and  the 
cone  sinks  into  the  soft  sediment  and  is  filled  with  it.  On  being 
raised,  the  disc  shuts  down  and  prevents  the  escape 
of  the  contents  of  the  cup,  and  also  the  access  of 
new  matter  from  higher  levels. 

Fig.  707  shows  a  dredge.  The  flaring  strip  of 
metal  E  is  dragged  along  the  bottom,  and  directs 
the  surface  sediment  into  the  sack.  Swabs  are 
attached  below  to  entangle  animals  missed  by  the 
dredge. 

The  bottom  of  the  sea  is  generally  covered  with 
sediment  which  is,  for  the  most  part,  in  a  loose 
or  uncemented  condition.  This  sediment  has 
come  from  various  sources.  Some  of  it  was  carried 
to  the  sea  by  rivers,  some  of  it  was  worn  from 
the  shores  by  the  waves,  some  of  it  was  blown  out 
from  the  land,  some  of  it  is  made  up  of  the  shells, 
etc.,  of  the  organisms  which  live  in  the  water,  and 
some  of  it  is  composed  of  fine  debris  thrown  out  F  _  __, 
from  submarine  volcanoes.  Cosmic  ("shooting-  cup  lead. 

star")   dust    is  also   an  element,  though  a  very   (Challenger    Re- 

J  port.) 

minor  one. 

The  sediments  derived  from  the  land  came  from  the  disinte- 
gration of  land  rock.  In  their  present  state,  however,  they  are  to 
be  looked  upon  as  rock  in  the  making,  for  all  sediments  in  the  sea 

753 


754 


PHYSIOGRAPHY 


Dredge  Rope, 
wivel. 


Dredge  Chain. 


may  become  solid  rock  by  cementation,  and  cementation  is  now 
taking  place  at  many  points  in  the  bottom  of  the  sea.  Locally,  it 
takes  place  as  fast  as  the  sediments  accumulate. 

Physically,  the  materials  of  the  sea  bottom  may  be  grouped 

into  several  classes,  namely: 
gravel,  sand,  mud,  shells,  coral, 
etc.,  and  ooze. 

Gravel  is  found  chiefly 
along  the  borders  of  the  land 
out  to  depths  of  a  few  fathoms, 
or  at  most  a  few  scores  of 
fathoms.  Gravel  and  bowlders, 
carried  out  by  icebergs,  are 
occasionally  found  at  great 
depths  and  far  from  land. 
Sand  also  is  generally  confined 
to  relatively  shallow  water, 
but  it  occurs  out  to  depths 
beyond  that  reached  by  gravel, 
but  rarely  out  to  100  fathoms. 
Mud  is  much  more  wide- 
spread. While  it  frequently 
occurs  in  shallow  water,  it  also 
extends  out  to  a  depth  far  be- 
yond that  reached  by  gravel 
and  sand.  In  general,  land- 
derived  mud  is  not  washed 
out  far  from  the  land,  but  m 
exceptional  cases,  as  off  the 
mouths  of  rivers,  it  is  carried  hundreds  or  even  a  thousand 
miles. 

Ooze  is  the  name  applied  to  the  loose  materials  of  the  sea  bottom 
composed  primarily  of  the  minute  shells  and  tests  of  organisms 
which  live  in  the  water.  Many  of  these  organisms  live  near  the 
surface  of  the  water,  and  their  shells,  etc.,  sink  when  they  die. 
The  distinctive  names  of  the  oozes  are  derived  from  the  names  of 
the  organisms  which  contributed  most  to  them.  Thus,  foramini- 
feral  ooze  is  the  ooze  in  which  shells  of  foraminifera  are  abundant, 
diatom  ooze  is  ooze  in  which  tests  of  diatoms  predominate,  etc. 
Foraminiferal  ooze  has  a  composition  very  similar  to  that  of 


FIG.  707.— The  dredge.     (Challenger 
Report.) 


MATERIALS   OF  THE  SEA  BOTTOM  755 

chalk.  Other  oozes,  such  as  diatom  ooze,  radiolarian  ooze,  etc., 
are  made  up  largely  of  silica. 

In  the  deepest  part  of  the  ocean,  below  the  depth  of  some 
2200  fathoms,  the  surface  is  covered  with  red  clay.  The  origin  of 
this  clay  has  long  been  in  question,  but  it  is  probably  made  up  of 
material  derived  from  many  sources.  A  considerable  part  was 
doubtless  derived  from  the  fine  material  ejected  from  volcanoes; 
another  part  was  probably  carried  out  from  the  land  by  the  wind, 
a  part  was  probably  derived  from  the  shells  and  tests  of  animals 
which  lived  in  the  ocean,  and  cosmic  dust  doubtless  enters  into 
its  composition.  The  materials  from  all  these  sources,  so  far  as 
they  enter  into  the  composition  of  the  red  clay,  are  probably  only 
the  insoluble  parts  of  the  original  material. 

On  the  lands  there  is  rock  (conglomerate)  composed  of  ce- 
mented gravel,  rock  (sandstone)  composed  of  cemented  sand,  rock 
(shale)  composed  of  cemented  mud,  and  rock  (limestone)  com- 
posed of  material  derived  from  shells,  corals,  etc.  None  of  these 
correspond  to  the  deep-sea  oozes,  and  none  correspond  to  the  red 
clay  of  the  abysmal  depths.  In  the  lands,  therefore,  there  are 
varieties  of  rock  corresponding  to  all  the  sediments  now  making 
in  the  shallow  water  of  the  sea,  but,  so  far  as  known,  none  corre- 
sponding to  those  of  the  deep  waters.  This  suggests  that  the  lands 
have  been  at  some  tune  beneath  the  sea,  a  conclusion  which  is 
borne  out  by  the  finding  of  the  shells  of  marine  species  imbedded 
in  the  sandstone,  shale,  etc.,  of  the  land;  but  it  also  indicates  that, 
so  far  as  now  known,  no  part  of  the  present  continents  ivas  ever  at 
the  bottom  of  the  deep  ocean. 


CHAPTER  XXVI 
RELATION  OF  THE  SEA  TO  THE  REST  OF  THE  EARTH 

THE  ocean  has  an  important  influence  on  the  rest  of  the  earth. 
This  is  felt  in  various  ways,  some  of  which  have  already  been 
noted.  By  way  of  summary  they  may  here  be  brought  to- 
gether. 

1.  Waves   affect  the  coast-line;    they  wear  away  the  land  in 
some  places  and  build  new  land  in  others.  On  the  whole,  destruc- 
tion exceeds  construction,  so  far  as  the  land  is  concerned,  so' that 
the  tendency  of  the  ocean  is  to  extend  itself  at  the  expense  o£  the 
land. 

2.  Oceans  modify  the  climate  of  the  land,  affecting  both  tem- 
perature and  precipitation.     The  general  influence  on  tempera- 
ture arises  from  the  fact  that  water  is  heated  and  cooled  more 
slowly  than  land  is.     The  air  over  the  sea,  therefore,  has  a  lesser 
range  of  temperature  than  that  over  the  land,  and  blowing  to  the 
iand  tends  to  carry  the  temperature  of  the  sea,  as  well  as  abun- 
dant moisture,  over  to  it.     Winds  from  the  ocean  therefore  temper 
the  climate  of  the  land  both  in  summer  and  winter.     The  warm 
currents  enhance  the  general  effect  of  the  sea  in  this  respect.     The 
climatic  effect  of  the  sea  on  the  land  is  felt  especially  on  the  west 
sides  of  the  continents,  in  the  temperate  zones,  because  of  the 
westerly  winds,  and  on  the  east  sides  of  the  continents  in  the  zone 
of  easterly  winds.     The  cold  currents  of  the  sea  have  much  less 
effect  than  warm  ones  on  the  climate  of  the  land,  because  they 
tend  to  hug  the  east  sides  of  the  continents,  so  far  as  they  stay 
at  the  surface;  and  in  the  latitudes  where  they  occur  the  winds 
blow  from  them  to  the  sea  rather  than  to  land. 

3.  The  ocean  is  the  great  source  of  the  water  for  rain  and  snow, 
and  its  precipitation  from  the  atmosphere  furnishes  the  conditions 
necessary  for  life  on  the  land. 

4.  Through  its  effects  on  rainfall,  snowfall,  and  temperature, 
the  ocean  has  an  important  effect  on  the  degradation  of  the  land. 

756 


RELATION  OF  THE  SEA  TO  THE  REST  OF  THE  EARTH  757 

The  total  amount  of  rainfall  for  the  earth  is  not  accurately 
known.  If  it  is  as  much  as  three  feet  per  year,  on  the  average, 
for  the  whole  earth,  and  if  all  this  were  derived  directly  from  the 
ocean,  an  amount  equal  to  all  the  water  in  the  ocean  would  be 
evaporated  in  about  3000  years.  Since  most  of  the  water  evap- 
orated from  the  ocean  falls  again  into  the  sea,  or  runs  to  it  in 
rivers,  or  issues  beneath  it  as  springs,  the  amount  of  the  ocean 
water  is  not,  so  far  as  known,  growing  less. 

5.  The  ocean  affords  an  enormous  harvest  of  foodstuff  annually, 
and  many  thousands  of  people  depend  on  this  harvest  for  their 
livelihood.     Fisheries  were  among  the  earliest  industries. 

6.  The  ocean  also  plays  an  important  part  in  the  commerce 
of  the  world  by  serving  as  a  great  highway.     The  obstacle  which 
the  oceans  long  seemed  to  interpose  to  quick  communication  be- 
tween continents  separated  by  them,  has  been  overcome  during  the 
last  half  century,  and  several  cables   now  connect  Europe  and 
America,  so  that  all  the  important  news  of  either  continent  is 
known  in  the  other  almost  as  soon  as  it  is  at  home.     The  Pacific,  too, 
is  bridged  by  cables,  though  their  number  is  small. 

Some  conception  of  the  role  which  the  ocean  plays  in  the  af- 
fairs of  the  earth  may  perhaps  be  gained  by  picturing  the  con- 
ditions which  would  exist  if  there  were  no  oceans. 

REFERENCES 

1.  WILD,  Thalassa:   Marcus  Ward  &  Co.,  London. 

2.  MAURY,  Physical  Geography  of  the  Sea. 

3.  CHAMBERLIN   AND  SALISBURY,  Chapter  VI,  Geologic  Processes,  and 
other  text-books  of  Geology. 

4.  AGASSIZ,  Three  Cruises  of  the  Blake:  Hough  ton  Mifflin  <fc  Co. 

5.  THOMSON,   The  Depths  of  the  Sea,  and  Voyage  of  the    Challenger: 
Macmillan. 

6.  Challenger  Reports,  especially  Narrative,  Vol.  I,  and  Summary,  First 
Part:  Eyre  &  Spottiswoode,  London. 

7.  MURRAY,  Important  articles  on  oceanography  in  Geog.  Jour.,  Vol. 
XII,  pp.  113-137,  and  Vol.  XIV,  pp.  34-50,  and  426-441;  and  Scot.  Geog. 
Mag.,  Vol.  XV,  pp.  505-522. 

8.  BIGSBEE,  Deep-sea    Soundings    and  Dredgings:    U.  S.  Coast   Surv* 
Washington. 

9.  TANNER,  Deep-sea  Exploration:   U.  S.  Fish  Commission,  Washington 

10.  BELKNAP,  Deep-sea    Soundings    in  the  North  Pacific:   U.  S.  Hydro- 
graphic  Office,  Washington. 

11.  FLINT,  Oceanography  of  the  Pacific:    Smithsonian  Institution. 
12!  Jour,  of  Geol.,  Vol.  XIII,  pp.  469-484. 


INDEX 


Abert  Lake,  40 

Abnormal  temperature,  546 

Abrasion  by  ground-water,  105 

by  the  wind,  70 
Absolute  humidity,  570 

temperature,  677 
Accidents  to  streams,  173 
Aconcagua,  445 
Agassiz,  A.,  474,  757 
Aggradation,  44 
Agonic  lines,  477 
Agriculture  and  rainfall,  701 
Air,  chemical  work  of,  71 
Aitkin,  J.,  290 
Alaska,  coast  of,  458 
Alden,  W.  C.,  (and  Salisbury),  R.  D., 

78 

Aldrich  deep,  711 
Alluvial  cone,  182 

fan,  183 

plains,  183,  187 
fertility  of,  191 

terraces,  203 
Alpine  glaciers,  219 
Alps,  glaciers  of,  233 
Alps  Mountains,  441 

section  of,  39 

Altitude  and  temperature,  537,  545 
Anchor-ice,  214 
Andes  Mountains,  441 
Aneroid  barometer,  583 
Annual  isobars,  585 

maximum  temperatures,  550 

minimum  temperatures,  551 

parallax,  488 

ran^e  of  temperature,  549,  552 
Antarctic  Circle,  502 
Antarctica,  ice-cap  of,  216,  240 
Antecedent  streams,  177 
Anticyclones,  620 

movements  of,  632 

origin  of,  648 

tracks  of,  in  United  States,  636, 645 


Ants,  effect  on  surface,  78 
Aperiodic  changes  of  pressure,  620 
Aphelion,  489 

Appalachian  Mountains,  39,  41 
Aral  Sea,  297 
Arctic  Circle,  502 

Areas  of  low  pressure  in  high  lati- 
tudes, 597,  601 
Argon,  512 

Aridity,  effect  on  temperature,  545 
Artesian  wells,  94,  95 
Ashes,  volcanic,  368 
Asteroids,  505 
Astronomic  latitude,  491 
Atlantic,      equatorial      temperature 

curves  for,  723,  724 
Atmosphere,  a  mixture  of  gases,  513 

carbonic-acid  gas  of,  514 

constitution  of,  512 

density  and  altitude,  507 

general  conception  of,  506 
.   heating  of,  521 

height  of,  508 

history  of,  510 

impurities  of,  513 

mass  of,  510 

oxygen  of,  514 

relation  to  rest  of  earth,  507 

temperature  of,  520 

volume  of,  510 

water-vapor  of,  517 

work  of,  55 

Atmospheric     moisture,     effect     on 
movements,  569 

function  of,  564 
Atmospheric  pressure,  582 

temperature,  effect  on  movement, 

561 

Atolls,  470 
Attraction  of  sun  and  moon,  cause  of 

ocean  movement,  728 
Aurora,  509 
Autumn,  531 

759 


760 


INDEX 


Avalanches,  253 
Axis  of  earth,  484 
inclination  of,  497 

Bad  lands,  160 
Balloon  ascents,  508 
Barometer,  582 

aneroid,  583 

Barometric  gradient,  613 
Bars,  327 
Base-level,  131 

temporary,  139 
B(ath,  springs  of,  99 
Batholiths,  375 
Bayous,  189 
Beaches,  324,  325 
Belknap,  G.  E.,  757 
Bigsbee,  Lieut.  Com.  C.  D.,  757 
Black  Hills,  23 
Blake  deep,  711 
Blizzards,  646 
Boiling  springs,  124 
Bonney,  T.  G.,  390 
Bore,  tidal,  735 
Bowlders,  251 
Braided  stream,  185 
Brigham,  Prof.  A.  P.,  337 
Buttes,  173 
Bysmalith,  375 

Caldera,  305 
California,  model  of,  37 

rainfall  in,  619 
Campbell,  M.  R.,  206 
Canyon  of  the  Yellowstone,  159 
Canyons  and  gorges,  156 
Carbon  dioxide  of  atmosphere,  512 
Carbonation,  72 
Carbonic-acid  gas,  512 
in  atmosphere,  514,  517 

effect  on  temperature,  517 
in  ocean,  719 
Cascade  Mountains,  35 
Caspian  Sea,  297 
Castle  Geyser,  92 
Catskill  Mountains,  440,  446 
Caucasus  Mountains,  442 
Caverns,  97 
Central    America,    earthquakes    in, 

428 

Challenger,  course  of,  710 
Chamberlin,  Prof.  T.  C.,  112, 289,  2£0, 

291,337,  757 

Changes  of  level,  43,  398,  400-404 
Changes  of  temperature,  effect    of, 

72 

Charleston  earthquake,  413 
Chemical  work  of  air,  71 


Chemical  work  of  ground-water,  96 
Chesapeake  Bay,  drainage  about,  174 
Chile,  coast  of,  332,  459 

earthquakes  in,  416 
Chimborazo,  445 
Chinook  winds,  673,  675 
Chittenden,  H.  M.,  113 
Cinder  buttes,  365 
Cinders,  volcanic,  368 
Circle  of  illumination,  497 
Circulation  of  the  atmosphere,  598 
Circumpolar  whirl,  605 
Cirques,  231,  249 
Cirrus  clouds,  576 
Cliff  glacier,  221,  222 
Climate,  676 
changes  of,  703 
continental,  683 
oceanic,  683 
of  polar  zones,  700 
of  tropical  zone,  693 
Climate  and  life,  702 
Climates,  classification  of,  683 
Climatic  changes,  causes  of,  705 
effects  of  ocean  currents,  733 
zones,  684 
Cloudbursts,  667 
Cloudiness,  precipitation,  etc.,  shown 

on  weather  maps,  623 
Clouds,  574 

forms  of,  576 

Coal  in  the  United  States,  distribu- 
tion of,  455 
Coastal  plains,  17 
Coast-line  irregularities,  distribution 

of,  461 
Coast-lines,  affected  by  diastrophism, 

463 

affected  by  gradation,  462 
affected  by  vulcanism,  464 
effect  on  history,  465 
irregularities  of,  457 
relief  of,  460 
Cold  currents,  721 
Cold  waves,  645 
Colorado,  Grand  Canyon  of,  32,  33, 

157 

Colorado  River,  delta  of,  199 
Columnar  structure,  388 
Comets,  505 

Compound  alluvial  fan,  183 
Condensation  of  water-vapor,  572 
Conduction,  526 
Conglomerate,  49 
Consequent  falls,  168 

streams,  177 

Continental    and    oceanic    tempera- 
tures, 560 
climate,  683,  690 


INDEX 


761 


Continental  glaciers,  234 

changes  produced  by,  274 

of  Europe,  272 

of  North  America,  270 
Continental  platforms,  10,  11 

shelves,  6,  22 

slopes,  713 
Continents,  grouping  of,  11 

outlines  of,  457 
Contour  interval,  20 

map,  explanation  of,  19 
Contours,  20 
Convection,  93,  527 
Convection  currents,  529 
Coon  butte,  380,  381 
Copper  ores  in  the   United   States, 

distribution  of,  454 
Coral  islands,  469 

reefs,  469 
Cornish,  V.,  78 
Corrasion,  129 
Corrosion,  129 
Coseismic  lines,  425 
Cotidal  lines,  747 
Counter  currents,  733 
Cowles,  H.  C.,  78 
Crater,  339 

Crater  Lake,  297,  305,  306,  382 
Craterlets,  410 
Creep,  108,  133 
Crevasses  of  glaciers,  224 
Crustal  deformation,  405 

movements,  392 
Cumulus  clouds,  576 
Currents,  oceanic,  721,  730 
Cycle  of  erosion,  153 
Cyclone,  621 

structure  of,  625 

Cyclones,  mean  tracks  of,  in  United 
States,  636 

movements  of,  632 

origin  of,  648 

paths  of,  645 

tropical,  648 
Cyclonic  winds,  618 

Daily    range    of    temperature     553, 

559 

Daly,  R.  A.,  206 
Dana,  Prof.  J.  D.,  473,  474 
Darwin,  Charles,  474 
Davis,  B.  M.,  113 

Davis,  Prof.  W.  M.,  205, 290, 51 1, 686 
Dead  Sea,  297 
Deepening  of  valleys,  129 
Deeps,  711 
Deflection  of  winds  by  rotation,  602, 

604 
Degrees,  length  of,  494 


Delta   of   the    Colorado  River,  199 
200 

the  Danube,  202 

the  Mississippi,  181,  197 

the  Nile,  201 

the  Rhone  River,  201 

the  St.  Clair,  197 
Delta  lakes,  310 
Delta-land,  330 
Deltas,  198,  199 
Density  and  movement  of  sea-water 

719 

Deposition    by   continental   glaciers 
276 

by  ground-water,  99 

by  running  water,  179,  180 
Depth-limit  of  valleys,  131 
Desert  climates,  690 
Dew,  573 

Dew-point,  570,  572 
Diastrophism ,  303,  392 

a  cause  of  change  of  sea-level,  401 

effects  on  coast -lines,  463 
Diatom  ooze,  754 
Dikes,  375 

Diller,  J.  S.,  337,  390,  391 
Dip  compass,  479 
Direction  of  winds,  602,  613 
Distributary  streams,  185 
Divides,  permanency  of,  140 
Dodge,  Prof.  R.  E.,  206 
Doldrums,  617 
Dolphin  ridge,  714 
Drainage  changed  by  glaciation,  285 
Drainage  of  the  upper  Ohio  basin 

285 

Dredging,  753 
Drift,  257 

disposition  of,  258 

oceanic,  730,  732 

topography,  260,  264 
Driftless  area,  272 
Drowning  of  valleys,  173 
Drumlins,  265,  278,  279 
Dry  farming,  615 
Dry  winds,  697 
Dunes,  63 

distribution  of,  63 

migration  of,  67 

shapes  of,  65 
Dust,  55 

in  atmosphere,  518 

distribution  of,  62 

sources  of,  56 
Dust-wells,  227,  228 
Dutton,  Major  C.  E.,  206,  391,  434 

Earth,  flattened  form  of,  482,  496 
motions  of,  484,  488 


762 


INDEX 


Earth  relations,  482 

Earth,  size  of,  484 

Eartnquake  fissures,!  431       t  i    ••.; 

Earthquake  wave,  424 

Earthquakes,  408 

beneath  the  sea,  420 

Calif ornian,  419 

causes  of,  430 

distribution  of,  429 

frequency  of,  427 

in  Italy,  418 

in  Japan,  412 

in  Mississippi  valley,  417 

in  Panama,  428 

strength  and  destructiveness,  408 

surface  changes  caused  by,  432 
Earthworms,  effect  on  surface,  78 
Ebb-tide,  735 
Eddying  streams,  124 
Elbruz  Mountains,  445 
Elk  Mountains,  36,  39 
Enchanted  Mesa,  172 
Englacial  drift,  253 
Entrenched  meanders,  174 
Eolian  sand,  63,  69 
Epicontinental  seas,  7 
Equator,  484 
Equatorial  calms,  603,  617 

currents,  730 

temperature    curves    for    the    At- 
lantic, 724 

temperature  curves  for  the  Pacific, 

724 

Equinoxes,  499 
Erosion,  129 

by  continental  glaciers,  274 

by  streams,  120 

by  wind,  55 

conditions  affecting  rate  of,  155 
Eskers,  267 
Evaporation,  80,  565 

affected  by  temperature,  568 

amount  of,  567 

effect  of  temperature  on,  568 

influenced  by  wind,  568 

rate  of,  566 
Exfoliation,  74 
Extinct  lakes,  333 
Extra-tropical  belts  of  high  pressure, 
600 

Fairchild,  Prof.  H.  L.,  291 
Fall  Line,  22 
Fault,  normal,  406 

reversed,  406 
Faulted  mountain,  39 
Faulting,  405,  406 
Faults  and  folds,  406 
Fault-scarps,  407 


I   Fenneman,  Prof.  N.  M.,  337 
Ferrel,  W.,  511 
"Fetch"  of  waves,  321 
Filling  of  lake  basins,  302 
Fiords,  248,  331 
Fissure  eruptions,  371 
Fissures,  338 
Flint,  J.  M.,  757 
Floe-ice,  212 

Flood-plain  meanders,  187 
Floods  of  rivers,  115,  195 

of  Yellow  River,  202 
Flood-tide,  735 
Flowing  wells,  94 
"Fluvial  period"  of  history,  191 
Fluvio-glacial  deposits,  265 
Foehn  winds,  673 
Fog,  574,  734 
Folding,  405 
Folds  and  faults,  406 
Foraminiferal  ooze,  754 
Forests,  climatic  effect  of,  692 
Form  of  earth,  482 
Fossils  as  evidence  of  change  of  level, 

393 

Foucault's  pendulum,  486 
Freezing  and  thawing,  72 
Frigid  zones,  684 
Frost,  573 
Fuji-yama,  445 
Funnel-shaped  cloud,  668 

Galveston  storm,  648,  654 
Gannett,  H.,  206 

Geikie,  Prof.  J.,  78,  112,  290,  337 
Geikie,  Sir  Archibald,  290,  337 
General  circulation  and  precipitation, 

614 

Geodetic  latitude,  491 
Geographic  latitude,  491 
Geography  defined,  4 
Geology  defined,  3 
Geysers,  90 
Geyser  tube,  91 
Giant  Geyser,  91 
Gilbert,  G.  K.,  205,  290,  337,  391, 

434 
Glacial  bowlders,  252 

epochs,  cause  of,  273 

grooves,  249,  250,  263 

lakes,  311 

period,  270 

plains,  436 

stria?,  248,  250,  263 
Glaciated  valleys,  244 
Glaciation  and  drainage,  280,  285 

effects  of,  on  human  affairs,  287 
Glacier  movement,  nature  of,  231 
Glaciers,  219 


INDEX 


703 


Glaciers,  conditions  affecting  rate  of 
mo vement,  230 

crevasses  of,  224 

deposition  by,  255 

effect  on  shores,  330 

erosion  by,  242 

movement  of,  229 

size  of,  233 

types  of,  219 

waste  of,  229 

work  of,  242 
Glaciers   and    ice     sheets,     ancient, 

270 
Gold    and     silver,    distribution    of, 

454 
Gradation,  305 

agents  of,  44 

effects  on  coast-lines,  462 
Gradient  of  wind,  613 
Graham  Island,  379 
Grand  Canyon  of  the  Colorado,  32, 

33,  157 
Granite,  52 
Gravity  faults,  406 
Great  Ararat,  445 
Great  Bear  Lake,  297 
Great  Lakes,  the,  284 
Great  Plains,  23 
Great  Salt  Lake,  315 
Great  sea  waves,  412 
Greece,  earthquakes  of,  420 
Greely,  Gen.  A.  W.,  511 
Greenland,  ice-cap  of,  216,  235 
Ground-ice,  214 
Ground  moraine,  257,  278 
Ground-swell,  319 
Ground-water,  83 

abrasion  by,  105 

amount  of,  86 

deposition  by,  99 

descent  of,  84 

existence  of,  83 

movement  of,  86 

solution  by,  96 

source  of,  83 

surface,  85 

work  of,  80,  96 
Gulf  Stream,  544,  730 
Gullies,  119 
Gulliver,  F.  P.,  474 
Gully,  growth  of,  142 

Heat  received  in  different  latitudes, 

524 

Heating  of  land  and  water,  530 
Heilprin,  Prof.  A.,  390,  474 
Henry  Mountains,  373 
Hanging  valleys,  247 
Hann,  Prof.  J.,  511,  689 


Harrington,  M.  W.,  337 
Hawaiian  volcanoes,  301 
Hayden,  F.  V.,  511 
Hayes,  Dr.  C.  W.,  206 
"High."      (See  Anticyclone.) 
High-latitude  areas  of  low  pressure. 

601 

High-latitude  glaciers,  219 
High-pressure  belts,  594 
High  tides,  738 

variation  in  height  of,  744 
Hill,  R.  T.,  390 
Himalayas,  rainfall  in,  619 
Hitchcock,  A.  S.,  78 
Hogbacks,  171 
Hot  waves,  645 
Hovey,  E.  O.,  112,  390 
Humidity,  570 

absolute,  570 

effect  on  temperature,  545 

relative,  570 
Hurricanes,  648 
Hydration,  105 
Hydrography  denned,  3 

Ice  and  snow,  work  of,  207 
Ice  of  lakes,  207 

of  sea,  210 

in  soil,  207 
Icebergs,  269 
Ice-cap  of  Antarctica,  240 

of  Greenland,  235 
Ice-caps,  221,  234 
Ice  cascade,  224 
Ice  columns,  228 
Ice-fields,  218 
Ice-foot,  212 
Ice  jams,  213 
Ice-packs,  212 
Ice-sheets,  221 
Ice-shove,  212 
Igneous  rocks,  51 
Immediate  run-off,  83 
Inclination  of  earth's  axis,  effects  of, 

497 
India,  earthquakes  in,  416 

famines  in,  619 
Inequalities  of  level,  727 
Insolation,  522 

in  different  latitudes,  524 
Interior  plains,  17,  22 
Intermediate  zones,  684,  686 

climate  of,  695 
Intermittent  streams,  118 
Intrusions  of  lava,  374 
Iron  ores  in  the  United  States,  dis- 
tribution of,  455 
Irrigation,  193,  451 
Is-abnormal  lines,  546 


764 


INDEX 


Is-abnormal  temperatures  for  Janu- 
ary, 547 

for  July,  548 
Islands,  origin  of,  466 
Isobaric  chart  for  January,  591 

for  July,  593 
Isobaric  charts,  584 
Isobaric  gradient,  587 
Isobaric  surfaces,  587,  596 
Isobars,  584,  620 

annual,  585 

courses  of,  588 

and  humidity,  594 

and  parallels,  589 

and  temperature,  590,  592 

relation  of,  to  land  and  water,  589 
Isogonic  lines,  477 
Isothermal  chart  of  United  States, 
annual,  553 

for  April,  554 

for  January,  554 

for  July,  555 

for  October,  555 
Isothermal  charts,  539 
Isothermal  surfaces,  546 
Isotherms,  539 

courses  of,  540,  624 

Japan,  earthquakes  in,  412 
Jefferson,  M.  W.,  206 
Jetties  of  the  Mississippi,  128 
Johnson,  L.  C.,  206 
Joints,  53 

Jordan,  Pres.  D.  S.,  434 
Jordan  craters,  364 
Judd,  Prof.  J.  W.,  390 
Jupiter,  504 

Kames,  268 
Karst,  98 

Karst  topography,  98 
Kaskaskia,  189 
Kemp,  Prof.  J.  F.,  337 
Kenai,  445 
Kilauea,  363,  364 
Kilimanjaro,  445 
King,  F.  H.,  112 
Krakatoa,  348 
Kiimmel,  H.  B.,  291 

Laccoliths,  374 
Lacustrine  plains,  333,  437 
Lag  of  tides,  741 
Lake  Agassiz,  282,  333 

Agnes,  35 

Baikal,  297 

Balkash,  297 

Bonne ville,  314 

Chad,  297 


lake  Chelan,  297 

Como,  297 

Erie,  297 

Garda,  297 

Huron,  297 

Michigan.  297 

Nyassa,  297 

Ontario,  297 

Pepin,  292 

Pontchartrain,  310 

St.  Clair,  delta  of,  197 

Superior,  297 

Tanganyika,  297 

Titicaca,  297 

Victoria  Nyanza,  297 

Winnipeg,  297 
Lake  basins,  filling  of,  302 

origin  of,  303 
Lake  water,  movements  of,  300 

sources  of,  301 
Lakes,  292 

area  of,  295 

changes  of  level  of,  300 

changes  taking  place  in,  302 

climatic  effects  of,  316 

conditions  necessary  for,  301 

depth  of,  295,  296 

distribution  of,  293 

economic    advantages   and   disail* 
vantages  of,  316 

fate  of,  303 

lowering  of  outlets,  303 

origin  of,  313 

sections  of,  298 

topographic  position  of,  295 

volume  of  water  in,  297 
Lakes  of  Red  River  (La.)  Valley,  301 
Land,  materials  of,  45 
Land  and  water,  effect  on  temper* ,~ 
ture,  541 

heating  of,  530 
Land-breezes,  561,  610 
Land  degradation,  rate  of,  154 
Landslides,  106 
Landslip  Mountain,  106 
Land-tied  islands,  329 
Land-water,  source,  80 
Lateral  moraines,  254 

mode  of  origin,  257 
Latitude,  490 

and  sun  altitude,  502 

length  of  degrees,  494 
Lava,  338,  367 

Lava-flows  of  the  northwest,  372 
Lead  and   zinc  ores  in  the  United 

States,  distribution  of,  456 
Le  Conte,  Prof.  J.,  473 
Leuk,  springs  of,  98 
Leverett,  F.,  290 


LXDEX 


765 


Life  and  climate,  702 
Lightning,  665 
Limestone,  49 

origin  of,  718 
Limestone  sinks,  97 
Lithosphere,  relief  of,  9 
Littoral  climate,  691 

currents,  319,  326,  729 
Load  of  streams,  122,  128 
Local  variations  of  pressure,  597 
Loess,  58,  59,  61 
Lone  Star  Geyser,  92 
Longitude,  491 

and  time,  490,  492 
Louisville  tornado,  670,  671 
"Low."     (See  Cyclones) 
Low  tides,  738 

Magnetic  declination,  477,  478 

inclination,  479 
.    intensity,  479 

meridians,  475,  476 
Magnetism,  terrestrial,  475 
Malaspina  Glacier,  210 
Mammoth  Cave,  97 
Mammoth  Springs,  102 
Mantle  rock,  46 
Mars,  504 

Maryland  vein,  100 
Marysville  buttes,  384 
Massive  rocks,  52 
Materials  of  the  land,  45 
Mature  topography,  152 

valley,  149 

Maturity  of  rivers,  150 
Mauna  Kea,  442-443 
Mauna  Loa,  339,  362,  365 
McGee,  W  J,  434 
Mean  monthly  temperatures,  680 
Medial  moraine,  255 
Medicinal  springs,  90 
Mercury,  504 
Meridians,  484,  491 
Merrill,  G.  P.,  78,  206 
Mesa,  34,  173 
Metamprphic  rocks,  53 
Meteorites,  514 
Meteorology  defined,  3 
Meteors,  509 
Mill,  H.  R.,  337 
Milne,  J.,  434 
Mineral  matter  of  sea,  716 

amount  of,  716 

source  of,  717 

withdrawal  of,  718 
Mineral  springs,  90 
Mississippi   River,  amount   of  sedi- 
ment carried  by,  121 

delta  of,  181,  197 


Mississippi  River,  flat,  136 

rate  01  erosion,  154 
Mistral,  648 

Moisture,  effect  on  temperature,  545 
Moisture  of  the  air,  564 
Monadnocks,  153,  171 
Monoclinal  folds,  406 
Monsoon  winds,  562,  605 

of  Chicago,  609 

of  India,  608 

of  Spain,  610 
Mont  Pelee,  350 
Monument  Park,  71 
Moore,  W.,  511 
Moraine-dams,  258 
Motions  of  earth,  484 
Moulton,  Prof.  F.  R.,  740 
Mount  Hood,  381 

Mazama,  307 

McKinley,  441 

Rainier,  381,  383,  441 

Shasta,  380,  381,  382 

Whitney,  441 

Wrangell,  445 
Mountain  breezes,  562 

chain,  439 

climate,  691 

group,  439 

range,  438 

ridge,  438 

system,  439 
Mountains,  15,  33,  437 

barriers  to  animals  and  plants,  452 

barriers  to  transportation,  452 

changes  taking  place  in,  443 

distribution  of,  439 

effects  of,  450 

produced  by  erosion,  445 
by  faulting,  448 
by  folding,  447 
by  intrusion,  447 

height  of,  440 

in  history,  38 

in  ocean,  442 

origin  of,  39,  445 
Movement  of  sea-water,  719 

causes  of,  727 
Mud  cones,  389 

volcanoes,  388 
Murdoch,  L.  H.,  337 
Murray,  Sir  John,  337,  757 

Nansen,  F.,  291 

Narrows,  169 

Natural  Bridge  of  Virginia,  163 

Natural  bridges,  98,  161 

Natural  levees,  187 

Neap  tides,  744 

Needle  Mountains,  34 


766 


INDEX 


Neptune,  504 

New  Madrid  earthquake,  418 

Niagara  Falls,  167 

Nimbus  clouds,  bt\j 

Nitrogen,  512,  513 

Nitrogenous  compounds,  513 

Normal  fault,  406 

North  Atlantic,   temperature   curve 

for,  723 

Northern  lights,  509 
Northers,  646 
Nunatak,  237 

Obsidian,  367 
Ocean,  the,  706 

age  of,  718 
Ocean  basins,  708 

continuity  of,  11 

Ocean  bottom,  topography  of,  713 
Ocean  currents,  721,  730 

cause  of,  733 

climatic  effects  of,  733 

effect  on  temperature,  544 

gradational  effects  of,  734 
Ocean,  depth  of,  711 
Ocean  life,  749 
Ocean,  mass  of,  712 
Ocean  movements,  types  of,  729 
Ocean,  reasons  for  low  temperature 
of,  724 

relation  to  rest  of  earth,  756 

temperature  of,  721,  722 

volume  of,  712 

Ocean  water,  temperature  and  move- 
ment, 722 

Oceanic  climates,  683,  688 
Oceanography,  defined,  3 
Oceans,  area  of,  6 
Old  age  of  rivers,  150 
Ores,  distribution  of,  452 
Orizaba,  445 
Ouachita  Mountains,  23 
Outwash  plain,  266 
Ox-bow  lakes,  189 
Oxygen  of  atmosphere,  512,  514 
Ozark  Mountains,  23 

Pacific,       equatorial       temperature 

curves  for,  724 
Paint-pots,  390 
Palisade  Ridge,  385,  387 
Parallels   4SO 
Peary,  Lieut.  R.  E.,  291 
Pelee,  58 
Peneplains,  153 
Perched  bowlder,  262,  264 
Perihelion,  489 
Periodic  winds,  £98 
Petrifaction,  104 


Petrified  tree-trunks,  104 
Physical  geography  of  the  sea,  709 
Piedmont  alluvial  plain,  183 
Piedmont  glaciers,  222,  240 
Piedmont  plateau,  28 
Piracy,  176 
Plains,  15,  16,  435 

topography  of,  24 
Planetary  winds,  604 
Plateau  climate,  691 
Plateaus,  15,  28,  437 

origin  of,  31 

position  and  area  of,  30 
Polar  circles,  502 

zones,  684,  686 

climate  of,  700 
Poles  of  earth,  484 
Ponding,  175 
Popocatepetl,  445 

Population  of  the  United  States,  26 
Pore  space,  82 
Pot-holes,  168,  169 
Powell,  Major  J.  W.,  474 
Precipitation,  80,  573,  616 

and  general  circulation,  614 

necessary  for  agriculture,  615 
Pressure,  aperiodic  changes  of,  620 

atmospheric,  532 

inequalities  of,  583 
Prevailing  winds,  598,  604 

Radiation,  526 

Rafts,  308 

Railway  map  of  the  United  States, 

453 

Rain,  580 
Rainfall,  614 

and  agriculture,  701 

in  California,  619 

distribution  of,  615 

and  sugar  crops,  702 

of  the  United  States,  452,  61& 

zone  of  trades,  616 

zones  of  westerlies,  617 
Rain-making,  580 
Rain-water,  fate  of,  81 
Raised  beaches,  394 
Ravines,  119 
Reade,  T.  M.,  474 
Recessional  moraines,  277 
Reconstructed  glacier,  222 
Red  Sea,  temperature  of,  725 
Red  snow,  239 
Reefs,  325 

Reid,  Prof.  H.  F.,  290 
Regolith,  46 

Rejuvenation  of  streams,  174 
Relative-  humidity,  570 
Relief  of  iithosphere,  9 


INDEX 


767 


Relief  features  of  first  order,  5,  12 

of  the  land, 15 

of  sea  bottom,  li> 

of  second  order,  1 5 
Relief-map  of  United  States,  27 
Reversed  fault,  406 
Revolution  of  earth,  488 
Rise  of  land  (relative),  evidences  of. 

393 

River  floods,  195 
River  ice,  213 
River  lakes,  308 
River  plains,  436 
River  system,  history  of,  141 
Rivers,  amount  of  water  in,  114 

effect  on  shores,  329 

load  of,  122 
Rock,  47 
Rock-basins,  275 
Rock-breaking,  73,  111 
Rock  decay,  111 
Rock  flour,  251 
Rock  terraces,  170,  171 
Rock  waste,  46,  47 
Roots,  agents  of  weathering,  76 
Rotation  of  earth,  484 
Rotation,  effect  of,  488 

winds  deflected  by,  602,  604 
Running  water,  work  of,  114 
Run-off,  83 

Russell,  Prof.  I.  C.,  205, 289, 290, 337, 
390,  511 

Salinity  and  movement,  719 
Salt  Lake,  315 
Salt  lakes,  314 
Sand,  62 

lodgment  of,  62 

sources  of,  62 
Sandstone,  49 

San  Francisco  earthquake,  419 
San  Francisco  Mountain,  381,  384, 

386 

Satellites,  505 
Saturation,  569 
Saturn,  504 

Scandinavia,  changes  of  level  in,  393 
Sea,  temperature  of,  721 
Sea  bottom,  materials  of,  753 
Sea-breezes,  561,  610 
Sea  caves,  394 

ice  of,  726 

Sea  cliff,  321,  322,  323,  394 
Sea-level,  400.  707 

distortion  by  attraction,  707 

inequalities  of ,  707,727 
Seasonal  range  of  temperature.  559 
Seasons,  531 

change  of,  533 


Seasons  in  different  latitudes,  534 
Sea-water,  composition  of,  710 

gases  in,  718 

movements,  727 

salinity  and  color,  720 
Secular  changes  of  level,  392 
Sedimentary  rocks,  48 
Sedimentation,  a  cause  of  change  of 

sea-level,  400 
Seepage,  89 
Seiches,  300 
Semi-arid  lands,  618 
Sensible  temperature,  677 
Seward  glacier,  234 
Shale,  49 
Shaler,  Prof.   N.  S.,   112,  205    290, 

337,  434 

Shepard,  E.  M.  Prof.,  434 
"Shooting-star"  dust,  753 
Shooting-stars,  509,  514 
Shore  current,  318,  319 

deposition  by,  324 
Shore-drift,  325 
Shore  ice,  333 

lakes,  311 

terraces,  334 
Shore  lines,  292 

effect  of  glaciers  on,  330 

effect  of  rivers  on,  329 

effect  of  wind  on,  330 

topographic  features  of.  317 
Sierra  el  Late  Mount  aias,  34 
Sills,  374,  375 

Sinking  of  coasts,  evidences  of,  395 
Sirocco,  645 

Slichter,  Prof.  C.  S.,  112 
Sliding,  105 

Slumping,  105,  107,  135 
Snake  river,  31 
Snow,  214,  580 
Snow  and  ice,  work  of,  207 
Snow-eaters,  674 
Snowfall,  682 

at  Chicago,  680 

in  the  United  States,  556 
Snow-fields,  215 
Snowflakes,  81 
Snow  line,  215,  217 
Soil,  46 
Solar  climate,  683 

system,  504 

tides,  743 
Solstices,  499 

Solution  by  ground-water,  96 
Soufridre,  57,  350 
Sounding  line,  712 
South  Atlantic,  temperatures  in,  725 

temperature  curve  for,  725 
Spanish  Peaks,  445 


768 


INDEX 


Spits,  327 

Spring,  defined,  531 
Spring  tides,  736,  744 
Springs,  89 

mineral  and  medicinal,  90 

temperature  of,  89 
Stalactites,  101 
Stalagmites,  101 
Standard-time  zones,  495 
St.  Louis  tornado,  669 
Stone,  G.  H.,  2^0 
Storms,  621 

special  types  of,  663    • 
Stratified  drift,  287 
Stratified  rocks,  48 
Stratus  clouds,  576 
Stream  water,  sources  of,  116 
Streams,  accidents  to,  173 

antecedent,  177 

consequent,  177 

deposition  by,  179 

erosive  work  of,  120 

load  of,  122 
Stromboli,  341 

Submarine    volcanic    extrusions,    a 
cause    of    change    of   sea-level, 

401 

Submerged  forests,  396 
Submerged  valleys,  397,  714 
Subsoil,  47 
Summer,  defined,  531 
Summer     and     winter,    differences, 

532 

Summer  solstice,  499 
Sun,  varying  distance  of,  536 
Sun-cracks,  135 

Sun's  distance,  effect   on   tempera- 
ture. 536 
Supan,  A.,  688 
Surf,  319 

Suspension   of   sediment    in    rivers. 
127 

Tajamulco,  445 
Talus,  74,  134 
Talus  glacier,  107 
Tanner,  Lieut.  Com.,  Z.  L.,  757 
Tarr,  Prof.  R.  8.,  337,  474 
Taylor,  F.  B.,  291,  337 
Temperature  of  air,  520 
effect  on  movement,  561 
vertical  movements,  563 
Temperature  and  altitude,  537 
and  condensation  of  water-vapor, 

572 
Temperature  changes  with  rise  of  air, 

537 

curve    for    the     North    Atlantic, 
723 


Temperature   curve    for   the   South 

Atlantic,  723 

Temperature,  effect  on  evaporation, 
568 

of  springs,  89 

of  sea,  721 

on  weather  map,  624 

zones,  688 

Temperatures  in  South  Atlantic,  725 
Temporary  base-level,  139 
Terminal  moraines,  257,  276 
Terrestrial  magnetism,  475 
Texas,  coast  of,  461 

model  of,  36 
Thermal  equator,  602 
Thermometer,  520 

self-registering,  726 
Thomson,  Dr.  C.  Wyville,  757 
Thrust-fault,  406 
"Thunder-squall,"  664 
Thunder-storms,  610,  663,  666 
Tidal  currents,  736 
Tidal  poles,  741 
Tidal  races,  736 
Tides,  729,  735 

cause  of,  736 

effects  of  on  shores,  748 

lag  of,  741 

monthly  variation  of,  746 

periodicity  of,  736 

rate  of  movement,  748 
Timber  jams,  308 

Topographic  features  of  shores,  317 
Topographic  relations,  effect  on  tem- 
perature, 545 

Topographic  unconformity,  332 
Tornado  near  Chicago,  672 

at  Louisville,  670 

at  St.  Louis,  669 

at  Rochester,  Minn.,  672 
Tornadoes,  611,  657 
Tower,  N.  S.,  206 
Trade-winds,  603,  617 
Trade-wind  zone,  687 
Transpoitation  by  rivers,  127 
Travertine,  101 
Tributaries,  growth  of,  147 
Tropic  of  Cancer,  501 

of  Capricorn,  501 
Tropical  belts  of  high  pressure,  594 

calms,  601,  617 

cyclones,  611,  648 

zone,  684,  685 

climate  of,  693 

Turtle  Mountain  landslide,  106 
Tuscarora  deep,  711 
Tyndall,  J.,  290 
Typhoons.  655 
Typhoon  tracks,  659 


INDEX 


769 


Udden,  Prof.  J.  A.,  78 

Undertow,  318,  319,  729 

Unequal  heating  of  land  and  water, 

effect  on  wind,  605 
Unequal  insolation,  effect  on  winds, 

598 

Upham,  Warren,  291,  337 
Uranus,  504 


Valley,  stages  in  history  of,  148 
Valley  breezes,  562 

flats,  135 

glacier,  219,  223 

train,  266 
Valleys,  118 

courses  of,  144 

deepening  of   129 

depth-limit  of,  131 

lengthening  of,  140 

widening  of,  131 

width-limit  of,  134 
Variable  climate,  677 
Veins,  100,  101 
Velocity  of  wind,  613 
Venus,  504 
Vesuvius,  341 
Volcanic  "ash,"  56,  368 
Volcanic  belts,  368 

cinders,  368 

cones,  378 

dust,  56 

eruptions,  phenomena  of,  366 

gases,  368 

glass,  367 

mountains,  445 

necks,  385 

plugs,  385 

vapors,  368 
Volcano,  338 
Volcanoes,  active,  341 

distribution,  368 

explosive  type,  340 

number,  368 

products  of,  367 

quiet  type,  340 

topographic  effects  of,  378 
Vulcanism,  305,  338 

causes  of,  375 

effect  on  coast-lines,  464 


Walcott,  Dr.  C.  D.,  206 
Waldo,  Frank,  511 
Wall  Lake ,  Iowa,  210 
Ward,  Prof.  R-  D.  C.,  511,  684 
Warm  currents,  721 ,  732 
Warped  shore-lines,  394 
Warping,  405 


Washington,  rainfall  in,  619 
Watchung  Mountains,  387 
Waterspouts,  673 
Water  surface,  85 
Water  table,  85 
Water-vapor,  564 

amount  of,  568 

of  atmosphere,  512,  517 

distribution  of,  569 

sources  of,  565 
Wave-cut  terrace,  324 
Waves,  318,  729 

crest  of,  318 

deposition  by,  324 

erosive  work  of,  320 

"fetch  "  of,  321 

length  of,  318 

of  translation,  319 

period  of,  318 

trough  of,  318 
Weather  Bureau,  622,  663 
Weathering,  72,  76,  110,  129 

conditions  affecting,  111 
Weather  maps,  620,  627 
Weather  predictions,  656 

failure  of,  660 

value  of,  663 
Weed,  W.  H.,  113 
Westerly  winds,  603 
West  Indian  storms,  653 
Whirlwinds,  611,  666 
Width-limit  of  valleys,  134 
Wild,  J.  J.,  757 
Willis,  Bailey,  206,  473 
Wind,  abrasion  by,  70 
Wind-carving,  70 
Wind,  cause  of  ocean  movements, 

728 
Wind,  direction,  613 

in  upper  air,  606 

effect  on  evaporation,  568 

effect  on  shores,  330 
Wind  equator,  602 
Wind -gap,  177 
WTind  gradient,  613 
WTind,  mechanical  work  of,  55 
Wind  roses,  609 
Wind    shown    on     weather     maps. 

622 

Wind  velocities,  613,  614 
Wind  zones,  686 
Winds,  affected  by  moisture,  569 

deflected  by  rotation,  602,  604 

due  to  unequal  temperature,  561 

effect  on  temperature,  543 

gradational  effect  of,  62 
Winter,  denned,  531 
Winter    and     summer,    difference*, 
532 


770 


INDEX 


Winter  solstice,  499 
Woonsocket,  artesian  well  at,  94 
Wyanaotte  Cave,  97,  101 

Yellow  River  of  China,  196 
Yellowstone  Canyon,  159 
Yellowstone  Lake,  102 
Yellowstone  National  Park,  90,  103 


Young  valley,  148 
Youthful  topography,  152 
Youth  of  rivers,  15U 

Zones  of  climate,  684 
defined  by  isotherms,  687 
by  latitude,  685 
by  winds,  686 


Chamberlin  and  Salisbury's  Geology 

By  THOMAS  C.  CHAMBERLIN  AND  ROLLIN  D.  SALISBURY, 
Professors  in  the  University  of  Chicago.  (American  Science  Se- 
ries, Advanced  Course.)  3  Volumes  8vo. 

Volume    I.     Processes    and    Their    Results.      xix-f654    pp.    $4.00 
Volumes    II    and    III.      Earth    History.      xxvi-|-692-(-xi-)-624    pp. 
Vols.  II  and  HI  not  sold  separately.    $8.00. 

Chas.  D.  Walcott,  Director  of  U.  S.  Geological  Survey:— \  am 
impressed  with  the  admirable  plan  of  the  work  and  with  the  thorough 
manner  in  which  geological  principles  and  processes  and  their  results 
have  been  presented.  The  text  is  written  in  an  entertaining  style, 
and  is  supplemented  by  admirable  illustrations,  so  that  the  student 
cannot  fail  to  obtain  a  clear  idea  of  the  nature  and  work  of  geological 
agencies,  of  the  present  status  of  the  science,  and  of  the  spirit  which 
actuates  the  working  geologist. 

Israel  C.  Russell,  Professor  in  the  University  of  Michigan: — 
The  work  is  certainly  monumental,  and,  like  Lyell's  Principles  of 
Geology,  will  in  the  future,  I  am  confident,  be  recognized  as  marking 
the  beginning  of  a  new  period  in  the  development  of  the  science  of 
the  earth. 

R.  S.  Woodward,  Director  of  the  Carnegie  Institution: — It  is 
admirable  for  its  science,  admirable  for  its  literary  perfection,  and 
admirable  for  its  unequalled  illustrations. 


College  Geology 


By  T.   C.   CHAMBERLIN  AND  R.   D.   SALISBURY.     (American 
Science  Series.)  xvi-f-9/8  pp.    8vo.    $3.50. 

A  one-volume  text-book  on  the  lines  of  the  authors'  widely 
known  three-volume  work.  It  supplies  ample  material  for  the  av- 
erage college  course  in  geology. 

T.  C.  Hopkins,  Professor  in  Syracuse  University:  —  I  find  it  so 
far  in  advance  of  the  other  text-books  I  have  seen  that  I  decided  to 
use  it  in  my  class. 

A.  H.  Purdue,  Professor  in  thz  University  of  Arkansas:—  It 
probably  will  be  the  standard  college  text-book  in  Geology  for 
several  years  to  come. 

E.  N.  Lowe,  Director  of  Geological  Survey,  Jackson,  Miss.:  —  It 
is  an  unrivaled  text-book  for  a  brief  course  in  general  geology. 

£      rT>  34  West  33d  Street,  Hew  York 

&    L/\J.          623  So.  Wabash  Avenue,  Chicago 


Salisbury's  Physiography.  Advanced  Course 

By  ROLLIN  D.  SALISBURY,  Professor  in  the  University  of  Chi- 
cago. (American  Science  Series,  Advanced  Course.)  xx-f-77o 
pp.  8vo.  $3.50. 

While  planned  for  courses  in  colleges  and  normal  schools,  it 
presupposes  no  previous  study  of  physical  geography. 

W.  M.  Davis,  Harvard  University:— It  is  an  important  addition 
to  college  literature,  the  first  book  of  college  grade  on  physiography. 
The  illustrations  are  for  the  most  part  remarkably  successful  and 
appropriate. 


Salisbury's  Physiography  for  High  Schools 

By  ROLLIN  D.  SALISBURY,  Professor  in  the  University  of  Chi- 
cago. (American  Science  Series,  Briefer  Course.)  viii-f53i 
pp.  Large  I2mo.  $1.50. 

This    is    intended    to    cover   the   work    in   physical   geography    as 
given  in  the  first  or  second  year  of  the  high  school. 

W.  H.  Hawkes,  Ann  Arbor  (Mich.*)  High  School:— I  am  of  the 
opinion,  after  careful  examination  of  the  leading  Physiographies 
on  the  market,  that  Salisbury's  Briefer  Course  of  Physiography  is 
in  the  lead,  in  the  method  of  treatment  of  the  subject  of  Earth 
Science,  its  clearness  of  presentation  of  detail,  and  especially  in  the 
ways  in  which  the  subject  matter  is  discussed  in  illustrative  dia- 
grams, 

Salisbury's  Elementary  Physiography 

By  ROLLIN  D.  SALISBURY,  Professor  in  the  University  of 
Chicago.  (American  Science  Series,  Elementary  Course.) 
X'  +  359PP-  47  plates.  Large  i2mo.  $1.30. 

This  is  particularly  well  adapted  to  first  year  pupils  and  short  enough 
for  a  half-year  course  if  need  be. 

C.  L.  Walton,  Lake  Visiu  High  School.  Chicago: — "It  would  seem 
that  nothing  further  could  be  done  in  the  way  of  perfecting  a  text-book  in 
elementary  physiography.  As  we  are  to  have  but  five  months  of  this  study 
hereafter,  I  think  no  other  book  is  so  well  adapted  to  o-ir  needs  as  this  one." 

HENRY    HOT^T    &    CO  3*  West  33d  Street.  Hew  York 

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Elements  of  Geography 

By   ROLLIN   D.    SALISBURY,    HARLAN    H.    BARROWS 
and    WALTER    S.    TOWER,    of  the    Department  of  Geog- 
raphy,   University  of  Chicago.        (American    Science    Series.) 
ix+6i6  pp.    7  maps  in  color.     I2mo.    $1.50. 

A  course  in  geography  which  is  adapted  to  courses  in  colleges, 
normal  schools  and  in  the  later  years  of  the  high  school.  It  treats 
physiographic  processes  and  features  briefly  and  develops  at  greater 
length  the  relations  of  earth,  air,  and  water  to  life  and  especially  to 
human  affairs. 

J.  C.  Branner,  Stanford  University: — It  is  a  remarkably  well 
written,  well  arranged,  and  well  illustrated  book,  and  is  fairly  burst- 
ing with  information  of  the  most  trustworthy  kind,  and  references 
to  the  best  books  and  papers  on  special  topics. 

Modern  Geography 

The  Effects  of  Physical  Features  on  Living  Things 

By  ROLLIN  D.  SALISBURY,  HARLAN  H.  BARROWS, 
and  WALTER  S.  TOWER,  of  the  Department  of  Geog- 
raphy, University  of  Chicago.  ix+4Q6  pp.  7  maps  in  color. 
I2mo.  $1.25. 

This  book  is  planned  for  use  in  the  first  or  second  year  of  the 
high  school.  Representing  the  "new"  geography  which  stresses 
the  geographic  factor  in  human  life,  it  lays  a  solid  foundation  of 
physiography  but  vitalizes  it  by  constantly  showing  that  broad  phy- 
siographic principles  underlie  all  commercial,  industrial  and  econo- 
mic activities  of  mankind. 

Caroline  C.  Haney,  Eastern  High  School,  Detroit,  Mich.:— 
What  we  need  in  our  curriculum  is  the  humanizing  of  the  technical 
so  that  the  child  may  co-ordinate  his  everyday  life  and  interests  with 
his  school  work.  This  has  been  most  successfully  accomplished  in 
Modern  Geography. 

Laboratory  Manuals  of  Geology  and 
Physiography 

By  ROLLIN  D.  SALISBURY,  Professor  in  the  University  of 
Chicago,  and  ARTHUR  C.  TROWBRIDGE,  Professor  in 
the  State  University  of  Iowa.  I2mo.  Paper,  25  cents  each. 

I.  The  Interpretarion  of  Topographic  Maps.     64  pp. 

For  beginning  courses  in  physiography. 
II.  Studies  in  Geology.    68  pp. 

Based    on   topographic    maps  and    folios  for  classes   in 
physiographic  and  structural  geology. 
III.  Historical  and  Structural  Geology.     76pp. 

TTTJi\TT>V     TTf>T   rP   XT    PO  34  West  33d  Street,  New  York 

±lH(lM±t  I      tlULi  1    <X    l^LJ.  623  So.  Wabash  Ave.,  Chicago 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 

Los  Angeles 
This  book  is  DUE  on  the  last  date  stamped  below. 


MOV  9    1951 
2  1  1954 


NOV 


1  1959 


Form  L9-50m-ll,'50  (2554)444 


—  •  — 

The  RALPH  D.  RFED  UBRARY 

DEPARTMENT  OF  GEOLOGY 

UNIVERSITY  of  CALIFORNIA 

U»  AXOELES.  CALIF. 


